Physical Resources
The topography, settlement patterns, and mineral resources of Dutchess County are all influenced by the underlying geology. The highest mountains contain the hardest rocks, and communities in the county are generally located in areas of sand and gravel because of the relatively level terrain and abundant water supplies they contain. Construction aggregates are mined where suitable deposits are found. Understanding geologic materials and processes is essential to sound resource management because the geology affects the quality and quantity of groundwater resources, the migration of pollutants, potential hazards to inhabitants, drainage patterns, mineral resources, and soil characteristics.
Geology is the study of the earth, including all materials found at and below the earth’s surface. Geologists analyze the composition, origin, and ongoing changes in the rocks and sediments that compose the earth. The natural processes that shape the land – uplift, erosion, deposition of sediments, and faulting – are as vigorous today as they were in the past. These processes have resulted in a continual recycling of materials formed over the past 4.6 billion years of Earth’s history. Here in Dutchess County, we can see the effects of this rock cycle in the ancient gneisses exposed in Stissing Mountain and the Hudson Highlands, in the schists and marbles of the Harlem Valley, in the sandstone and shale that underlie Rhinebeck, and in the glacial deposits found throughout the county.
The geologic structure of Dutchess County is complex. Its history extends over one and a half billion years and has included several periods of major mountain building, ocean invasion and retreat, and glaciation. These events are part of the dynamic evolution of the earth’s crust. However, there are also large gaps in the record of the geological history of the county; vast segments of time are unaccounted for within its borders due to erosion or simply lack of deposition. Today’s topography is the result of the interaction between internal forces that uplift the land and external elements (weather, water, gravity, and human efforts) that continually erode it away.
Geologic History & Context
All of the rocks in Dutchess County were formed during the Precambrian Eon, and the Cambrian and Ordovician periods of the early Paleozoic Era, or between about 1.5 billion years and 400 million years ago. The oldest rocks in the county were deposited as sediments 1.5 billion years ago and around 1.0 billion years ago these sediments were caught in a collision between ancestral North America and another continent (perhaps an ancestral version of Europe or Africa). The collision, resulting in a mountain building event that is referred to as the Grenville Orogeny, caused the sediments to be deformed, heated, and changed into the high-grade metamorphic rock known as gneiss. In places, the sediments were heated to the point of melting and an igneous rock known as granite was formed. The resulting mixture of granite and gneiss is very resistant to erosion and forms the bedrock of the Hudson Highlands in the southern portion of the county and throughout neighboring Putnam County.
At times when sea level was higher, Dutchess County was covered by deeper water, and fine-grained black shales were formed, representing deposition in deep ocean waters of the abyssal plain (the deepest and flattest part of the ocean basins). These shales underlie much of the county, but since they are relatively soft, they are not often seen at the surface. Both the limestones and shales are autochthonous, meaning that they were formed in their present location.
About 450 million years ago, the east coast of North America began to feel the effects of a collision of continental plates, that would continue in two major pulses over the next 200 million years. During the initial stages of this collision (known as the Taconic Orogeny), interbedded sandstones and shales were deposited by submarine landslides along the continental shelf and slope. Later, large masses of sandstone and shale deposited in the open ocean at or just beyond the edge of the continental shelf were pushed from an area far to the south and east up and over the autochthonous shales and limestones of Dutchess County in a process known as thrust faulting. The Taconic Allochthon, which was created by this thrust faulting, formed a high range of mountains, which would come to be the Taconic Mountains in the vicinity of the present New York-Connecticut border.
The mountain building of the Taconic Orogeny squeezed and heated rocks caught within the collision zone causing them to be changed or “metamorphosed.” The limestones in the western part of the county were metamorphosed to become the marbles today found in the Harlem Valley. Shales were gradually metamorphosed to become schists and gneisses in different parts of the county. The intensity of metamorphism of the rocks increases from northwest to southeast with low-grade metamorphism in the northwestern part of the county (mostly shale), medium-grade metamorphism in the central part of the county, (mostly schist), and high-grade metamorphism in the southeastern part of the county (gneiss).
The latest notable geologic force to influence Dutchess County was the advance of the massive Laurentide ice sheet southward out of Canada, which extended as far south as Long Island approximately 20,000 years ago. The retreat of the ice sheet left behind the surficial deposits of the county and the topography that we see today. As the Laurentide Ice Sheet retreated, it left behind sediment ridges that dammed glacial meltwater to form lakes. The largest of these lakes occupied the Hudson River valley and was called Lake Albany. It gradually lengthened as the ice margin retreated northward, eventually merging with a precursor of Lake Champlain called glacial Lake Vermont [1]. A much larger lake, Lake Iroquois, to the west of Lake Albany formed in the area where Lake Ontario exists today. At around 13,000 years ago, the ice dam separating the two lakes was breached, leading Lake Iroquois to catastrophically drain through Lake Albany to the south, breaching the moraine dam that impounded Lake Albany and scouring out the lake sediments within the Hudson Valley [2].
Topography & Elevation
Dutchess County, being part of the Mid-Hudson Valley, has a fairly diverse mix of terrain, ranging from near sea level at the Hudson River to the rolling hills and farmlands of the central and northern parts of the county to the highlands of the eastern and southern parts of the county. Being in a river valley, the majority of the county is at relatively low elevations. Exceptions include the Hudson Highlands along the southern border with Putnam County, West, East, and Stissing Mountain ridges in the county’s center, and the Taconic Mountains in eastern Dutchess County along the borders with Connecticut and Massachusetts. While hilltop elevations in the lowlands are generally below 500 feet above sea level, some elevations in these highland areas exceed 1,500 feet.
