Glaciers of the Northeastern U.S.: a brief review

Figure 3.1: Geologic time scale (not to scale).

The Quaternary period began 1.8 million years ago and was marked by a series of advances and retreats of successive enormous ice sheets that originated in the Hudson Bay area of Canada. The Quaternary period is divided into two epochs: the Pleistocene and Holocene (Figure 3.1). The Pleistocene is simply the equivalent of the Quaternary minus the most recent (and current) interglacial interval, the Holocene. Ice age conditions existed when the ice sheet advanced over the North American continent; interglacial or warming periods existed when the ice sheet retreated north. Advances of the ice sheet over the northern United States occurred several dozen times over the course of the Pleistocene epoch of the Quaternary.

The most recent glacial advance reached its maximum extent 25-20,000 years ago and had an enormous impact on the Northeast. The glaciers blanketed much of the region with glacial deposits, challenging agriculture with rocky fields; limestone ridges, however, were ground and spread, increasing soil quality south of limestone outcrops. The topography was sculpted and drainage patterns shifted by the scouring action and deposits of the glacier. Abundant and easily mined sand and gravel also resulted from glacial deposits. Marks left behind by the glaciers on the high peaks of the Adirondacks and New England mountains tell us that the glaciers reached a thickness of between 1 and 2 kilometers, covering the tallest peaks in the Northeast. By 10,000 years ago, the ice had fully retreated from the Northeast. This ice-free interval, which we are in currently, is called the Holocene or Recent. Although all glacial advances had impacts on the surface of the Northeast, the effects of only the last ice sheet are well documented, since each succeeding glacial advance erodes and smears the record of the previous advance. 

The ice sheets are a form of glacial ice. As snow falls and is compacted, individual snowflakes become smaller, rounder and thicker, changing to granular snow. Upon further burial, compaction and cementation from recrystallized meltwater, the granular ice is changed to firn. When the firn has been buried to a depth greater than 30 meters, ice flow occurs, causing subsequent deformation. The firn recrystalizes to glacial ice, forming interlocking ice crystals, just as sedimentary rocks are recrystalized to form metamorphic rocks. As snow accumulates, packs down, and is converted to glacial ice, the weight of the accumulating snow causes the underlying glacial ice to flow out in all directions from the center. Like water, ice flow is driven by gravity, and moves downhill. 

There are two types of glaciers: smaller-scale valley glaciers and large-scale ice sheets. Found in mountainous regions at high altitudes, valley glaciers form by erosive action in bowl-shaped scours called cirques and flow down preexisting valleys on high altitude mountains. Ice sheets occur on a much larger scale, spreading from a central point outward in all directions across a continent. Greenland and Antarctica currently have ice sheets similar to the one that stretched over North America 20,000 years ago.

Glaciers will only form in specific environments. They require adequate snowfall so that each year more snow is accumulating than melting. This allows for the build-up and compaction of snow that will gradually become glacial ice. Thus, cold climate and sufficient moisture in the air for the precipitation of snow are both necessary for the formation of a glacier. Cold climate conditions exist at high altitudes and high latitudes. It is not surprising that the ice sheets of today are in the high latitude polar regions of Greenland and Antarctica, where temperatures are low. For continental ice sheets to occur, there must be landmasses over the high latitudes, since flowing ice will not form over open water. 

Glacial Scouring

The ice sheet left its mark in many ways on the Northeast, resulting in many noticeable topographic features. As the 1-2 kilometer thick glacier advanced forward, flowing under its own weight from the center of accumulation, it scraped and scoured the crust beneath. Boulder- to clay-sized sediments were plucked from the underlying bedrock and soil. The glaciers incorporated this sediment into the glacial ice or bulldozed it forward in front of the advancing ice. Sediments in the glacial ice acted like coarse sand paper, scouring and scraping the bedrock beneath. Sediments and less resistant sedimentary rocks over which the glacier moved were often eroded and ground-up into very fine sediment and clay (called rock flour). More resistant igneous and metamorphic rock was often polished and scratched by the grinding action of the sediments in the glacial ice. Knobs of resistant rocks, polished by the glaciers, are common in the Northeast. Streams of meltwater from the glacier, frequently gushing and full of sediment, caused significant amounts of scour as well. The abrasive sediments in the flowing water created potholes in the bedrock and plunge pools at the base of waterfalls.

