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Cape Cod

National Seashore

Massachusetts

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park geology subheading
Satellite photo of the Cape Cod peninsula
Cape Cod National Seashore, Massachusetts

Most of Cape Cod was shaped by the last great glaciation in North America, the Wisconsin glacial stage of the Pleistocene, approximately 75,000 to 10,000 years ago. A vast ice sheet (the Laurentide ice sheet) advanced south from northern New England and Canada and transported eroded rock debris scoured from the underlying Paleozoic crystalline bedrock until it reached its southernmost limit at Martha's Vineyard and Nantucket Island. Late in this time period, the coalescing Buzzard's Bay, Cape Cod Bay, and South Channel glacial lobes of the Laurentide ice sheet deposited the glacial drift that now comprises much of Cape Cod (Oldale, 1980; 1992) (Figure 2.1).

The glacial history of the Cape Cod area was rapid in geologic terms. The minimum radiocarbon age of material found in the glacial drift indicates that the ice had reached Cape Cod more than 21,000 years ago. The maximum advance of the Laurentide ice sheet in the New England area is marked by the terminal moraines on Martha's Vineyard and Nantucket. At the time of maximum ice advance, sea level was about 300 feet lower than its present level, and the coastal plain extended far to the south of Cape Cod, out to the present edge of the continental shelf. South of the ice margin, meltwater streams flowed across the coastal plain to the sea (Oldale, 1980; 1992) (Figure 2.1). Retreat of the ice must have begun earlier than 18,000 years ago as the ice is thought to have retreated as far north as the Gulf of Maine by that time. This means that the ice had vanished from the Cape Cod area in less than 3,000 years and that most of Cape Cod's glacial landforms were created within about 1,000 years. Individual features may have formed in as little as several hundred years (Oldale, 1992).

Through the interpretation of landforms, the relative timing of depositional events during glacial retreat has been fairly well determined. Forward progress of the ice sheet was often balanced by melting at the leading edge, so that the ice front maintained its position (ablation) even as it began to retreat. While the ice front was stationary, frequent warm periods caused large amounts of water to melt from the glaciers. The meltwater from the Buzzards Bay Lobe and Cape Cod Bay Lobe carried huge quantities of sediment from the glacier. This sediment formed the gently sloping outwash plains of stratified drift, several miles long, that now comprise much of the inner Cape (Oldale, 1992) (Figure 2.2). Minor re-advances of the ice sheets formed the thrusted Buzzards Bay and Sandwich Moraines located along the north and west margins of the upper Cape (Figure 2.2). No moraine deposits have been identified on the lower Cape.

When ice retreat resumed, the central Cape Cod Bay Lobe of the Laurentide ice sheet retreated faster than the surrounding lobes, and meltwater flooded the newly vacated lowlands to form glacial Lake Cape Cod in the area currently occupied by Cape Cod Bay. The lake was dammed to the north by the Cape Cod Bay Lobe, to the east by the South Channel Lobe, to the west by the Buzzards Bay Lobe, and to the south by the moraine and outwash plain deposits of Cape Cod (Figure 2.2). Fine grained clay and silt settled to the lake bottom leaving behind seasonal bands which together represent annual layers or varves as evidence for the lake in the Cape Cod Bay area. The lake periodically broke through the moraine and outwash deposits and partially drained. The escaping water left eroded lowlands, one of which would later be exploited for the construction of the Cape Cod Canal. The lake drained for the last time when both the Cape Cod Bay Lobe and the South Channel Lobe retreated far enough to allow the water to escape to the ocean (Oldale, 1980).

Later stagnations of the South Channel Lobe to the east of Glacial Lake Cape Cod allowed the four outwash plains of the lower Cape to be built. The Eastham, Wellfleet, Truro and the Highland outwash plains are the dominant morphologic features of the lower Cape. This outwash material was built up by deposits from braided meltwater streams flowing west into Glacial Lake Cape Cod. Isolated blocks of ice buried in the outwash deposits of both the inner and the lower Cape melted slowly, long after the glacial lobes had retreated far to the north. As sediments collapsed around the melting ice blocks, kettle holes formed within the outwash plains (Figure 2.3) (Oldale 1980; 1992).