Topography is intimately related to the underlying geology. Much of the Mid-Hudson Valley is underlain by soft shale or carbonate bedrock, while the Hudson Highlands and the Taconic Mountains are underlain by much harder granites, gneisses, schists, and other hard rocks. Carbonate rocks often form localized valleys in the eastern part of the county and ridges in the west. For example, the Harlem and Clove Valleys in the east, as well as the ridge beneath the Galleria Mall in Poughkeepsie, are all under underlain by carbonates. Major streams commonly occur over soft carbonate rocks in Dutchess County. Webatuck Creek, Wappinger Creek, and Fishkill Creek all occur on carbonate bedrock.
Low and relatively flat terrain can be found in the county’s major valleys and floodplain areas such as the Fishkill Creek/Sprout Creek Valley, the Wappinger Creek Valley, the Harlem Valley, Clove Valley, and the plain created by a glacial melt-water lake west of Red Hook and Rhinebeck. While these valley bottoms have low relief, adjacent areas are often hilly with relief of several hundred feet. The highest terrain in the county can be found in the Taconic Mountains, with Brace Mountain being the highest point in the county at 2,311 feet, as well as the Hudson Highlands to the south. Stissing Mountain in Pine Plains, at 1,388 feet, also stands out, as it provides contrast from the lower elevation rolling hills in its immediate vicinity.
Steep SlopesGenerally speaking, lowlands were developed first in Dutchess County. Early development occurred along the Hudson River, along major streams and along major roads and railroads, which were built in lowland areas first. Development of level areas is often relatively easy and inexpensive. Today, much of the development occurs on moderate to steep slopes. Development in areas of steep slope is more expensive, more difficult, and is prone to serious environmental impacts. Care must be taken to ensure that development on slopes does not adversely affect pre-existing development in the lowlands or create hazards. Development on steep slopes can lead to significantly increased soil erosion and greatly increased stormwater runoff which may lead to increased flooding in local streams and a variety of other problems. For this reason, a number of Dutchess County municipalities have passed steep slope ordinances in recent years, seeking to reduce these problems.
Glacial LandformsHistoric glaciation has has a tremendous impact on the topographic features of Dutchess County and the region. In some places, glacial till was sculpted into ridges parallel to the margin of the Laurentide ice sheet, known as glacial moraines. These ridges reflect both the bulldozing of material by the advancing glacier and the continuous delivery of new material to the glacier’s edge by the movement of ice as the edge remained fixed in position for centuries. In Dutchess County, these moraines are traceable for 5 to 15 miles from west to east and rise up to 180 feet above the surrounding landscape [3]. Glacially streamlined rock knobs called drumlins are also mantled by glacial till. Examples of such hills include Spy Hill between Route 55 and Route 376 in the Town of Poughkeepsie and the College Hill Park area in the City of Poughkeepsie.
Flat valley floors testify to the former presence of glacial lakes and outwash plains in the county. These easily eroded sands and gravels provided a ready pathway along which post-glacial streams currently flow. Streams like the Wappinger Creek, Fishkill Creek, and Tenmile River flow through many of these deposits and have subsequently left their own in the form of channel floodplain sediments.
Dutchess County Geology
Dutchess County contains rocks from all three of the major rock groups: metamorphic, sedimentary, and igneous. Metamorphic rocks are those that have been changed in texture and composition by heat, pressure, or chemically active solutions deep within the earth but without melting. In contrast, igneous rocks are formed from the solidification of molten material, either at Earth’s surface (volcanic rocks), or deep in Earth’s interior (plutonic rocks). Sedimentary rocks are formed at or near the earth's surface by depositional and biological processes. Clastic sedimentary rocks are composed of rock fragments and clay minerals that have been cemented together and include shale and sandstone. Carbonate rocks (sedimentary rocks composed of calcium carbonate) may be formed by biological processes, such as the formation of coral reefs, or by physical processes, such as the precipitation of carbonate minerals out of saturated sea water. Carbonates that contain a significant component of calcium-magnesium carbonate are labeled “dolomite” or “dolostone.”
The bedrock of Dutchess County includes all of the solid rocks found in road cuts, valleys, and mountaintops. It can be divided into two basic groups: 1) older, highly altered (metamorphosed) former sedimentary and igneous rocks found primarily in the Hudson Highlands and in scattered outcrops elsewhere in the county, and 2) younger, slightly to highly metamorphosed sedimentary rocks found throughout the rest of the county. As mentioned in the geologic history section, the degree of metamorphism in the younger rocks increases from the northwest (Red Hook and Rhinebeck) to the southeast (Dover and Pawling). The rocks with the highest metamorphic grade (most altered) are found in the uplands east of the Harlem Valley.
It is common for geologists to attach proper names to each distinctive geological unit. For example, there are many carbonates in the world, but only one Wappinger Group. The names come from localities where the unit was first or best described. There are many exposures of the Wappinger Group throughout the county, but the formation was first described in and around the Town of Wappinger.
Bedrock formations of Dutchess County are described below:
Older metamorphic rocks of the County (geological symbol: pЄg)The oldest rocks in Dutchess County are in the Hudson Highlands, an upland area composed primarily of various gneisses (metamorphic rocks made up of discrete bands of light and dark minerals). These rocks, which were formed more than one billion years ago during the Grenville Orogeny, are most common along the southern border of Dutchess County, between the Hudson River and the western border of the town of Pawling [4]. The second largest occurrence of these rocks underlies a group of prominent hills, the Housatonic Highlands, east of Dover Plains. Isolated, uprooted blocks of gneiss also crop out at Todd Hill along the Taconic Parkway in the town of LaGrange, Corbin Hill north of the Village of Pawling, Stissing Mountain in the Town of Pine Plains, and in a series of small fault slivers between the City of Beacon and the Town of Fishkill.
Most of the gneiss consists of light and dark colored minerals arranged in layers with a banded, streaky, or speckled appearance. Gneisses containing light colored minerals such as quartz, feldspar, and muscovite (white mica) predominate. Various types of gneisses containing dark minerals such as hornblende, garnet, and biotite (black mica) also occur. Extensive outcrops of gneiss are generally more resistant to weathering than sedimentary rocks. As a result, gneiss outcrop areas are usually part of more rugged terrain and exist at higher elevations. Granitic gneiss, which occurs at North Beacon Mountain, is the most durable of these types and is sometimes quarried for crushed stone and building stone.
