Supercontinents, Ice Ages, and the North Shore

In 1633, John Winthrop Jr. led a group of settlers to the Native American area known as Agawam, and began building the town that they would name Ipswich. They chose a spot with half-billion year old granite outcroppings as a worthy location for their meeting house. It is still known today as Meeting House Green, or North Green. Yet most of the landforms in our town were formed less than 20,000 years ago.


North Shore Terraines

When the supercontinent Rodinia broke up 750 million years ago, the Laurentian craton became the early North American continent. Other pieces reassembled in the Southern Hemisphere into a new supercontinent, Gondwana.

Eventually Gondwana reunited with Laurentia to form Pangaea, the third supercontinent. The early stages of this collision created the Appalachian chain, including the Taconic mountain range that separates Massachusetts from New York State. The breakup of Pangaea 200 million years ago defined today’s North America and created the Atlantic Ocean.

The Merrimack, Nashoba, Avalon and Meguma terranes that make up the eastern half of Massachusetts were probably volcanic island chains that broke off of Gondwana and collided with Laurentia between 550 and 370 million years ago. Ipswich and Cape Ann are in the Avalon Terrane. The granite underlying the Avalon land mass crystallized while the volcanic chain was still attached to Gondwana.

Ice Ages

The Wisconsinian ice sheet began its advance about 80,000 years ago, eventually covering all of New England. Water was locked up as ice, which caused the sea level to be about 300 ft. lower than it is today. As it moved over the landscape, the glacier scraped up and transported rock and soil. Its retreat 20,000 years ago left glacial land forms that dominate our landscape:

  • Moraines are linear mounds of debris that build at the toe of the glacier. Cape Ann is broadly defined as the rocky 160 square mile area between Rockport, Danvers, Ipswich and Manchester by the Sea, and is a terminal moraine.
  • Eskers are long sinuous ridges of debris deposited by streams of glacial meltwater. Various trails in Willowdale State Forest, Appleton Farms near Cutler Road and the Ipswich River Sanctuary follow the crest of eskers.
  • Drumlins are elliptical hills formed when moving ice rides over the top of glacial material instead of transporting it. The Necks and Castle Hill are excellent examples of Drumlins.
  • Glacial erratics are large boulders carried by the glaciers. These are most obvious in the area from West Gloucester to Rockport.
  • Till is the unsorted glacial debris deposited directly under the glacier. It constitutes much of the soil in this area.

The animation below shows the ebb and flow of the Ice Ages over the last 120,000 years. The timeline in the upper right corner is measured in thousands of years before the present (kaPB).

Post-Glacial Rebound

The sheer weight of mile-high glacial ice caused our 450,000 granite bedrock to sink. Then as the glaciers melted, the sea level rose, putting much of the North Shore under water. As the ice-free land began to decompress and rebound upwards, the ocean receded to the shoreline we see today. Much of the surface soil in our area was deposited during the post-glacial period by these events. Marshland, drumlins and bedrock hills mix with broad flat areas of marine sand, gravel deposits and glacial till, creating the landscape of Ipswich and the North Shore area today.

The most recent Ice Age glaciation started two and a half million years ago with ice sheets advancing and retreating on 40,000- and 100,000-year glacial periods. Twenty thousand years ago the Laurentide Ice Sheet covered New England. The Topsfield hills, Great Neck and Castle Hill are but a few of the local geological features left by the retreating glacier.


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