17.9 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.

17.10 Tides

The term tide is applied to the periodical rising and falling of the water of the ocean caused by the attraction of the sun and moon. Periodical alterations in the direction of the wind, and periodical variations in atmospheric pressure, may give rise to alterations in the level of the sea, but true tides are attributed (are due to) to astronomical causes. It is supposed that the attraction of the sun and moon may affect not only the waters of the ocean but also the solid crust of the earth, producing an alternating change in its shape, but so small as to be difficult of detection.

Anyone living at the seaside must have observed the gradual advance and retreat of the sea about twice in the 24 hours, or to be more exact, twice in 24 hours 50 minutes, the average interval between two successive high waters being 12 hours 25 minutes. The time of high-water thus changes from day to day, and is evidently related to the position of the moon, which passes the meridian on an average 50 minutes later on each succeeding day. The height to which the water rises varies also from day to day, the range from high-water to low-water being greatest about the time of full moon and new moon, when the tides are called “spring-tides”, and least about the time of the moon’s first and third quarters, when the tides are called “neap-tides”. The tide generating effect of the moon is more than double that of the sun, because of the very much greater distance of the sun, in spite of its greater mass. When the sun and the moon are both on the same side of the earth and when they are diametrically opposed to each other their tide-generating effects are additive, but when they are at right angles to each other the effects are subtractive, so that the spring-tides have a range three times greater than the neap-tides.