Younger sedimentary/metamorphic rocks (Є and O) The younger sequence of bedrock units in the county includes:
- Carbonate rocks of the Wappinger Group (OЄw) and its metamorphic equivalent, the Stockbridge marble.
- Poughquag quartzite (Єp), a body of sandstone known from its extensive outcrops near the hamlet of Poughquag.
- Metamorphosed clastic sedimentary rocks (now schists) of the Taconic Sequence (Єt), which were deposited at a point several hundred kilometers east of North America and were subsequently shoved over younger rocks to their present position. The Taconic sequence is very similar to the clastic rocks that it rests upon (Osh) which were deposited in about the location we find them now. The Osh rocks are autochthonous shales, whereas the Taconic sequence is allochthonous shales.
- Relatively unaltered to moderately metamorphosed autochthonous clastic sedimentary rocks, including the Poughkeepsie (or Taconic) Melange (Otm) and the Austin Glen greywacke (Oag).
Carbonate rocks
Carbonate rocks are formed primarily from the precipitation of calcium and magnesium carbonate in seawater, commonly through the action of algae and other organisms like corals and mollusks. The Wappinger Group, found in the central and western part of the county, consists of Cambrian- and Ordovician-age carbonate strata (rocks composed of layered sediments). Its metamorphic equivalent, the Stockbridge marble, underlies the Harlem Valley. Outcrops of carbonate rocks are found scattered throughout the southern, central, and eastern parts of the county. In the southeast, carbonate rocks form valleys, as the marble is softer and more susceptible to erosion than the surrounding metamorphic rocks. Elsewhere, the carbonates form small ridges, as they are relatively harder than the adjacent unmetamorphosed clastic rocks. Some of these rocks can be seen south of Poughkeepsie in road cuts along Route 9 near the Galleria and South Hills Malls, and along the river in the large quarry operating at Clinton Point.
Carbonate rocks are economically important in Dutchess County as a source of construction aggregate. One of the largest quarries in New York State is located in the Town of Poughkeepsie at Clinton Point. Glimpses of the carbonate rocks and the quarry operations can be seen from the Metro-North train between Poughkeepsie and New Hamburg. The carbonate rock of this quarry has an average magnesium carbonate content of about 40 percent. For this reason, it is classified not as a limestone, but as a dolostone, which is slightly harder than limestone. The rock is blasted, crushed, and sorted to sizes ranging from large individual rock fragments (riprap) weighing up to 15 tons each, to sand used in the production of asphaltic paving and the manufacturing of concrete blocks. Most of the production from the quarry is shipped by barge to New York City. Carbonate rocks are also mined in the Town of Pleasant Valley to produce construction aggregate. During World War II, marble in the Town of Dover was mined to recover magnesium for the war effort.
The metamorphism of the Wappinger Group generally increases in intensity from the northwest to the southeast. In the town of Milan and the valley of the Wappinger Creek, the original bedding (layering) is readily visible because the rocks are relatively unaltered. Farther east, in the Harlem Valley, the formation has been metamorphosed into marble and the beds are severely folded. The marble in the southeastern part of the county has been deformed several times by plastic flow so that it appears to wrap around stronger rocks. South of Pawling, masses of schist (a metamorphic rock made of parallel layers of mica and other minerals) have been folded and shoved into the carbonate, appearing as inclusions.
It is difficult to determine the exact thickness of the carbonate rocks because of the amount of faulting and metamorphism that have occurred. However, these rocks are believed to be approximately 1,000 feet thick in the western part of the county, and to thicken to the east. A thickness of 2,800 feet has been measured near Stissing Mountain [5], and they are estimated to be nearly 4,000 feet thick in the Harlem Valley [4].
Carbonate rocks are susceptible to internal erosion by the movement of groundwater along fractures and faults. Groundwater dissolves carbonate deposits, producing solution channels and voids; these openings provide storage cavities for groundwater supplies. This stored water can easily be polluted by contamination sources, such as septic tanks, where there are not enough sediment deposits on top of the carbonate bedrock to filter the waste materials. Although cave-ins may occur elsewhere in carbonate rocks, they are rare in Dutchess County.
Clastic Rocks
Clastic sedimentary rocks were formed by the compaction and cementing together of muds and sands at shallow depths beneath the Earth’s surface. All of the clastic rocks of Dutchess County were originally formed in an ocean; however, much of the Catskill Mountains, to the west, is composed of clastic rocks formed on land. There are two distinct groups of clastic rocks in the county: the Poughquag quartzite, found in close proximity to the Wappinger Group carbonate rocks, and shales and clay-rich sandstones (and their metamorphic equivalents) found throughout the county.
- Poughkeepsie quartzite: The Poughquag quartzite (Єp) is a very hard, compact, white to gray sandstone with a quartz content greater than 90 percent. Very clean sandstones (almost pure quartz) are typically believed to be former beach deposits; winnowing by waves removed clays and other minerals. The quartzite is so hard that Native Americans mined it for stone tools and points. Although it is relatively thin (less than 50 feet), equivalent sedimentary layers extend across the United States as far west as Nevada.
- Shales and clay-rich sandstones: Most of Dutchess County is occupied by what were originally shales and clay-rich sandstones (map units Єt-allochthonous Taconic sequence, Osh-autochthonous shales, and Oag-Austin Glen formation). These are found throughout the county, extending into Columbia County to the north, and Orange and Ulster counties to the west. Like the carbonates, the intensity of metamorphism increases from the northwestern part of the county to the southeast. These rocks were originally formed in relatively deep water, during or after the Taconic Orogeny (see Geologic History section), as muds derived from land settled in ocean water.