Valley glaciers, flowing from the high mountains in the Adirondacks, Catskills and New England, originated near the peaks in bowl-shaped scours called cirques. Though the mountains are now free of glacial ice, the distinctive scoop-like cirques are still visible in some peaks in the Northeast. (Tuckerman’s ravine in New Hampshire is a cirque.) Scouring by the valley glaciers and the ice sheet that covered the mountains eroded a great deal of bedrock, rounding out and shortening the mountains, sometimes by hundreds of meters. 

How do we know the mark of glaciers?

How do we know that striations, polish, scoured basins, U-shaped valleys and the variety of deposits attributed to glaciers are in fact a result of glacial action? Before the modern understanding of the ice ages, many believed that the features now attributed to glaciers were the result of a great flood similar to the story of Noah and the Ark in the Bible. By studying modern glaciers, however, geologists have come to understand the resulting features of glacial scour and deposition that are readily identified in much of the Northeast. Modern glaciers include the large-scale ice sheets in Greenland and Antarctica as well as the small-scale valley glaciers found in mountain ranges in places such as Alaska, Canada and the Alps. 

Glacial Deposits

In an action similar to a bulldozer, the glacier plowed over the land. As it moved forward, the glaciers scraped up earth and pushed ahead piles of sand, gravel and broken rock to form characteristic glacial deposits (Figure 3.2). The unsorted mixture of boulders, gravel, sand, silt and clay picked up and later deposited by glaciers is called till. Where the bulldozing glacier stopped its advance for a time and then melted back, the ridge of till that had been pushed in front of the glacier was left behind, marking the end or terminus of the glacial advance. The ridge of till is called a moraine and ranges in length from hundreds to thousands of meters. Till that has been molded and reshaped by the underside of an advancing glacier into a streamlined, elongated hill is called a drumlin. This is till that has been trapped underneath the glacier, and has thus been deformed by the ice flowing above. The elongated shape of a drumlin is parallel to the direction of ice flow, and thus an excellent clue to determine the flow of the ice sheet during its most recent advance.

Figure 3.2: Glacial features. Figure by J. Houghton.

Meltwater flowing off the glacier also left behind deposits. Unlike till deposits, meltwater deposits are well-sorted, just as other rivers and streams have well sorted layers of sediment. As the glacier melted, streams of sediment-laden meltwater poured off the ice, often creating networks of braided streams in front of the glacier. Streams of meltwater flowing under the glacier deposited sand and gravel. When the ice sheet retreated, these ridges of meltwater stream deposits, known as eskers, were left standing.

Other glacial features include kettles, kames and erratics. Kettles are ponds or depressions left behind by the melting glacier. Blocks of ice broken off from the glacier often were buried or surrounded by meltwater sediments (Figure 3.3). When the ice eventually melted, the overlying sediments had no support, collapsing to form a depression that often filled with water to become a lake. Many kettle lakes and ponds are found throughout the glaciated Northeast. Kames are mound-like deposits of sediment from the melting glacier. Erratics are rocks that the ice sheet picked up and transported further south as it moved over the continents.

Figure 3.3: Formation of a kettle lake. Figure by J. Houghton.

Periglacial Environments

Though not all of the Northeast was covered by the ice sheet, the entire region felt its effects. The region covered by the ice sheet was scoured and covered with glacial deposits; the region south of the ice sheet has its own distinctive landscape and features because it was next to the ice margin. This unglaciated but still affected zone south of the ice sheet is called the periglacial zone.

Figure 3.4: Physical weathering from freeze-thaw cycle. Figure by J. Houghton.

