Types of Glaciers

Glaciers may be divided into four principal types: continental, ice caps, valley glaciers and piedmont glaciers.

Continental glaciers. These are the largest of all glaciers. There are good examples today in Antarctica and Greenland. Continental glaciers may form regardless of topography, on plains, plateaus or mountains. From the centre of accumulation the ice moves slowly outward in all directions.

Ice caps. An ice cap is the covering of snow and ice on mountains from which alpine glaciers spring and move in different directions.

Valley glaciers. There are glaciers which rise in ice caps or single snow fields and occupy mountain valleys. They are sometimes called alpine because this type of a glacier was first studied in the Alps. There is a great difference in the size of these glaciers. Some are many miles long and hundreds of feet thick near their heads. Others are only a fraction of a mile in length, nearly as wide as they are long and only a few score of feet thick. Many modern valley glaciers are but tiny remnants of their former size.

Piedmont glaciers. Two or more valley glaciers that combine on a plain or in a broad intermontane valley at the foot of a mountain constitute a piedmont glacier. There were many glaciers of this type on the plains which border the Northern Rocky Mountains during the Pleistocene ice age, and there are fine examples in Alaska at the present time.

The Malaspina glacier in Alaska is probably the most typical and certainly is the most interesting piedmont glacier known. Situated immediately west of Yakutat Bay and south-east of Mount St. Elias, it is fed by numerous alpine glaciers, some of which are very large. The total area of this great ice sheet is about 1,500 square miles. Its central portion is a great plateau of clear white ice cut by thousands of shallow crevasses. Its margins, except where the larger glaciers come in, are covered with a thick mantle of morainal debris. Proceeding from the clear ice toward the sea, on the outer margin of this belt of morainal material there are, first, scattered flowers then clumps of alder and finally, thick forest of large spruces. Yet the whole area is underlain by glacial ice stagnant in some places, but moving in many others. The movement is plainly shown by new crevasses and great trees that have been overturned in the forested areas. The surface slope from the mountain front to the outer margin is about 70 feet to the mile. The morainal belt shows characteristic kettle and hummock topography (бугристо-котловинный рельеф).

Crevasses are numerous, as are small lakes of peculiar hour-glass shape formed in the underlying ice. Beneath the marginal ice are subglacial streams of large size. Hundreds of such streams, all loaded with silt flow out from the south margin of the glacier. One, the Yahtre, flows through a tunnel 6 to 8 miles long.

Дата добавления: 2015-01-03