Glaciers are slow moving masses or rivers of ice formed by the accumulation and compaction of snow on mountains or near the poles. Glaciers only form where the amount of snow that falls in an area exceeds the amount of snow that melts. There are two primary types of glaciers:
- Continental glaciers or ice sheets are dome-shaped glaciers that flow away from a central region in all directions and are largely unaffected by underlying topography. Examples include Greenland and the Antarctic ice sheet.
- Alpine or valley glaciers occur in mountains and flow down valleys causing the valley to be shaped like the letter U. Examples include the Columbia Glacier in Alaska and the Mt. Kilimanjaro in Tanzania.
Features of Glaciers
Because all glaciers by definition are moving ice masses, even slowly, they cause erosion of the surfaces on which they move. Like flowing water, glaciers loosen and carry away rock material on their sides and terminus. When continental glaciers have moved to a place where the climate is warmer than where they form, the glaciers melt and the debris, or till, is deposited. These deposits are named for their location relative to the ice sheet. Terminal moraines are found at the terminus or the furthest (end) point reached by a glacier. Lateral moraines are found deposited along the sides of the glacier. Medial moraines are found at the junction between two glaciers.
During the last Ice Age, about 18,000 years ago, a massive continental glacier covered all of the Arctic land region and extended far south into North America (see below).
Extent of Most Recent Ice Age Glaciation (courtesy, USGS)
One outcome of this continental glaciation was the formation of the Great Lakes of North America, which began forming near the end of the last glacial period around 14,000 years ago (see below).
Evolution of the Great Lakes (courtesy, NOAA)
As the glacier began retreating, ice sheets carved basins into the land and they became filled with melt water. Charlie and I were able to visit four of the five great lakes in 2013 while on a “bucket list” trip to finish seeing all fifty states . All were beautiful, but Lake Huron, below, was my favorite.
Lake Huron (my photo)
Evidence of this continental glaciation can also be seen in Long Island, NY where the most recent glaciation extended. Very course grained sediments from the terminal moraine for this glaciation make up most of the island (see below).
Beach “Sand” on Long Island (my photo)
Further evidence can be found in Acadia National Park, Maine, where huge boulders (glacial erratics) are lying atop the low mountain peaks of the park, and the composition of the boulders and underlying bedrock are completely different (see below).
Glacial Erratic in Acadia National Park (my photo)
Other features of Acadia include a series of barren ridges separated by glacially deepened U-shaped valleys. Mountains within the park rise to heights of only 1,530 ft above sea level. Glacial scouring (scratches in the bed rock caused by rocks carried by the glacier abrading the bed rock) are also common features. Many of Acadia’s landforms have a distinctly asymmetric profile, with gentler slopes to the north and northwest and steeper slopes to the south and southeast (roches moutonnees) that were caused by glacial action on resistant bedrock (see below).
Roches Moutonnees in Acadia National Park, Maine (my photo)
When alpine glaciers move to lower and warmer elevations, they, too, melt and deposit the rock material they are carrying. A glacier never actually retreats. But when melting of the glacial ice is faster than the downhill velocity of the glacier, the glacier appears to be moving uphill. Unlike continental glaciers, which no longer can be seen in North America, alpine glaciers can be seen in many states, including California, Alaska, Washington, Oregon, Montana, Wyoming, Colorado and Utah. Alpine glaciers are responsible for valleys that are U-shaped, rather than the V-shaped valleys that are formed in mountainous regions by rivers. Spectacular examples of U-shaped valleys can be seen in the Sierra Nevada Mountains in California, as seen below.
U-shaped valley in Kings Canyon National Park, California (my photo)
Other spectacular features associated with glaciers are fjords. Geologically, fjords are most frequently long, narrow, deep inlets of the sea between high cliffs. They are typically formed by submergence of a glaciated U-shaped valley and are often surrounded by dramatic mountain scenery. Fjords are most shallow where the terminal moraine was deposited, usually in front of the glacier arm. As the glacier which carved the U-shaped valley melts, the earth rebounds from the removal of the ice load. While fjords are most typically associated with Norway, fjords can be found on the coasts of British Columbia, Chile, Greenland, Iceland, New Zealand, Labrador and Newfoundland.
In the US, fjords occur in Alaska and Washington state. Charlie and I were fortunate to visit the Kenai Fjords National Park, which is located at the edge of the Kenai Peninsula of Alaska, in 2005. Here we saw not just the glacial valleys, but also the glacial ice which has formed the valley spilling into Prince William Sound (see below).
Holgate Glacier, Kenai Fjords National Park, Alaska (my photo)
We also flew to the Yanert Glacier near Denali National Park, where we were able to explore the glacier on foot (see below).
John and Charlie Carpenter on Yanert Glacier, near Denali National Park, Alaska
It must be noted before leaving that all glaciers, continental and alpine, are disappearing as a result of global climate change. In the words of the National Snow and Ice Data Center, “Scientists are also finding that glaciers reveal clues about global warming. How much does our atmosphere naturally warm up between Ice Ages? How does human activity affect climate? Because glaciers are so sensitive to temperature fluctuations accompanying climate change, direct glacier observation may help answer these questions. Since the early twentieth century, with few exceptions, glaciers around the world have been retreating at unprecedented rates. Some scientists attribute this massive glacial retreat to the Industrial Revolution, which began around 1760. In fact, several ice caps, glaciers and ice shelves have disappeared altogether in this century. Many more are retreating so rapidly that they may vanish within a matter of decades.”
Geologic Principles, Practices and Features 