A distinct subset of these rocks is the Poughkeepsie or Taconic Melange (Otm). This unit formed as the westward-moving allochthons slid into a deep basin lying in the approximate position of the Hudson River, forming a jumbled mix of sandstone blocks in a matrix of mud. Good exposures of the melange are seen in Kaal Rock Park, at the eastern end of the Mid-Hudson Bridge. The melange also underlies much of the bank of the Hudson River from Poughkeepsie to Hyde Park.
The mineral composition and structure of the shale and sandstone units change from the northwest to the southeast. Quartz and mica are found chiefly in the northwest and central parts of Dutchess County. Feldspar is an additional component in the southeast. Bedding plane openings that serve as channels for the storage and movement of groundwater are apparent between the Fishkill Creek and Wappinger Creek valleys. Also, between the two creeks, slaty cleavage (a texture produced by rock compression) has resulted in numerous small, closely spaced parallel fractures within the rock. Such cleavage is absent, and the rocks are more massive in the southeastern part of the county. Garnet has been mined in the Harlem Valley, as described below. Because these garnet-bearing rocks are relatively soft, they are not used commercially except locally as fill.
A variety of names have been applied to the clay-rich rocks in the county, which may have formed at different times and in different geologic settings. The lack of well-preserved fossils or other diagnostic features, combined with the complex deformation to which they have been subjected, has resulted in less certainty as to the correlation amongst the various named units. Generally, the rocks located in the higher elevations east of the Taconic Parkway are allochthonous (moved), whereas those to the west of the Parkway are autochthonous (in-place).
Structural Geology
Although sedimentary rocks are generally formed in horizontal layers, parallel to the Earth's surface, today we find them oriented in many directions. Their present configuration is the result of multiple mountain-building events, as described in the section on previous section. Additional signs of mountain-building are found in folds and faults that can be observed in road cuts throughout the county. Similar, but larger-scale features can be inferred from the distribution of different types of bedrock. For example, higher-grade metamorphic schists overlie lower-grade slates in the eastern part of the county. Although the contacts between these contrasting rock types are only exposed in a few places, they are inferred to be low-angle thrust faults (faults that allow one rock unit to ride up and over another rock unit, a movement that occurs during the compression associated with mountain building).
The distribution of rock units generally forms a northeast to southwest pattern across the county. Areas of similar bedrock types are inferred to be bounded by faults that separate them from areas of differing bedrock types. No doubt there are many faults within the blocks of similar rock types, but it is difficult to identify them because of the soil and vegetative cover.
The fracturing and crushing that occurs along faults creates channels that can carry large volumes of groundwater. These channels may be enlarged in carbonate rocks as a result of dissolution from acidic waters. As a result, wells drilled into fault and fracture zones may yield large quantities of water. The faults in the region are very old and considered to be inactive. They were formed more than 200 million years ago and have experienced little or no activity since then. Consequently, the county has historically had very few, mostly small, earthquakes. The tectonic history of the Taconic Orogeny has resulted in a complex structural picture for Dutchess County. The map and the cross section indicate a number of faults where rock masses have moved in relation to one another during the long geologic history of the area.
Important features of the cross section include remnants of the Taconic Allochthon (Єt), which underlie topographic highs in the central part of the county. These remnants are floored by relatively low-angle thrust faults active during the Taconic Orogeny. Also shown are relatively high-angle reverse faults, another type of faulting typical of the compressional forces active during mountain building events. These faults bring very old Precambrian gneisses and their Paleozoic cover rocks, including the Poughquag quartzite (Єp), Wappinger Group limestones (OЄw), and autochthonous shales (Osh), to the surface at various places across the county. Some of these faults may have originally formed as extensional fractures known as normal faults during the initial breakup of North America 500 million years ago and may have been reactivated during the Taconic Orogeny (400 million years ago) as reverse faults. During the breakup of Pangea (200 million years ago) they may have become normal faults once again [4].
In addition to faults, several large folds that bend the autochthonous rock into downwarps known as synclines are present. Finally, the cross section suggests an interesting topographic inversion whereby the limestones are ridge-forming units in the western part of the county. This is because the limestone is more resistant to erosion than the surrounding shales, and valley-forming units to the east where the situation is reversed. In the humid northeast United States, it is more common for limestones to floor valleys because they are more susceptible to weathering and erosion than other rock types.
Surficial Geology
Unconsolidated materials overlie the bedrock in most parts of the county. These include glacial deposits formed by the Laurentide ice sheet during the last Ice Age as well as more recent stream deposits. Many of the surficial geologic deposits have been altered to produce soils.
The sedimentary deposits produced by the advance and retreat of ice sheets vary depending on whether the deposits were formed under the ice sheet or adjacent to it as climate warmed and the ice margin retreated northward. Rocks embedded in the bottom of ice sheets were pushed over the underlying landscape, scratching bedrock surfaces and creating particles that range in size from large boulders (erratics) to fine clay. Resulting glacial deposits are classified into three main categories: till, outwash, and clay. Unlike the bedrock, the processes that formed Dutchess County’s glacial deposits can be observed today in association with modern glaciers, streams, and lakes.
TillUnsorted mixtures of fine material, sand, and larger cobble-sized rocks make up glacial till found throughout the county. This material blankets the hills of the county and underlies the other glacial deposits in the valleys. It was deposited directly from the melting ice without further modification by moving water. In some areas, till was pushed up into linear ridges (moraines) by the episodic re-advance of the ice sheet.
OutwashMeltwater issuing from the edge of a glacier can transport large volumes of debris, much as modern streams do during flooding. The coarser-grained fraction of the stream load is deposited where the flow velocity of the stream ebbs because it enters a lake or flood plain. In contrast, fine-grained sediments are carried far away. The resulting glacial outwash is relatively free of very large boulders and fine silts and clays.
