LAURENTIDE GLACIATION OF THE MASSACHUSETTS COAST

By: Margaret Martin

ES 767, Quaternary Geology, Fall 2008

Introduction

The most recent continental ice sheet to cover New England was the Laurentide, which formed during the late Wisconsinan glacial stage. The Laurentide ice sheet formed in Canada approximately 75,000 years ago. During the early and middle Wisconsinan stages the ice sheet advanced, then retreated from northern New England. Late Wisconsinan cooling 25,000 years ago initiated another advance of the ice sheet. This time it reached southern New England, stopping just south of the Cape Cod peninsula in the the vicinity of Martha's Vineyard and Nantucket Islands about 21,000 years ago. As the global climate warmed 18,000 years ago, the ice sheet began its retreat from southern New England. The portion of the ice front that covered eastern Massachusetts had a lobate form.

Three lobes covered the area of the Massachusetts coast and Massachusetts Bay to the east. The lobes retreated at different rates, which caused proglacial lakes to form in front of the ice. As deglaciation continued, the sea covered lowlands that had been isostatically depressed by the weight of the glacier (Oldale, 2001).

By 14,000 years ago, about the same time the Laurentide sheet separated from the western North American Cordilleran ice sheet, the Laurentide ice front receded to a location near the present northern border of Massachusetts (Figure 1). Around 5,000 years ago the Laurentide had retreated as far as Baffin Island, Canada. When the ice sheet was at its maximum extent, global sea level was 120 m lower than today's level and the continental shelf south of Cape Cod was exposed as a coastal plain (Oldale, 2001).

Figure 1 - Laurentide Ice Sheet, approximately 14,000 years ago. (Short, 2008)

Regional Setting

Most of coastal Massachusetts lies within the Seaboard Lowlands of the New England Physiographic Province (Figure 2). The islands of Nantucket and Martha's Vineyard, south of the Cape Cod Peninsula, are located in the Atlantic Coastal Plain Province. The Seaboard Lowlands topography is characterized by low topographic relief; most features are less than 60 m above sealevel. Small rivers and streams flow toward the bays. The shape of the Seaboard Lowlands boundary is similar to the area trangressed by the ocean or occupied by proglacial lakes when the last ice sheet retreated from the area (Figure 3) (Flanagan, 1999).

Figure 2. Physiographic regions of New England (Flanagan, 1999)

Figure 3. Surficial geology of New England coast. (Flanagan, 1999)

Erosional and depositional features formed by the Laurentide ice sheet shape the surficial landscape of the coast. Debris was scraped, carried and deposited by the glacier. Proglacial lakes formed and river courses were altered. Glacier deposits along the coastal area reflect the direction of ice movement, from northwest to southeast, the opposite of the northeast-southwest bedrock orientation. During the retreat of the ice sheet deposition dominated.

Till is the most common and widespread glacial deposit found in the area. Till, a mixture of gravel, boulders, clay, silt and sand can be found in the region as a thin layer over upland area or buried in valleys under stratified deposits. These deposits range in thickness from an average of 6 m overall up to 60 m in drumlin formations. Sediments deposited by the retreating glacier in the coastal area north of Boston were released into the ocean. The ocean had transgressed inland when the ice sheet retreated. Deltas and debris flows formed near the ice front and finer grained deposits were carried out by meltwaters and formed marine clay in the sea. Meltwater also deposited sediments into proglacial lakes that formed during ice front retreat. In southeastern Massachusetts, especially the Cape Cod peninsula, outwash plain and moraine deposits dominate the landscape. Figure 3 illustrates the surficial deposits of the New England coast (Flanagan, 1999).

The remainder of this report will focus on the glaciation of the following areas of the Massachusetts coast: The Cape Ann peninsula, The Boston Basin, and Cape Cod (see Figure 4.).

