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Geologic Origins of Stellwagen Bank

by Robert Oldale

From The Cape Naturalist, 1993-94.

Just six miles north of the Provincetown spit lies a shallow platform which is known as Stellwagen Bank. Now famous for whale watching, Stellwagen Bank was once dry land where mastodon and mammoth roamed

Located in the western Gulf of Maine, the large bay north of Georges Bank and between New England and Nova Scotia, the bank is about 26 miles (40 kilometers) long, about 13 miles (21 kilometers) wide at its south end, and narrows to about 3 miles (5 kilometers) wide at its northern end (Fig. 1). Water depths to the top of the bank range from 65 to 100 feet (20 to 30 meters). The bank rises about 165 feet (50 meters) above the surrounding seafloor. Stellwagen Basin, with water depths of about 260 feet (80 meters), is adjacent to the west flank of the bank and is part of the Massachusetts Bay seafloor. Stellwagen Bank is separated from the Provincetown spit by Little Stellwagen Basin with water depths ranging from 130 to 200 feet (40 to 60 meters). 

3-D image of Stellwagen Bank

A computer-generated northward-looking oblique view of Lower Cape Cod and Stellwagen Bank. Boston Harbor lies in the upper left corner of the view. Cape Ann is located north of the northern tip of Stellwagen Bank. Vertical exaggeration is about 100 times. Generated by Richard Signell

Stellwagen Bank mostly owes its existence to the last great ice sheet (known as the Laurentide Ice Sheet) and to changes in sea level that accompanied and followed deglaciation. In a computer-generated oblique view (see figure), Stellwagen Bank looks like an extension of Lower Cape Cod. This similarity is not accidental, for Lower Cape Cod and Stellwagen Bank share, at least in part, the same geologic history. The submergence of the bank has hidden much of the evidence of its origin, but Lower Cape Cod provides us with a model so that we can speculate on how and when Stellwagen Bank was formed.

Although the events that shaped the present bank occurred over about the last 25,000 years and involve the last continental glaciation and sea-level change that accompanied and followed deglaciation, it is likely that rocks deeply buried by glacial and postglacial deposits record a geologic history that spans more than500 million years. A geologic cross section , based on seismic soundings, shows the inferred structure of the bank. The deepest rock below the bank (called bedrock) is likely to resemble the bedrock exposed at the surface in Eastern Massachusetts. If this is so, the oldest rocks consist of granite (solidified molten rock formed deep in the earth’s crust) and volcanic rocks that formed during explosive eruptions (similar to the eruption of Mount St. Helen's), or as lava flows. The bedrock also would include sedimentary rocks that formed from deposits laid down in ancient streams, lakes, and oceans and metamorphic rocks that were formed from existing rocks by heat and pressure caused by deep burial and mountain building.

The bedrock beneath the bank may range from more than 570 million years, when there was little or no life on Earth, to as little as about 150 million years when dinosaurs ranged the landscape. Together these rocks contain a record of the breakup of super continents, collisions between continents as they drifted around the globe, and the birth and death of oceans. The youngest bedrock beneath Cape Cod contains a record of the earliest opening of the Atlantic Ocean, which occurred about 200 million years ago. It is likely that some of the bedrock beneath Stellwagen Bank contains a similar record.

Above the bedrock and beneath the glacial deposits of Stellwagen Bank, there are rock layers that are thought to resemble the strata under the coastal plain of New Jersey. The coastal plain strata were deposited in the streams, lakes, and swamps or on the shallow seafloor adjacent to the land. They may range in age from 5 to 140 million years old and were formed during the last part of the age of dinosaurs and the early part of the age of mammals. Coastal plain deposits filled the Gulf of Maine. The most seaward waterfalls of present coastal rivers mark the fall line that probably delineates the former landward edge of the coastal plain sedimentary wedge. About 5 million years ago, sea level fell, and streams and rivers began to erode the coastal plain deposits. Stellwagen Bank began to take shape as the streams eroded away the coastal plain deposits adjacent to the bank.

Glaciers, a much more powerful force in erosion than running water, finished what the streams had started. They removed most of the coastal plain deposits in the Gulf of Maine, eventually leaving behind only remnants and carved deep basins in the bedrock. We know that continental glaciers reached southern New England on two occasions, but we don’t know how many more times the region was glaciated. Over the past 1 1/2 million years — roughly the length of the Pleistocene Epoch or the Ice Age — it was likely that the Gulf of Maine was glaciated several more times. However, it was the last continental glacier that deposited most, if not all, the glacial sediments that underlie Stellwagen Bank.

The Laurentide Ice Sheet advanced out of Canada about 25,000 years ago and reached southern New England about 21,000 years ago. Shortly after its arrival, the ice sheet began its retreat as melting along the glacier front exceeded the advance of the ice. During the retreat, the ice sheet built Cape Cod, and studies of the geology of the Cape provide the clues needed to infer how Stellwagen Bank was built.

