The smaller Greenland glaciers.  Many of the smaller glaciers of Greenland do not reach the sea, but deploy on plains of sand and gravel.  The edges of these ice tongues are often as abrupt as if sliced away with a knife (Fig. 92), and their structure is thus readily seen.  They are stratified, their layers representing in part the successive snowfalls of the interior of the country.  The upper layers are commonly white and free from stones; but the lower layers, to the height of a hundred feet or more, are dark with debris which is being slowly carried on.  So thickly studded with stones is the base of the ice that it is sometimes difficult to distinguish it from the rock waste which has been slowly dragged beneath the glacier or left about its edges.  The waste beneath and about the glacier is unsorted.  The stones are of many kinds, and numbers of them have been ground to flat faces.  Where the front of the ice has retreated the rock surface is seen to be planed and scored in places by the stones frozen fast in the sole of the glacier.

We have now found in glacier ice an agent able to produce the drift of North America.  The ice sheet of Greenland is now doing what we have seen was done in the recent past in our own land.  It is carrying for long distances rocks of many kinds gathered, we may infer, over a large extent of country.  It is laying down its load without assortment in unstratified deposits.  It grinds down and scores the rock over which it moves, and in the process many of the pebbles of its load are themselves also ground smooth and scratched.  Since this work can be done by no other agent, we must conclude that the northeastern part of our own continent was covered in the recent past by glacier ice, as Greenland is to-day.

VALLEY GLACIERS

The work of glacier ice can be most conveniently studied in the separate ice streams which creep down mountain valleys in many regions such as Alaska, the western mountains of the United States and Canada, the Himalayas, and the Alps.  As the glaciers of the Alps have been studied longer and more thoroughly than any others, we shall describe them in some detail as examples of valley glaciers in all parts of the world.

Conditions of glacier formation.  The condition of the great accumulation of snow to which glaciers are due—­that more or less of each winter’s snow should be left over unmelted and unevaporated to the next—­is fully met in the Alps.  There is abundant moisture brought by the winds from neighboring seas.  The currents of moist air driven up the mountain slopes are cooled by their own expansion as they rise, and the moisture which they contain is condensed at a temperature at or below 32 degrees F., and therefore is precipitated in the form of snow.  The summers are cool and their heat does not suffice to completely melt the heavy snow of the preceding winter.  On the Alps the snow line—­the lower limit of permanent snow—­is drawn at about eight thousand five hundred feet above sea level.  Above the snow line on the slopes and crests, where these are not too steep, the snow lies the year round and gathers in valley heads to a depth of hundreds of feet.