Figure 4. Massachusetts Coast. (Einstein, N., 2005)

Cape Ann Peninsula

The Cape Ann peninsula area, bordering the western Gulf of Maine, is comprised of rocky headlands and islands. The bedrock of the area consists of faulted Paleozoic instrusive rocks. Massive intrusive granite is exposed at the surface and the rocky coast is indented with small harbors and beaches. Thin, discontinuous glacial deposits cover the bedrock (Barnhardt, 2005). The granite of the Cape Ann peninsula is extremely dense and was once valued for quarrying. The unique density of this granite is due to the overlying pressure of the sedimentary rock that once covered the bedrock. The retreating Laurentide ice sheet eroded the sedimentary rock overlying the granite about 15,000 years ago and brought the granite close to the surface. Evidence of glacial erosion is seen in Figure 5 in the striations on a boulder at Halibut State Park on the Cape Ann peninsula (Ratti, 2007).

Figure 5. Striations on boulder, Halibut State Park. (Ratti, 2007)

The Cape Ann peninsula is a terminal moraine. The melting ice sheet left behind extensive areas of rocky debris including huge erratic boulders, some carried by the glacier from Newfoundland. In addition to the rocky debris, drumlins of molded till, and eskers are common in the area (Essex County Greenbelt Association, 2008). As the glacier wasted away large areas of rocky glacial till was deposited in the region. The Dogtown Common area of central Gloucester is littered with the rocks and boulders of the terminal moraine. This 1600-hectare site, once a prosperous settlement in the late 1600s, is now protected as a municipal watershed. The site includes unusual erratics and remnants of the terminal moraine (Carlsen, 2008).

As the ice sheet began its retreat from the area, the sea transgressed inland. When the ocean reached about 33 m above present sea level, lowlands were flooded and the Cape Ann Peninsula became an island. Glacial meltwater plumes deposited large quantities of glacial-marine sediments onto the ice-marginal ocean floor (Barnhardt, 2005).

The inland portion of Cape Ann has a poor drainage system due to glacial debris blocking and overloading streams on the outwash terrain. Kettle holes, formed from melted ice blocks, and kames are evident in this area. The present course of the Merrimack River is a result of glacial debris blocking the river's former southern drainage route. The river now turns sharply from its southern direction toward the northeast where it enters the ocean north of Cape Ann. A submerged delta from this river is present offshore at Newburyport (Dennen). The submerged delta and other features from the last glacial stage have been preserved offshore on the continental shelf. These offshore features are relics of the relative sea level changes northeastern coastal Massachusetts has experienced. During ice sheet retreat, the area experienced first a transgression of the ocean, then a regression as the land rebounded. Subsequently, another marine transgression has covered these continental shelf features (Barnhardt, 2005).

Boston Basin

The Boston Basin is a topographic lowland bordering Boston Bay. The basin is underlain by Precambrian volcanic and sedimentary rocks. Repeated advances of the ice sheet preferentially eroded softer rocks. The basin slopes below sea level as it extends into Massachusetts Bay. Topography in the area shows signs of ice stagnation during glacial retreat. Proglacial lakes formed in front of the ice in the Charles River valley. Drumlins are common in the region, rising 30-60 m above the basin. The famous landmark, Charlestown's Bunker Hill, is a drumlin. Another glacially formed landmark, the Boston Harbor Islands, are a group islands in Boston Harbor formed of moraines and drumlins (Peragallo, 1989).

The Boston Harbor Islands drumlins are part of the Boston Harbor Island National Recreation Area (Figure 6). These islands have been protected by the National Park Service because they represent a geological rarity. The 30 drumlins occur in a swarm, or a cluster. The drumlin swarm is the only cluster in the United States to intersect a marine coastline. Rising Holocene sea level covered the harbor portion of the Boston Basin. The drumlim swarm now appears as rounded isolated islands in the harbor. The drumlins are asymmetrical and many of them do not have an elongated shape. Geologists suspect that the islands were subject to two periods of glacial activity (National Park Service, 2007).

Figure 6. Boston Harbor Islands drumlins. (National Park Service, 2007)

Cape Cod

The Cape Cod Peninsula and the islands of Martha's Vineyard and Nantucket extend into the Atlantic Ocean in the far southeastern portion of Massachusetts. Cape Cod Bay lies to the north and Buzzards Bay is to the west, north of the Elizabeth Islands. The surficial geology of Cape Cod and the islands was formed by the Laurentide continental ice sheet. Glacial deposits 60 to 180 m thick bury the bedrock of Cape Cod. The deposits are shaped mainly into ice-thrust moraines and outwash plains. Three ice lobes existed along the Laurentide ice front over the Cape Cod vicinity and influenced the positions of the moraines and outwash plains of the Cape. The lobe to the west, the Buzzards Bay lobe formed the upper cape and the Cape Cod Bay lobe shaped the middle cape. The South Cannel lobe, located east of the cape, influenced the eastern, or lower Cape Cod (Oldale, 2001a).

