Background on Glaciers
A glacier is a thick mass of ice that originates on land. It is formed by the accumulation, compaction, and recrystallization of snow. A glacier advances when there is more accumulation than ablation occurring; this means there is more accumulation of snow and ice than there is melting of it (Pidwirny). _
Glacial History
The last ice sheet to cover New England was the Laurentide, during the end of the Wisconsin Glacial period. This glacial period was the last major advance of the North American ice sheet complex and occurred between 85,000 to 11,000 years ago. There was two periods of pleistocene glaciation. The Laurentide ice sheet formed in Canada about 75,000 years ago, and advanced to Northern New England. It then retreated during the beginning to middle of the Wisconsin period. The Laurentide advanced once again during a cooling climate 25,000 years ago, at the end of the Wisconsin period, this time reaching Southern New England. Throughout this glaciation, the Boston Basin area was covered by an ice sheet more than one mile thick. Eighteen thousand years ago the climate warmed and the ice sheet began its final retreat. The part of the ice that covered Massachusetts had three projections, or lobes which all retreated at different rates (Martin).
Even after the Laurentide Ice Sheet retreated, it left behind many erosional and depositional features. Deposits along the coast show the northwest to southeast ice movements that contrasts the northeast-southwest bedrock orientation. The most common depositional feature found in the Boston Basin is till, which is an unstratified mixture of sand, gravel, boulders and clay (Martin).
Boston Harbor formed because its bedrock, Cambridge Argilite, is softer and erodes more easily than other bedrock to the North and South. During the glacial period, thousands of tons of ice were moved over the Boston Harbor, and since the bedrock is soft, a depression was created. Once the glaciers melted, 18,000 years ago, sea levels rose and the Boston Harbor was filled with water (Clifford).
To learn more about features formed by glaciers, click below!