After the last of the ice had retreated from the area, winds deposited eolian layers on top of the drift and sea level rose nearly 300 feet (Masterson and Barlow, 1994). Approximately 6,000 years ago, sea level rose high enough to flood Vineyard and Nantucket sounds. Marine reworking of the glacial sediments became an important process. The coastline was smoothed as glacial headlands were eroded back. Marine scarps were formed by the attack of storm surge waves, and the sediment was carried by long-shore drift to form bars and spits. In the early period of deglaciation, sea level rise was about 50 feet per 1000 years. From 6,000 to 2,000 years ago, when most of the ice sheets had vanished, sea level rise had slowed to about 11 feet per 1000 years. Since then sea level rise has been approximately 3 feet per 1000 years. At the current rate of sea level rise, Cape Cod will continue to battle the waves for about another 5,000 years before succumbing to the sea (Oldale, 1980; 1992; Strahler, 1966).

Geomorphology

The landforms of the lower Cape are either glacially derived or a product of later marine and eolian reworking of glacial sediments (Oldale, 1980; 1992). Outwash plain deposits comprise the major geologic features of the lower Cape. They are predominantly stratified fine to medium sand and medium to coarse sand and gravel with lenses of fine silt and scattered boulders. Although lithologic variations over short distances can be extreme, grain size generally decreases with depth and distance from the former ice margin (Masterson and Barlow, 1994). Outwash plain surfaces are commonly pocked and pitted by kettle holes (e.g., the Wellfleet pitted outwash plain). When the kettles are deep enough to intersect the water table, a pond is formed. Thus pond level provides a close approximation of the water table. A kettle pond in Wellfleet yielded the oldest radiocarbon dated material at 12,000 years. (Winkler, 1985). This date, as much as 5,000 years after the ice retreated north, indicates that the buried ice blocks may have persisted for several thousand years after the glaciers retreated (Oldale, 1980; 1992).

Small streams and rivers, like the Pamet River, currently occupy oversized valleys within the outwash plains. The valleys were likely cut by ground water springs contacting the land surface at a time when a large proglacial lake, formed by large volumes of trapped meltwater, supported higher water tables. Later, with glacial retreat, catastrophic lake drainages enlarged the channels. Today, the streams appear undersized for the older valleys (Oldale, 1980; 1992).

The portion of Truro north of High Head and all of the Provincetown land area are not glacially derived. These areas consist of material derived from coastal erosion of the glacial outwash plains, transported northward, and redeposited by marine and eolian action as a series of recurved sand spits and dunes during the last 6,000 years (Ziegler et al., 1965).

Soils

The soils on the lower Cape are relatively young, having formed since the end of the last glaciation approximately 16,000 to 18,000 years ago. They exhibit only slight alteration of the original parent sand and gravel material and are well drained (U.S. Soil Conservation Service, 1993). Depth of soils on the Cape range from just a few inches in new dune and beach areas of the Province Lands to several feet in others; however, average depth is less than 6 inches. The soil on lower Cape Cod is predominantly a podzol, characteristic of climates that are both cold and humid. Cold temperatures inhibit bacteria and promote frost action, while humid conditions leach water soluble materials downward and support the growth of a vegetative cover (U.S. Soil Conservation Service, 1993; Oldale, 1992). A podzol soil profile typically consists of an upper organic layer undergoing decay, a middle layer of mixed humus and mineral grains, and a lower layer of mostly mineral grains (Oldale, 1992). The historic cultivation and burning of the land on the lower Cape, the associated current abundance of conifers, and the near shore ammonium loss through cation exchange with sea salts create acidic and nutrient poor soil conditions which contribute to stunted vegetative growth (Barnstable County Soil Survey, 1993; Brownlow, 1979; Blood et al., 1991; Valiela et al., 1997).

Soil type on the Cape is very important because it has a direct relationship to the rate at which infiltrating waters are purified. Soils which are coarse and sandy are highly permeable and allow effluent waters to travel quickly over large distances. Low organic matter and clay content provide little contaminant removal through soil sorption or cation exchange. Low organic content of the soils also decreases bacterial immobilization of nutrients as well as denitrification of nitrate-nitrogen. As a result, Cape Cod ground water is susceptible to contamination (Brownlow, 1979). According to the Barnstable County Soil Survey General Soil Map (1993), there are three principle soil types on the lower Cape: (1) Carver soil is characteristic of outwash deposits. It is the most common soil type on the lower Cape and is a poor filter for septic systems, sewage lagoons, and sanitary landfills.; (2) Hooksan- Beaches-Dune Land soil is characteristic of wind-blown deposits found in the Province Lands and on beaches. It is a poor filter for septic systems, sewage lagoons, and sanitary landfills.; (3) Ipswich-Pawcatuck-Matanuck soil is poorly drained and limited to lowland areas (e.g., the Pamet River, Little Pamet River, Herring River and Salt Meadow) (Figure 2.4). It has flooding and ponding potential when used for septic systems, sewage lagoons, and sanitary landfills.

