Around one million years ago, North America began experiencing regular ice ages (Skehan 18). Most of what is the northeastern United States was covered with increasingly deep snowfall which compacted to form glaciers.
The most recent continental ice sheet to cover Boston area was the Laurentide, which formed during the late Wisconsinan glacial stage. The Laurentide ice sheet formed in Canada approximately 75,000 years ago (Martin). 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 (Martin). 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. By 14,000 years ago, the Laurentide ice front receded to a location near the present northern border of Massachusetts (Martin).
The most recent continental ice sheet to cover Boston area was the Laurentide, which formed during the late Wisconsinan glacial stage. The Laurentide ice sheet formed in Canada approximately 75,000 years ago (Martin). 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 (Martin). 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. By 14,000 years ago, the Laurentide ice front receded to a location near the present northern border of Massachusetts (Martin).
Glacial Features
Erosional and depositional features were formed by the Laurentide ice sheet. Debris was scraped, carried and deposited by the glacier. Kettle lakes formed and moraines were deposited. Glacier deposits reflect the direction of ice movement, from northwest to southeast (Martin). 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 (Martin).
The basin is underlain by Precambrian volcanic and sedimentary rocks. Repeated advances of the ice sheet preferentially eroded softer rocks (Martin). The basin slopes below sea level as it extends into Massachusetts Bay. Topography in the area shows signs of ice stagnation during glacial retreat.
The basin is underlain by Precambrian volcanic and sedimentary rocks. Repeated advances of the ice sheet preferentially eroded softer rocks (Martin). The basin slopes below sea level as it extends into Massachusetts Bay. Topography in the area shows signs of ice stagnation during glacial retreat.