There are a variety of features associated with a periglacial zone that also provide clues to the extent of the most recent ice sheet. In the tundra-like environment of a periglacial zone, eolian deposits are common. Sand dunes and wind-transported sediments are found in former periglacial areas and in glacial lake bottoms of the Northeast. The permafrost associated with the periglacial area, in which the ground is frozen much of the year, can cause mass movement of sediment. When the surface layer of the permafrost ground thaws, it is full of moisture. This water-heavy layer of soil may move rapidly down a hill in a process called solifluction. Physical weathering of the bedrock is magnified in the periglacial environment because of the freeze-thaw cycles associated with permafrost. When water enters the cracks and fissures in the ground and subsequently freezes, the ice wedges the cracks further and further apart (Figure 3.4). Because ice takes up more space than water, the pre-existing cracks and fractures are widened when the water freezes. Along ridges, rocks are eventually broken off as ice wedges continue to expand in joints and fractures. The boulders and blocks of bedrock roll downhill and are deposited along the slope or as fields of talus. Especially in the Appalachian/Piedmont region, talus blocks are carried far down slope and are found as fields of boulders. Frost action also brings cobbles and pebbles to the surface to form nets, circles, polygons and garlands of rocks. These unusual patterns of sorted rock are known as patterned ground. Solifluction and ice wedging are found exclusively where the ground remains perennially frozen, yet is not insulated by an ice sheet. Such conditions only occur in areas adjacent to ice sheets.

The Pleistocene

Whatever its cause, a cooling climate triggered the start of a series of ice ages shortly before the Pleistocene began. The most recent ice age before the present interglacial period began 65,000 years ago and affected the Northeast until 10,000 years before the present. Initially, the ice was spreading from a single dome located in northern Canada over the Hudson Bay. Twenty thousand years ago, this ice sheet reached its maximum extent, as far south as Long Island and northern Pennsylvania in the Northeast.

Figure 3.5: The approximate position of the ice sheet 20,000 years ago. After Hughes, T., et al., 1985.

The formation of glaciers comes from precipitation of water originating from evaporation of ocean water. Thus, significant glacial build-up ties up water in ice sheets, causing a sea level drop. During the Pleistocene glacial advances, sea level dropped an estimated 110 meters! The coastline of the east coast was an estimated 100 kilometers east of its present location 20,000 years ago (Figure 3.5).

Figure 3.6: The approximate position of the ice sheet 18,000 years ago. After Hughes, T., et al., 1985.

By 18,000 years ago, the ice sheet was in retreat because of a slight warming of the climate (Figure 3.6). Melting ice caused the ice sheet to begin calving into the St. Lawrence River and the Gulf of Maine, raising sea level. Though the ice sheet alternately moved forward and melted backward, overall it was on the retreat. Even during full glacial times, the glacier was always melting at its fringes. During times of glacial retreat, the ice sheet was not flowing backwards. The glacier continued to flow forward, but it was melting faster than it was advancing.

Figure 3.7: The Northeast 12,000 years ago. After Hughes, T., et al., 1985.

By 14,000 years ago, sea level had risen so high that the ocean flooded the St. Lawrence River. The formation of the St. Lawrence Seaway cut off the glacial ice that covered much of Maine. Continued melting left the Northeast free from the ice sheet 10,000 years ago. Though the crust was rebounding now that the heavy glacial ice was gone, continued melting of the ice sheet caused sea level to rise faster than the crust.

Sea level rise and the slowly rebounding crust caused the Northeast coastline and inland lakes to be flooded. Lake Champlain, many times larger than it is now, was flooded by ocean water to become the Champlain Seaway. The basins scoured by the glaciers to form the Great Lakes were flooded by meltwater and formed lakes with boundaries much larger than today (Figure 3.7).

Rebounding of the crust

A 2 kilometer thick ice sheet can weigh quite a bit. The enormous weight of the ice sheet over the continent depressed the crust into the asthenosphere just as the weight of a person in a canoe causes the boat to ride lower in the water. When the person steps out of the canoe, the buoyancy of the canoe allows it to once again rise. When the ice sheet retreated from the Northeast during the most recent ice age, the crust rebounded and continues to do so today. However, the crust could not rebound as fast as sea level was rising from the melting glaciers. The result was flooding of the coast and glacial lakes. The rebound of the crust when it is freed from overlying ice is known as isostacy.

Figure by J. Houghton.