Hydrogeology

Geologic materials that are saturated with abundant freshwater are called aquifers. The ability of a material to hold and transmit water is largely dependent on its porosity, that is the number, size, and interconnectedness of the pore spaces between particles. Deposits consisting primarily of large sized sand and gravel particles (e.g., outwash deposits) transmit more water than deposits of finer grained silt and clay size deposits (e.g., lake deposits). Well sorted, stratified sediments (e.g., outwash deposits) are not so easily compacted and are described as having a high hydraulic conductivity or transmissivity (Fetter, 1994). Poorly sorted sediments of mixed grain sizes (e.g., till) have a poor capacity to transmit water because the smaller particles fill the voids between the larger particles.

The thick, glacial sand and gravel outwash plain of the lower Cape can be thought of as a huge sponge with a large capacity for water storage. Precipitation on the land surface easily percolates down through the soil until it comes to a level saturated with water. This level is the water table. Pore space above the water table, where water and air mix, is known as the unsaturated zone. Below the water table is the ground water or saturated zone, where all pore space is completely filled by water. An unconfined aquifer is one in which the water table forms the upper boundary (Freeze and Cherry, 1979). A confined aquifer is one that lies between two layers of geological material having very poor capacity to transmit water, such as silt and clay. Unconfined aquifers occur near land surface, whereas confined aquifers tend to occur at depth. Most of the ground water on the lower Cape occurs under unconfined conditions, although there are small areas of confinement in the vicinity of localized silt and clay lenses (Strahler, 1966). Lenses of silt and clay commonly exhibit conductivities of less than 1 foot per day creating a serious impediment to vertical flow (Martin, 1993).

The outwash deposits present by far the best opportunities for ground water development on the lower Cape. They are not only thick, but consist of sand and gravel which has high hydraulic conductivities of 100 to 500 feet per day and provides excellent well yields. In these conditions, two foot diameter wells with a 10 foot screened length commonly yield 250 to 1000 gallons per minute (LeBlanc et al., 1986; Guswa and LeBlanc, 1981).

Thousands of years of melting glacial water and precipitation have built up four distinct subsurface reservoirs of fresh ground water hundreds of feet thick on the lower Cape. Since fresh water is less dense than salt water, rain infiltrating the subsurface rests atop and depresses the surface of the salt water. In each of the lower Cape's four aquifers, a lensshaped body of fresh water exists, which is thickest at its center. A vertical cross section of the lower Cape's aquifers would show that the fresh and salt waters meet on a surface that starts near the shoreline and slopes steeply down below the center of the peninsula from both sides (Figure 2.5). The upper surface of the freshwater lens, defined by the water table, is convex up and the lower surface, defined by the fresh water-salt water interface, is convex down. The maximum thickness of fresh water, therefore, is toward the center of each lens (Oldale, 1992). The top of the aquifer is marked by the water table and the bottom by the contact between fresh and salt water (depth to bedrock on the lower Cape is far below the deepest extent of fresh water).

The Ghyben-Herzberg principle states that in unconfined coastal aquifers, the fresh ground water will extend below mean sea level about forty times deeper than the height that the water table rises above mean sea level. This principle is based on a mathematical relationship between the relative densities of fresh and salt water (Fetter, 1994), and can be applied to Cape Cod ground water. For example, in Wellfleet, water levels in the ponds are about 8 feet above sea level, and fresh ground water extends to about 320 feet below sea level. Freshwater lenses are as much as 200 feet thick in Truro, 250 feet thick in Wellfleet, and 275 feet thick in Eastham (Oldale, 1992).