The plains of Lower Cape Cod, from Eastham to Truro, were formed mostly by meltwater streams that drained a lobe of the Laurentide Ice Sheet that lay to the east of Lower Cape Cod. Known as the South Channel lobe, it is named after Great South Channel, the western entrance to the Gulf of Maine that lies between Nantucket and Georges Bank. To the west of Lower Cape Cod, the Cape Cod Bay lobe occupied Cape Cod Bay. Evidence from Cape Cod shows that the retreat of these lobes was not simultaneous and that the lobe in Cape Cod Bay retreated before the South Channel lobe.

During the retreat of the Cape Cod Bay lobe, a lake developed in Cape Cod Bay. The lake was contained by glacial deposits to the south and west and by ice on the north and east. Meltwater streams of the Lower Cape Cod outwash plains flowed from the South Channel lobe westward to the lake and deposited sand and gravel on the outwash plain surface. When the streams reached the lakeshore, they deposited sand to form deltas in the lake. As the deltas advanced into the lake, the outwash streams lengthened and the outwash plains grew westward. Therefore, the outwash plains of Lower Caper Cod are composed of glacial stream deposits that cap delta deposits that, in turn, overlie glacial lake deposits of silt and clay.

When the worldwide volume of glacial ice was at its maximum, about 20,000 years ago, sea level around the world was about 300 feet below its present level. This was caused by the continental glaciers being made mostly of water removed from the world's oceans. In the Gulf of Maine, the weight of the glacier depressed the crust of the Earth to below the glacial sea level. Thus, as the Laurentide Ice Sheet retreated from the Gulf it was immediately replaced by the sea, which flooded this depressed region.

The shape, size, and location of Stellwagen Bank suggests that it may have had an origin similar to that of Lower Cape Cod, but the bank is somewhat younger. We can postulate that the meltwater streams that built the bank probably drained the South Channel lobe of glacial ice and that the Cape Cod Bay lobe probably had retreated away from the bank and into Stellwagen Basin. However, instead of the meltwater streams entering a glacial lake, as they did to build Lower Cape Cod, they entered the sea that had invaded Stellwagen Basin. As on Lower Cape Cod, meltwater streams deposited sand and gravel on the bank top, and sand along the seashore, to form deltas.

Thus, the glacial composition of the bank is likely to be very similar to Lower Cape Cod, with glacial outwash stream deposits overlying outwash delta deposits. However, beneath Stellwagen Bank, the delta deposits mix with and overlie glacial marine mud that was deposited in Stellwagen Basin. Eventually, as the ice melted, the South Channel lobe retreated, and the glacial development of Stellwagen Bank ended.

When did the Laurentide Ice Sheet build Stellwagen Bank? The answer to this question comes from Wilkinson Basin, a deep basin in the Gulf of Maine, located about 40 miles (65 kilometers) northeast of Stellwagen Basin. Microscopic, hard-shelled single-celled animals, called foraminifera, which lived in the sea and on the sea floor, were deposited along with the glacial marine mud. The remains of these animals can be radiocarbon-dated. The age of the oldest foraminifera in Wilkinson Basin is about 18,000 years. Because the glacial marine mud was deposited in the open sea in front of the retreating glacier, the age of these animals indicates that the glacial part of Stellwagen Bank can be no younger than the oldest foraminifera, or about 18,000 years.

A short time later, local sea level began to fall when, along with the release of the glacial load, the crust rebounded. By 12,000 years ago, the bank stood well above sea level and may even have been connected to Lower Cape Cod or, at the least, separated from the Cape by a shallow strait in Little Stellwagen Basin. Stellwagen Bank, then, closely resembled present-day Lower Cape Cod. Lakes, swamps, and marshes probably dotted the landscape. Along the shore, there were beaches, sea cliffs, spits, and lagoons. The climate was colder than it now is, and spruce and poplar forests and park lands of tundra shrubs and grasses may have covered the bank top. Teeth of wooly mammoths and mastodons have been dredged up in Stellwagen Basin, evidence of the animal life of the time. Early humans arrived in New England about 11,000 years ago, and they may have witnessed the beginning of the final chapter in the history of Stellwagen Bank. By then, local sea level was rising as crustal rebound slowed and as the melting glaciers continued to return water to the ocean basins. About 10,000 years ago, Stellwagen Bank slipped beneath the sea.

As the bank drowned, waves and currents eroded and reworked the glacial sediments atop the bank. The reworked sediments were redeposited on the bank or were transported to the edge of the bank and deposited on the flank. In this way, the bank took on its final shape.

Even though Stellwagen Bank is now covered by at least 20 meters (65 feet) of water, the bank top continues to change. Currents from large waves, generated by northeast storms and hurricanes, reach the surface of the bank and move sand and, at times, even gravel. The sand forms sand waves and sand ridges on the bank top, and some are likely transported to the bank edge and deposited on the flanks. However, compared to the dynamic events of glaciation and submergence, Stellwagen Bank is now a quiet place where whales, not mammoths and mastodons, find refuge.

Robert Oldale, recently retired, was a geologist with the US Geological Survey in Woods Hole for many years.

This article is reprinted from The Cape Naturalist 1993-94 by permission of the Cape Cod Museum of Natural History.