The glacier reached its farthest southern extent in Massachusetts about 21,000 years ago, terminating on Martha's Vineyard and Nantucket. Moraines mark this maximum ice advance. Within a few thousand years it receded back across Nantucket Bay to the peninsula where another moraine extends across the upper, (the westernly), portion of the peninsula (See figure 7). (Hutchinson, 2001)

Figure 7 - Cape Cod 21,000 years ago to present. (Hutchinson, 2001)

The Buzzards Bay and Sandwich moraines of the upper Cape are unique because they are ice shoved moraine formations. These glaciotectonic moraines were built by the ice front thrusting sheets of previously deposited sediments forward to form the ridges of the moraine (Oldale, 2001a).

Outwash plains cover most of the peninsula. Gravel and sand carried by meltwater streams away from the ice front was deposited on broad flat areas. The outwash plains of Cape Cod began as deltas that formed at the edge of glacial lakes to the north and south of the peninsula. Outwash plains of the upper Cape formed as deltas in lakes occupying Vinyard and Nantucket Sounds (Figure 8) (Oldale, 2001a). The eastern outwash plains of the lower Cape are comprised of stream deposits overlying delta and glacial lake deposits. These plains formed when meltwater streams drained the South Channel ice lobe. The streams flowed westward and sand and gravel accumulated on a plain. As the streams continued west, sand was deposited as deltas in Glacial Lake Cape Cod. Glacial Lake Cape Cod had developed as a proglacial lake in Cape Cod Bay when the Cape Cod Bay ice lobe retreated. As the delta grew out into the lake, the outwash plains of the lower Cape expanded in a western direction (Figure 9) (Oldale, 1993-94).

Figure 8. Formation of upper Cape outwash plains. (Poppe, 2007)

Figure 9. Formation of lower Cape outwash plains (Poppe, 2007)(modified from Uchupi, 1996)

The outwash plains have a kame-and-kettle morphology. Kames are created when outwash deposits fill a hole in the ice and it collapses into a hill shape when the ice melts. Kettle holes form when a block of ice melts and the sediments which surround the block collapse into the hole(Oldale, 2001a). Some kettle holes on Cape Cod are large and their depth reaches to the ground-water table, forming ponds or harbors. Great Harbor in Woods Hole on the upper cape is a kettle hole 21 m deep. A line of deep kettle holes form Woods Hole Channel (Hutchinson, 2001). On the lower Cape, kettle hole ponds are the most common glacial feature (Figure 10). Ice blocks left behind by the Cape Cod Bay ice lobe were buried by delta sediments. Now the depressions of the kettle holes are filled with ground water due to the rise in sea level (Masterson, 2005).

Figure 10. Kettle hole ponds. (Masterson, 2005)

Relict valleys are other unique features of the outwash plains. Most of the valleys are without rivers or streams; only their outlets near the ocean are filled with water. The valleys probably formed by spring sapping, a process that involves spring migration headward, which carves the valley. The water table during the time of Glacial Lake Cape Cod was probably higher than today, which would have allowed the headward migration to occur. The Pamet River Valley crosses the entire width of the lower Cape, connecting the Atlantic Ocean and Cape Cod Bay. This valley may have widened and extended to completely cross the cape when spring sapping reached a glacial lake east of the lower Cape. The breach would have caused a sudden draining of the lake (Oldale, 2001).

Conclusion

The Laurentide ice sheet was the last continental glacier to advance across New England during the Pleistocene Epoch. The erosional and depositional processes of this ice sheet formed most of the present day surficial geology of the region. The coast of Massachusetts exhibits a wide range of glacial landforms due to the glacial advance and retreat of the Laurentide. Marine transgression and regression associated with the glaciation has also shaped the coast. Rising sea levels during the Holocene continue to reshape the coastal landscape.

References

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