The water table on the lower Cape is not a perfectly horizontal surface, but has a gentle slope or hydraulic gradient. Ground water moves slowly down slope under the influence of gravity. The lower the hydraulic conductivity of the materials through which the water seeks to travel, the greater the energy required to accomplish that movement and the steeper the resultant slope of the water table. Flow through the very highly conductive materials of the lower Cape outwash plains requires very little hydraulic gradient. Therefore, the slope of the water table is less steep than it would be in less conductive materials. The highest ground water levels occur in the center of each ground water lens and create a linear band of high water table along the center of the outer Cape. The hydraulic conductivity in localized areas of silt and clay may be several orders of magnitude less and produce a steeper hydraulic gradient (Oldale, 1992). Ground water flows slowly and radially from higher areas to lower areas down-gradient towards the perimeter of the aquifer where it finally discharges to the sea, salt water bays, inlets, canals and streams (Figure 2.5) (Oldale, 1992; Strahler, 1966).

Source National Park Service, Water Resources Division

References

Blood, D., L. Lawrence and J. Gray. 1991. Fisheries-oceanography coordinated investigations. U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Technical Information Service, Seattle , WA .

Brownlow, A.H. 1979. Cape Cod environmental atlas. Department of Geology, Boston University , Boston , MA .

Fetter, C.W. 1994. Applied Hydrogeology. Macmillan College Publishing Company, New York , NY

Guswa, J.H. and D.R LeBlanc. 1981. Digital models of ground water flow in the Cape Cod aquifer system, Massachusetts . U.S. Geological Survey Water Resources Investigations Open File Report 80-67. U.S. Geological Survey, Boston , MA.

LeBlanc, D., J. Guswa, M. Frimpter and C. Londquist. 1986. Ground water resources of Cape Cod , Massachusetts . U.S. Geological Survey Hydrologic Investigations Atlas HA-692. U.S. Geological Survey, Reston , VA.

Masterson, J.P. and P.M. Barlow. 1994. Effects of simulated groundwater pumping and recharge on groundwater flow in Cape Cod , Martha's Vineyard , and Nantucket Island basins, Massachusetts . U.S. Geological Survey Open File Report 94-316. U.S. Geological Survey, Marlborough , MA .

Oldale, R. 1980. A geologic history of Cape Cod . U.S. Geological Survey, Washington , D.C. Oldale, R. 1992. Cape Cod and the islands, the geologic story. Parnassus Imprints, East

Strahler, A. 1966. A geologist , s view of Cape Cod . The Natural History Press, Garden City, NY.

U.S. Soil Conservation Service. 1993. Barnstable County soil survey. U.S. Department of Agriculture ; Soil Survey. Washington , D.C.

Valiela, L, G. Collins, J. Kremer, K. Lajtha, M. Geist, B. Seely, J. Brawly and C.H. Sham. 1997. Nitrogen loading from coastal watersheds to receiving estuaries: New method and application. Ecology Applied, 7:358-380.

Winkler, M.G. 1985. A 12,000 year history of vegetation and climate for Cape Cod , Massachusetts . -,' Quaternary Research, 23:301.

Zeigler, J.M., S.D. Tuttle, G.S. Giese, and H.J. Tesha. 1965. The age and development of the Provincelands Hook, outer Cape Cod , Massachusetts . Limnology and Oceanography 10:298­' 311.

Park Geology

Cape Cod resembles a flexed arm of sand thrust out into the Atlantic Ocean. It owes its origin to glaciers, which were active in the area as recently as 14,000 years ago. Since that time, waves and nearshore currents have extensively reshaped the sedimentary deposits left by these glaciers into a variety of coastal environments, for example, sandy beaches flanked by towering sea cliffs and bluffs and discontinuous chains of barrier islands, many with elegantly curved sand spits. Remarkably, the 40-mile-long eastern coastline of Cape Cod, despite its proximity to Boston, possesses few shore-protection structures; it is the longest, pristine shoreline of sand in New England (Pinet, 1992).

About 15,300 years ago, a huge ice sheet, which flowed southward from Canada, covered all of New England. As the ice mass crept across the continental shelf, one of its ice lobes—the Cape Cod Bay Lobe—deposited sediment at its margin and formed a morainal ridge—the terminal moraine—that can now be traced across Martha’s Vineyard and Nantucket, the two principal islands south of the Cape. In addition to the terminal moraine, recessional moraines also indicate the presence of the former ice sheet in southeastern Massachusetts. As the ice sheet retreated northward, meltwater trapped by the recessional moraine formed Glacial Lake Cape Cod. Stratified muds, silts, and deltaic sands accumulated in this glacial lake, which covered an area amounting to about 400 square miles. A river outlet cutting into the recessional moraine drained water out of the lake, presumably in the area of Eastham and Town Cove section of Nauset Beach. The South Channel lobe was just to the east, and its meltwater carried huge quantities of sediment from the glacier. This sediment formed the gently sloping (towards the west) outwash plains that are several miles long and now comprise much of the Outer Cape.