During the retreat of the Laurentide ice sheet, sands and gravels were deposited in deltas where meltwater streams issuing from the front of the ice sheet entered glacial lakes and dropped their coarse-grained loads. These deposits are characterized by stratification (layering) that is inclined at 5 to 30 degrees from horizontal [6] .
Outwash also fills many of the major valleys of the county, including those currently occupied by Wappinger Creek and the Tenmile River. Horizontally stratified sand and gravel were deposited on flood plains of the glacial predecessors of these modern streams. The layers within the outwash terraces reflect changes in grain size due to changes in stream flow volume and velocity.
Lacustrine (lake)Clay Finer silt- and clay-sized particles, winnowed from the sand and gravel fraction of stream loads, were carried farther downstream, eventually being deposited into local lakes and ponds. These deposits are very dense and uniform in grain-size, although some of the lake deposits are finely layered (varved), possibly reflecting seasonal changes in flow. The lake deposits are located in the lowest parts of the glacial terrain and are commonly found above a thin layer of till and beneath glacial outwash or modern wetland soils.
Remnants of Lake Albany sediments can today be found along the river’s edge on the lands of many of the large estates, such as the Franklin Delano Roosevelt home, the Vanderbilt Mansion, and Locust Grove. Additional lake sediments underlie the Vassar College farm, the Dutchess Plaza shopping center in the Town of Poughkeepsie, and the athletic fields of the Dutchess Day School in Millbrook.
Soils
Soil is a mixture of organic matter, minerals, gases, liquids, and organisms that together support the life of plants and soil organisms. It impacted by a number of factors including climate, topography, the organisms present, and the soil's parent or original minerals and how these factors interact over time. Soil is constantly in flux – undergoing changes and development as a result of physical, chemical, and biological processes [7].
The Soil Survey of Dutchess County was initially prepared in 1936 and updated in 1955 and 1972. A new survey, published in 2001, describes and maps 134 different soil series each with distinct characteristics and qualities. No single soil series covers more than three percent of the county. The 2001 survey mapped the county in 6 acres increments, instead of the 3-acre increment of previous surveys, and was one of the first digitized soil surveys in the nation. A web-based version of the Dutchess County soil survey is available at the National Resource Conservation Service (NRCS) Web Soil Survey. Dutchess County soils vary greatly. Silty loam textures dominate, but textures vary from gravelly, sandy loam to fine, silty clay. Most of the soils that have been cultivated are moderately eroded, except in certain nearly level areas. More than 70 percent of county soils are well drained, but small areas of poorly- and very poorly-drained soils can be found, often in complex associations that may limit the use of the well-drained soils associated with them.
Soil Hydrologic Properties
Soils play a very important role in ecosystems by filtering water, and by allowing infiltration and recharge of groundwater. The flow of water through a soil is governed by two different physical parameters, permeability and topography. Permeability is a measure of how well connected the pore spaces in a soil are. Permeability rates, which are usually given in inches per hour, measure the ease with which water flows through the soil layers. Values for permeability range from 0.06 inches of water per hour (very slow) to greater than 20 inches per hour (very rapid) and change according to the size of the soil particles and how closely they are packed. Soils dominated by very small (clay-sized) particles have low permeability rates simply because the grains are so small that surface tension limits the movement of water molecules through the pore spaces. Permeability combines with topography to determine the drainage characteristics of the soil. Drainage runs the gamut from a permeable soil located on a hillside that is said to be “excessively well drained” to a low permeability soil located in a depression with minimal external drainage that is said to be “poorly drained.”
The information given here is strictly general. Design manuals are available and should be used for site specific work. Septic fields, farming, and other uses requiring good internal soil drainage may not function properly in soils with low permeability rates, placing severe restrictions on development densities in areas not served by central water and sewer systems.
Prime and Important Agricultural Soils
The best and potentially most productive soils are classified by the Natural Resources Conservation Service (NRCS) as prime soils (USDA Handbook part 622.04). They are considered prime because they are suited to a wide variety of farm crops with relatively few limitations and represent an irreplaceable agricultural resource. Prime soils are well-drained, nearly-level, fertile, stable, and deep. These characteristics make them ideal for farming, but also easy to develop for residential and commercial uses.
“Prime” soils cover about 15 percent of Dutchess County. Significant concentrations occurred along the major stream valleys and throughout the towns of Red Hook and Rhinebeck, as well as major portions of Clinton and Pleasant Valley. High-quality soils also were abundant in the southwestern quarter of the county, but many have since been built over.
“Statewide Important” (USDA handbook part 657.5(c)) soils support good crop fields, but unlike prime soils, they have limitations that require special conservation measures and are suited to a smaller variety of crops. According to NRCS inventories, they cover about 32 percent of the county, and are usually found near prime agricultural soils. Smaller tracts of important soils are found in much of the county. Important and prime soils are noticeably absent from the Hudson Highlands, the ridges along the Harlem Valley, and other steeply sloping uplands where soils are characteristically shallow.
Erosion and SedimentationSoil erosion is an issue of concern in any area that is cultivated or otherwise cleared of vegetation for an appreciable portion of the year. By stripping topsoil, erosion robs the land of valuable natural nutrients, and washes soil, pesticides, and fertilizers into waterways. It also undermines soils and structures and chokes streams, lakes, rivers, and drainage systems with sediment.
The rate of soil loss varies dramatically with land use. Erosion rates from construction sites can be as much as 25 times higher than those from cropland, and as much as 75 times higher than those from pastures and woodlands. Proper conservation procedures can drastically reduce these rates.
Potential soil loss through erosion can be estimated using the Universal Soil Loss Equation, or USLE, which calculates the relative contribution of climatic, soil, and site-specific management factors to potential soil erosion. The specific soil characteristic is the erosion factor (K) which is summarized in Table 4.2 for the various soil types included in the map units. Erosion factors quantify the susceptibility of soils to sheet and rill erosion by water. The estimates are based primarily on the percentage of silt, sand and organic matter, and on soil structure and permeability. Values for K range from 0.02 (very low susceptibility to erosion) to 0.69 (very susceptible), and in Dutchess County the values are between 0.1 and 0.64.
