Tammy Marie Rittenour

For inquiries contact Professor Julie Brigham-Grette, Department of Geosciences, University of Massachusetts Amherst

Formation of Continental Glaciations

Ice began to accumulate in northern Canada to form the Laurentide ice sheet 120,000 years ago. This latest glaciation was initiated by a decrease in incoming solar radiation due to the elliptical nature of the Earth's orbit. Over the past 2.5 million years, variations in the axis of rotation and eccentricity of the Earth's orbit have caused many glaciations. These variations in the Earth's orbit are called Milankovitch orbital cycles after Milutin Milankovitch, an astronomer who first determined orbital variation frequencies and their influence on solar radiation received by the Earth.

The tilt of the Earth's axis is currently 23.50. In the past this tilt, or obliquity of the Earth's axis, has varied from 24.50 to 21.50 on a cycle of 40,000 years. Glacial conditions are most favorable when the tilt angle is lowest.

The Earth's axis also wobbles over time due to the gravitational pull of the Sun and Moon on the equatorial bulge of the Earth. (This wobble, or precession of the equinox, causes the time of year when the Earth is closest to the Sun to change in a cycle of 23,000 years. Prior to 900,000 years ago these orbital parameters strongly influenced glaciations, creating many small short-lived (23 40,000 year) glaciations.

After 900,000 years ago, glaciations became larger and lasted longer. Throughout the past 900,000 years climate has been dominated by the 100,000-year eccentricity cycle of the Earth's orbit around the sun. Incoming solar insolation has increased and decreased as the Earth's orbit has changed from elliptical to circular. The last several large glaciations were stimulated by low solar insolation during more circular orbits.

Glaciation in New England

During the latest advance of the Laurentide ice sheet, ice extended to cover most of Canada and the northern United States. In New England, the ice reached its maximum extent at Long Island around 21,000 years ago (ages presented here have been obtained from radiocarbon dating). During the last glacial maximum (LGM) global sea level was greatly reduced due to the large volume of water trapped in continental ice sheets in North America and other parts of the world. Ice in New England extended out onto the continental shelf, which at that time was not covered by water due to low sea level. While at this maximum position a series of terminal moraines were deposited.

These moraines formed the topography of Long Island, Block Island, Nantucket, Martha's Vineyard and Cape Cod. Only after retreat of the Laurentide ice sheet and subsequent sea level rise did these landforms become islands.

The Laurentide ice sheet left many indications of its former presence in New England. All of the bedrock hills and mountains in New England have been glacially modified and abraded. The flowing ice removed all loose and easily eroded rocks leaving many smooth sculpted surfaces covered with striations. Striation orientation can be mapped and has been used to determine past ice flow direction.

The advancing and retreating glacier left till deposited across the landscape of New England. Glacial till is formed beneath and within glaciers as rocks are carried and ground up by the flowing ice. Till is composed of grain sizes ranging from boulders to clay and is usually found resting directly upon bedrock in New England. Till deposits can range from a very thin mantle to tens of meters thick.

In many low-lying areas, glacial-fluvial, glacial-lacustrine or glacial-marine sediments were deposited after ice recession. Glacial-fluvial or outwash sediments are composed of sand and gravel deposited in meltwater streams near the glacier. These coarse-grained deposits can be found in many New England valleys. Glacial-lacustrine and glacial-marine deposits are composed of silt and clay deposited in lakes and marine inlets. Much of the eastern coasts of Massachusetts, New Hampshire and Maine are covered by glacial-marine clay. The Connecticut and Hudson River valleys were once occupied by glacial lakes and contain abundant glacial-lacustrine sediments.