When the ice sheet disappeared, the landforms of the Cape looked quite different than they do today. As the ice melted, sea level rose and flooded the area. Paleogeographic reconstructions of the shoreline indicate it was quite irregular at that time—a series of headlands and embayments composed of unconsolidated glacial sediments (glacial drift). This original coastline was located as much as three miles seaward of the present shoreline. Since then, sediment redistribution by waves and nearshore currents has changed the morphology of the landforms.

Landscapes change quickly in Cape Cod, and the retreat of the ice sheet is no exception, taking less than 3,000 years. Likewise, the creation of landforms after glacial retreat happened quickly, some taking as little as several hundred years. Outwash plain deposits, which are commonly pocked and pitted by kettle holes (e.g., the Wellfleet pitted outwash plain), are the major geologic feature of the lower Cape. When the kettles are deep enough to intersect the water table, a pond is formed. Pond level provides a close approximation of groundwater level.

The encroachment of the sea following deglaciation permitted wave currents to erode and rework the glacial drift. As waves refracted, energy was focused on the headlands. Consequently, peaks of land were worn down by wave erosion, creating a system of steep, wave-cut cliffs. The sediment moved by nearshore currents sequentially formed a series of sand spits and barrier islands (Uchupi et al., 1996). Prior to 6,000 years ago, the longshore drift of sand was predominantly to the south. This prevailing pattern of sediment movement formed the southern barrier island system of Nauset Spit, and eventually, Monomoy Island. The crest of Georges Bank, far offshore, still stood above sea level and afforded the northern shoreline of the Cape protection from erosion by large ocean waves approaching from the southeast. About 6,000 years ago, however, the rising sea submerged Georges Bank, exposing the Cape to wave attack from the southeast, resulting in the northerly transport of sand that eventually formed the curved spit system of Province Lands surrounding Provincetown. The appearance of the spit sheltered the northern shoreline and resulted in a northward transport direction on the bayside, whereas further south littoral transport was directed southward along Cape Cod Bay.

Erosion of the glacial deposits produced imposing marine cliffs, many of which are currently retreating at alarming rates. Although scarp retreat of the eastern shoreline averages 0.67 m/yr, specific coastal sites are losing land to the sea at higher rates. For example, the cliffs below Wellfleet-by-the-Sea are retreating approximately 1.0 m/yr (Pinet, 1992). Because most of this erosion occurs during storm events, cliff retreat is not constant over time.

A summary of Cape Cod’s geology is not complete without mention of sand dunes. This feature epitomizes Cape Cod itself—migrating constantly yet somehow enduring. Dunes are shaped by the prevailing winds and migrate constantly. On the Provincetown spit, there are parabolic dunes, or “U” shaped dunes, with the open end facing the wind. These are formed when the wind blows away the sand in the middle of the dune, exposing the underlying beach deposits. The eroded sand is transported by the wind and deposited along the advancing leeward face of the dunes (Oldale, 1998). The parabolic dune orientation is driven by strong winds from the northwest predominantly in the winter, but occasionally important in the summer (Allen et al., 2001)

Active coastal dunes are dynamic landforms whose shape and location are ever-changing. Youthful, unvegetated dunes are on the move as the sand, exposed to the prevailing wind, is picked up, transported, and redeposited repeatedly. When the dunes become vegetated, they stabilize and tend to remain unchanged for a time. If the dunes lose the protective vegetation, they will move again. This can be seen along US Route 6 in Provincetown, where once stable dunes are advancing on the forest and highway and are filling Pilgrim Lake (Oldale, 1998).



park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

A geology photo album has not been prepared for this park.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.



geologic research subheading

Information about the park's research program is available on the park's research webpage.

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.



selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Currently, we do not have a listing for any park-specific geology education programs or activities.

General information about the park's education and intrepretive programs is available on the park's education webpage.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
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