Hydric Soils, Wetlands and AquifersHydric soils are defined as those soils that form under conditions that are sufficiently wet to support anaerobic conditions in the upper part of the soil profile for a significant portion of the growing season. A list of hydric soils was created by the USDA NRCS National Technical Committee for Hydric Soils and is published in the National Soil Information System (NASIS) database. Hydric soil series in Dutchess County include Carlisle, Fredon, Halsey, Livingston, Palms, Raynham, Sun, and Wayland.
Hydric soils are critical to the formation and definition of wetlands as they support the presence of wetland (hydrophytic) vegetation. Many soils that were originally classified as hydric have been artificially drained to support agriculture and construction activities. These soils are still designated as hydric, but they may no longer support hydrophytic vegetation and may not be considered wetlands. For a site to be considered a wetland it must have hydric soil, hydrophytic vegetation, and wetland hydrology as defined by local, state, or federal jurisdictions.
Wetlands are often directly connected to aquifers as either recharge or discharge areas. Hydric soils are therefore useful as a planning tool, indicating areas that should be evaluated carefully for aquifer protection.
Soils and Ecosystem ServicesThere are clear links between soils and all categories of ecosystem services in terms of production of food and fiber, as well as regulation of water quantity and quality through percolation, erosion, and hydric soil effects on aquifers. Recently, there has been a great increase in interest in the ability of soils to sequester carbon. Plants remove carbon dioxide from the atmosphere during photosynthesis and transfer some of this carbon to the soil when plant tissues die. Soil organic matter is the largest reservoir of carbon in most ecosystems and there is great interest in altering agriculture and forestry practices to increase soil carbon storage.
The role of soils in cultural services related to aesthetics and biodiversity are also important and are largely driven by the effects of soils on vegetation. Distinctive vegetation communities, with specific aesthetic and biodiversity values, are associated with specific soil types. In general in Dutchess County, more fertile (high pH, high base cations) soil series are associated with maple forests, while more coarse-textured, low fertility soils are associated with oak forests. Suites of plants (such as spring ephemeral plants) and animals (such as salamanders and birds) are associated with these different soil-vegetation associations and can be used as a starting point for biodiversity, aesthetic, and recreational assessments.
Regulation of SoilsExcept as the nature and limitations of different soil types might be included in local zoning ordinances, there is no county-wide regulation of activities based on particular soils. Prime and Important Agricultural Soils, for example, are not protected at the county level. In fact, their gentle topography and even-textures make them prime candidates for development. Development in wetland areas is restricted by state and local wetland ordinances enacted on a town-by-town basis; larger wetland development projects are regulated by US Army Corps of Engineers.
Soil erosion associated with clearing and grading is regulated by some municipalities as well as the NYS Department of Environmental Conservation, as is the practice of “soil mining” which removes unconsolidated glacial material for aggregate and bank-run fill material.
Mineral Resources
Mineral resources have played an important role in the development of Dutchess County since its settlement. Many of the oldest buildings in the county (including the 1765 stone Clinton House and the 1767 brick Glebe House) were constructed of locally mined materials. Historically, significant resources have included: construction aggregate (crushed stone, sand, and gravel), metallic minerals, and non-metallic minerals.
Such resources can only be utilized where suitable deposits occur. Thus, gravel mines can only be located where there is a substantial reserve of gravel, and rock quarries must be situated where the bedrock has characteristics appropriate for the purpose to which it is to be put. As a result, resource-rich towns such as Amenia and Fishkill have had a more active mining industry than resource-poor towns, such as Wappinger. Unfortunately, as residential development has spread across the county, land-use conflicts have arisen, and many potentially valuable resources have been lost.
Construction Aggregate
Sand and Gravel
Sand and gravel are mined for construction aggregate throughout New York and are an important economic resource in Dutchess County. Significant sand and gravel reserves are found primarily along stream valleys, where glacial outwash is located. The valley of Wappinger Creek (in the central part of the county) and the Harlem Valley are notable for their thick outwash deposits. These relatively level gravel terraces are the greatest areas of land use conflict because they are also valuable as farmland and suitable for easy residential development.
The unconsolidated sand and gravel are extracted from open pits, processed on site, and shipped by truck to its point of use. These uses include construction of septic systems, erosion and stormwater management projects, sand filters for purifying water, concrete for houses and roads, and ice-control sand for reducing hazardous winter driving conditions.
Crushed Stone
Rock aggregate, produced by crushing bedrock, is currently the leading mineral commodity in New York State. There are four active quarries in Dutchess County at this time: the Clinton Point quarry (carbonate, Town of Poughkeepsie), Dutchess Quarry (carbonate, Pleasant Valley), the Thalle Industries quarry (gneiss, Fishkill), and the Wingdale Materials quarry (schist, Dover). Historical records indicate that numerous, small quarry operations produced crushed rock and building stone in the county. The Clinton Point (locally known as “Trap Rock”) quarry was started in 1888 for this reason.
Much of the production from the quarries is used in highway construction. The noncarbonate aggregate is especially useful for creating relatively skid-resistant road surfaces.
Metallic Minerals
Iron
At one time, iron ore constituted the most valuable metallic mineral resource found in New York State. From the mid-1800s to the early 1900s the resource was very actively exploited, with approximately 40 iron mines in operation in the Hudson Highlands and Harlem Valley. The iron ore in the Harlem Valley was found in the metamorphic schists near the contact with the marble. Bog iron was also a source of raw materials for Dutchess County iron works. This iron was precipitated in wetlands where iron-rich groundwater discharged to the oxygen-rich surface environment.
