The Geology of Baxter State Park and Mt. Katahdin

Glaciers and how they form

A glacier is a mass of ice on land, which is moving or shows some evidence of having moved in the recent past. Glaciers form wherever more snow falls during a series of winters than melts during the intervening summers. Such conditions are met at high latitudes, near the North and South Poles, where the largest glaciers now exist, as in Greenland, Iceland and Antarctica. Small glaciers occur today at high elevations at middle and even low latitudes, as in the Rocky Mountains, the Alps, the Andes, and the Himalayas. For reasons yet undiscovered by geologists, climatic conditions at several times during the past one million years were such that glaciers formed at much lower latitudes and elevations than they do today. Glacial ice is produced by the weight of accumulated snow.

If accumulation occurs in a valley, when the ice reaches a sufficient thickness, probably about 100 feet, it moves slowly down the valley, partly for the same reason that water flows downhill: it is pulled downhill by gravity. Also partly responsible for the movement of glacier ice is the squeezing force produced by the accumulation of ice and snow upstream. Glacier motion has been measured in many parts of the world and ranges from less than an inch to more than 100 feet per day.

There are two principal types of glaciers, continental glaciers and valley glaciers. A continental glacier, also termed an ice sheet or ice cap, is so large that it completely covers the landscape, burying valleys and mountains alike. The ice in continental glaciers moves essentially outward from the center, radiating in all directions. Because continental glaciers are tremendously thick, over 1000 feet in most cases and sometimes exceeding a mile or more in thickness, motion is but locally hindered by hills and mountains, and these glaciers are able to flow over whatever may be in their paths.

Valley glaciers, on the other hand, are confined by the valley walls and can move only down the valley. Valley glaciers are sometimes called mountain glaciers, because they occur only in mountains. The well known glaciers of Switzerland and Alaska are mostly valley glaciers.

The movement of glaciers carries ice to lower elevations or latitudes, where it is eventually melted. A glacier can advance to a point where the rate of forward motion just equals the rate at which ice is melted, so that the downstream margin of the glacier remains stationary. Boulders, gravel, mud, and other debris is dumped at the margin of the glacier in the form of a moraine. In many valleys such moraines record the former positions of glaciers where glaciers no longer exist.

The former margins of continental glaciers also are recorded by moraines. The presence of prominent moraines in such places as Long Island in New York, Cape Cod in Massachusetts, and in much of the Midwest is evidence that continental glaciers once covered North America north of these moraines.

Studies of moraines and other glacial features, principally in the Midwest, have indicated that continental glaciers have formed and melted several times during the past one million years. Following each glacial episode the glaciers melted, perhaps completely, which would indicate that the climate was warmer than it is today. The most recent major advance of the continental glacier started about 30,000 years ago and continued, with minor fluctuations, until about 9,000 years ago, at which time general melting occurred. Even though glaciers are at present melting rapidly the world over, historic records in the Alps and geologic evidence from many other mountain areas indicate that glaciers are now more extensive than they were some 3,000 to 7,000 years ago. From this it may be inferred that the climate of these areas is colder than it once was, but is becoming warmer.

Introduction Bedrock Glacial geology Geologic features Acknowledgments Glossary References Plates

Last updated on January 11, 2008

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The Geology of Baxter State Park and Mt. Katahdin

Glaciers and how they form