An "ice time" or "Glacial Theory" was proposed in 1840 by Louis Agassiz and championed for the rest of his life. Does it make sense to you?

According to the Glacial Theory, during a time known as the 'Great Ice Age', huge areas of the Earth's land surface were deeply buried by ice. The map shows (in white) those areas of North America covered by glaciers.

According to the Glacial Theory, much of North America was repeatedly covered by great sheets of ice during the period from about two million years ago to 15,000 years ago. Before this 'Great Ice Age', there was a series of islands off the mainland coast where Long Island is now.

Glacial ice, carrying rock fragments, advances from the north. Melt water streams flow south towards the sea, distributing deposits of gravel, sand, silt and clay released from the ice. With water locked up as ice on the land, sea level drops, causing the coast line to shift.

The offshore islands expand and merge. In places, melt water cannot find a direct path to the sea and is impounded as lakes. The sediment carried by the melt water streams, deposited layer by layer, begins to bury the pre-existing topography.

The glacier continues to advance. As it moves over the land, it erodes the bedrock and incorporates the eroded material within the ice, carrying it along. Except for the one remaining island off the coast, all the land in front of the glacier is buried by the layers of sediment distributed by the melt water streams.

The glacier advances across land and also what was once ocean floor. In doing so, it bulldozes, excavates, and overrides both the bedrock and previously deposited melt water sediments. An apron of sediment continues to bury the land in front of the glacier.

The glacier reaches its greatest thickness and extent in this area. Within the glacier, rock fragments are carried forward. Where they scrape against bedrock, they gouge out grooves and striations. Where the ice melts at the glacial front, the fragments are lowered to the ground.

The ice front no longer advances. However, the ice itself, together with the rock fragments embedded in it, continue to flow from the north towards the glacial front. When they reach the glacial front, the ice melts and the rock fragments are lowered to the ground. As the ice continues to melt, the ratio of rock fragments to ice increases all along the glacial front.

The glacial front continues to remain stationary. Rock fragments are increasingly concentrated along the front. At the same time, the gravel, sand, silt and clay washed out by the melt waters from the glacier accumulate south of the front. They form a gently sloping, flat surface that extends the coastline seawards.

In the eastern region, the glacial front begins to retreat. It leaves behind a line of hills made up of rock fragments that were brought to the front and lowered to the ground while the front was stationary. The apron of sediment south of the front continues to thicken as glacial melt water streams continue to deposit sediment there.

The glacier continues its retreat in the eastern region. Although sea level rises due to the addition of glacial melt water, the coastline does not retreat significantly because the apron of sediment in front of the glacier continues to grow.

The glacial front stabilizes again and the volume of rock fragments increases along its entire length.

Rapid retreat of the glacier occurs along its entire length. A belt of hills made of rock fragments deposited when the glacial front was stationary is revealed. The belt splits in two at its eastern end where the glacial front stabilized twice, in different positions. The coast moves inland. An arm of the sea advances towards the west along the low land that lies within the split in the belt of hills.

The ice is almost gone. Arms of the sea advancing from the east and the west hint at the eventual shape of Long Island. In those areas where the ice melted back rapidly, a thin, discontinuous veneer of rock fragments patchily coats the bedrock.

The ice is gone. Long Island has separated from the mainland. The indented, northern margin of Long Island as well as its forked eastern end are defined by the junction of the sea with the belts of irregular hills made of rock fragments. The southern margin, formed where the flat apron of sediments meets the sea, is fairly straight. Its string of narrow offshore islands and peninsulas is the result of submerged sediment heaped up by storms.

David J. Leveson