The burgeoning demand for iron and steel during the Second Industrial Revolution and advances in technology encouraged the exploitation of lower grade ores in Minnesota and the abandonment of the mines in Dutchess County and vicinity. Iron furnaces are still visible in the Harlem Valley, however, such as along Dover Furnace Road.
Silver and Lead
Reserves of other metallic minerals are lacking in the county. There have been historic reports of small silver and lead mines, but these were primarily prospects without significant economic production.
Non-metallic Minerals
Garnet
The aluminum-rich mineral garnet is found in high-grade metamorphic schists in the uplands adjacent to the Harlem Valley in the towns of Amenia, Dover, and Pawling. This very hard mineral is commonly used as an industrial abrasive. Garnet is recovered as a by-product of crushed stone production at the Wingdale Materials quarry in Dover. The sale of the garnet helps to offset the costs of operating this underground mine.
Clay
Hudson Valley brick manufacturers primarily used clay from relict glacial lake sediments located along riverbanks. Away from the rivers, clay-rich glacial till was also mined to create bricks. The largest brickyards were located in the Beacon area. The Dennings Point Brick Works produced as many as 400,000 bricks a day in the late 19th century. The Brockway Brick Company had a yard north of Beacon. Another brickyard existed in the area presently occupied by the Dutchess Plaza shopping area in the Arlington neighborhood in the Town of Poughkeepsie. The brickyards closed in the early 20th century because of changing economic conditions.
Implications for Decision Making
Geology Implications
Three aspects of Dutchess County geology and topography have important implications for decision making: availability of mineral resources, steep slopes, and viewsheds.
Mineral Resources
Mineral resources are finite in extent and are rapidly being lost to development. As a result, aggregate has to be trucked increasing distances to satisfy continuing demand within the county. Mining Overlay Districts have been established in some Dutchess County towns to restrict mining rather than encourage it.
Municipalities elsewhere in the nation have recognized the importance of an adequate supply of aggregate to society and have taken steps to preserve mineral resources from conflicting land uses. In Oroville, California, for example, mineral resource areas have been identified to indicate the significance of mineral deposits and to reduce the threat of encroachment by incompatible land uses.
Steep Slopes
Generally speaking, lowlands were developed first in Dutchess County. Early development occurred along the Hudson River, along major streams and along major roads and railroads, which were built in lowland areas first. Development of level areas is often relatively easy and inexpensive. Today, much of the development occurs on moderate to steep slopes. Development in areas of steep slope is more expensive, more difficult, and is prone to serious environmental impacts. Care must be taken to ensure that development on slopes does not adversely affect pre-existing development in the lowlands. For this reason, a few municipalities have passed steep slope ordinances in recent years. Development on steep slopes can lead to significantly increased soil erosion and greatly increased stormwater Three aspects of Dutchess County geology and topography have important implications for decision making: availability of mineral resources, steep slopes, and viewsheds.
Viewsheds
Dutchess County has long been known for its spectacular views. A viewshed is the area that can be seen from a particular spot, the viewpoint, and its size and shape are determined by the local topography. Important viewsheds can be damaged by development. For example, a building on top of a hill may have a wonderful view but damage the view of everyone living in the nearby lowlands. The viewshed from a scenic overlook may be considered an important resource worthy of preservation by a community. Viewsheds along the Hudson River were deemed so important by many people that they formed an organization, Scenic Hudson, to protect them. Viewsheds along scenic highways, trails and parks may also be considered important to the public. Of course, the view from an individual house is also important to the owner of that house. Scenic views in Dutchess County contribute to residents’ sense of place and improve our quality of life. Fortunately, there are simple ways to build on high areas without damaging the community’s scenic views, and at least one municipality in Dutchess County has passed a Ridge Protection Ordinance to protect a scenic ridgeline from being damaged by development.
Soil-based Implications
Soil is a fundamental resource that is often taken for granted because of its abundance, low cash value, utilitarian functions, and lack of aesthetic charm. Soil makes it possible to use and live on the land. Without ample supplies of good, arable soil, food production would be vastly more difficult.
Soils have several characteristics, such as permeability, depth-to-bedrock, erodibility, and wetness, which limit the land uses they can support. All of these limiting characteristics should be considered during the land use decision-making process. Development proposals and local land use controls should be well-matched to soil features to ensure that the type, density, location, intensity, and design of all land uses are appropriate to the soils and other natural resources that must sustain them. No amount of mapping at the county-wide scale can substitute, however, for site-specific knowledge, so decisions about individual pieces of land should be made in consultation with soil-mapping professionals such as the Dutchess County Soil and Water Conservation District (DCSWCD). Specific policy considerations discussed below are primarily focused on the relationship of soils to development activities in terms of agriculture, permeability, thickness, and susceptibility to erosion.
Prime and Important Agricultural Soils
Much of Dutchess County’s prime and important soil acreage has been developed for residential or commercial uses since the middle of the 20th century and is no longer available for agricultural or open space use. The best of these soils that remain undeveloped are located mainly outside the southwestern core area, and form a critical resource on which Dutchess County’s current agricultural industry and future food-producing capability depend.
Agriculture is a significant and highly valued component of Dutchess County’s economy, historic, and visual identity. Prime and important soils support active farms throughout the northern and eastern communities, as well as a handful of farming operations within the urban area. Many of these farms are under intense development pressure, which threatens their continued viability. It is necessary, therefore, to devise ways to preserve the county’s best soils even as farming activity declines.
If land uses that can function satisfactorily on less valuable soils are allowed to continue to consume the best soils in Dutchess County, the county’s agricultural community will be further weakened, and its ability to respond to future changes in the nation’s food production system will be severely impaired. The loss of agricultural open land also threatens one of the most traditional and aesthetically pleasing contributors to the county’s high quality of life. Aggressive measures are needed to protect the soil resource. Communities must find equitable, effective ways to divert development to less valuable sites, to encourage open space preservation, to support agricultural activities, and to institute effective erosion control measures.
Permeability
Development densities and waste management practices should reflect the severely limited ability of impermeable soils to absorb and filter waste. Otherwise, intensive development without central sewage treatment facilities will saturate soils with wastes, causing untreated wastes to spread into nearby surface waters and groundwater supplies. In areas where such contamination occurs, expensive construction of central sewage and water treatment facilities and pipelines may be the only remedy.
A report by Gerber (1982), updated by Chazen (2006), used the permeability characteristics of surficial materials (which are primarily soils) to calculate the maximum theoretical density of residential developments that are based on wells and septic systems. (See the Dutchess County Department of Planning & Development for copies of the Chazen report). The scale of mapping in the Chazen (2006) report is not sufficiently detailed for site specific work; it is intended to give an overview of infiltration issues in the county.
Highly permeable soils should also be used carefully because of their ability to transmit hazardous materials into groundwater supplies. Landfills, petroleum storage tank farms, chemical manufacturers, and other facilities that handle such hazardous materials should not be located on top of the most permeable soils. These considerations underscore the fact that soils are important filters of water as it makes its way from the surface to the water table to enter the aquifer. Whether that water is rainwater, surface water flowing into, or out of, streams and lakes, or effluent from septic systems and/or sewage treatment facilities, making effective use of the biological and physical filtering capacity of soils is an important water resource protection strategy.
Depth
Like permeability, depth-to-bedrock affects the development suitability of soils and should be considered when development proposals and land use policies are reviewed. Shallow soils limit the placement of wells, septic systems, foundations, agricultural uses, roads, and utilities. Expensive blasting is often needed for construction on shallow soils, and the likelihood of erosion and septic failures is much greater than in areas with deeper soil. At the same time, shallow soils with bedrock outcrops on steep slopes often offer spectacular views, making them tempting sites for recreational developments and homes. They are ideal sites for natural recreation areas such as hiking trails, forest preserves, and open space. Intensive development may be possible on shallow soils with thoughtful site planning, central sewage systems, and stringent erosion control measures.
Erosion and Sedimentation
Although federal and county soil conservation programs have helped reduce cropland erosion significantly since the mid-1970s, erosion continues to damage the county’s soil and water resources. Erosion rates are especially severe on construction sites, road banks, and croplands that are not using erosion control practices. Soil erosion can be exacerbated by runoff from impervious surfaces such as parking lots or lawns where runoff gains momentum and erosive power before it flows off the surface onto the soil. Soil eroded from improperly managed construction and agriculture can result in increased public expense when it chokes drainage culverts and sediment traps. During extreme weather events, erosion causes deposition, which in turn can cause flooding. The agricultural community has embraced environmental practices resulting in significant reduction in per acre soil erosion.
References
[1] Rayburn, J.A., P.L.K. Knuepfer and D.A. Franzi, D.A. “A series of large, Late Wisconsinan meltwater floods through the Champlain and Hudson Valleys, New York State, USA”. Quaternary Science Reviews 24: 2410-2419. 2005.
[2] Donnelly, J.P., N. Driscoll, E. Uchupi, L. Keigwin, W. Schwab, E.R. Thieler and S. Swift. “Catastrophic meltwater discharge down the Hudson River Valley: a potential trigger for the Intra-Allerod cold period”. Geology 33: 89-92. 2005.
[3] Connally, G.G., and L. Sirkin. “Woodfordian ice margins, recessional events, and pollen stratigraphy of the mid-Hudson Valley.” The Wisconsinan Stage of the First Geological District, Eastern New York edited by D.H. Cadwell. Albany, NY: New York State Museum Bulletin 455: 50-72. 1986.
[4] Isachsen, Y.W., E. Landing, J.M. Lauber, L.V Rickard and W.B. Rogers. “Geology of New York, a Simplified Account.” New York State Museum/Geological Survey Educational Leaflet No. 28. Albany, NY: New York State Museum, 1991.
[5] Knopf, Eleanora Frances Bliss. Stratigraphy and Structure of the Stissing Mountain Area, Dutchess County, New York. Volume 7, Issue 1 of Stanford University publications: Geological sciences. 1962.
[6] Murray, D.J. “Pleistocene history of the Millbrook, New York region.” New York State Geological Association Guidebook to Field Excursions edited by J. H. Johnsen, p. B-5-1 to B-5-10. 1976.
[7] Gilluly, James; Waters, Aaron Clement; Woodford, Alfred Oswald. Principles of geology (4th ed.). San Francisco, California: W.H. Freeman. 1975.
Glossary
Allochthonous: Describing geology that has been moved from its original site of formation
Autochthonous: Describing geology that was created where it currently is located
Continental (Tectonic) Plates: Massive, irregularly shaped slabs of solid rock, generally composed of both continental and oceanic lithosphere that move separately
Glacial Moraine: An accumulation of unconsolidated debris that occurs along the sides or end terminus of both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sheet
Glacial Till: Unsorted glacial sediment, derived from the erosion and entrainment of material by the moving ice of a glacier
Glaciation: The process, condition, or result of being covered by glaciers or ice sheets
Igneous Rock: Rocks that are formed through the cooling and solidification of magma or lava.
Loam: Soil composed mostly of sand, silt, and a smaller amount of clay (often a 40%-40%-20% ratio)
Metamorphic Rock: Rock that was originally another type of rock, but has been substantially changed from its original igneous, sedimentary, or earlier metamorphic form by high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors
Mineral: A solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form and is often of use to humans
Orogeny: A mountain-building process that takes place at a convergent plate margin when plate motion compresses the margin
Rock Cycle: The basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous
Sedimentary Rock: Rock that has been formed by the accumulation or deposition of mineral or organic particles at Earth’s surface, followed by cementation
Topography: Relief, terrain and elevation of the land forms and features of a given area, or the description or depiction of this terrain in maps