bowlders, gravel and sand, with inclosed hollows between. This materialis unstratified and without any uniformity in its arrangement.
When the glacier has retreated, ponds and lakes are formed in the depressions, and streams wander about in the low places between the heaps and receive the overflow of some of the lakes and ponds. Others of these lakes and ponds are so fully inclosed and receive the drainage from so small a surface that not enough water enters to overflow the rim. The arrangement of the streams is unsymmetrical and without order. The whole surface is a hodgepodge of glacially dumped material, a terminal moraine country.
Further back from this morainic dumping ground may be found other kinds of glacially deposited material. If a glacier is pushing along under it a mass of material and it meets some obstruction, or if on account of melting or adecrease in the rate of its flow it
AN ESKER.
has not the power to carry its load, it deposits a part of it. The ice slides over the deposited material and rounds it off, but leaves it as a river leaves its sand bars.
But this material is not stratified, like the material left in water. When the glacier melts away, these rounded deposition heaps are left as hills of greater or less height. Since the material forming them has been continually brought from the direction from which the ice came, they will have their greatest extension in that direction. Such hills have received the name drumlins.
Where there are stream channels in the under surface of the ice, thestreams may aggrade or fill up their beds as rivers do when overloaded. When the glaciers retreat, ice walls which bordered the channels melt away, and the sand and gravel which the streams had laid down along their beds are left as long, irregular ridges, at the end of which sometimes an alluvial fan or delta may be found. Such long ridges are called eskers.
Where the glacier has little load, as near its source, the bed rockis stripped bare, smoothed, polished, and scratched by the materialGLACIAL ROCK LAKE.
which the ice has scraped over it and borne away. Where the rock is soft, it is scooped out, and hollows are formed, afterward making lakes; and where it is hard, rounded ridges are made.
The valleys through which glaciers go are rounded out and left shaped like a U. If side glaciers join the main glacier, they may not be able to wear down their valleys as fast as the main glacier, so the mouths of these U-shaped valleys may be much higher than the bottom of the main valley. These are called hanging valleys. (See section 144.)A U-SHAPED VALLEY IN NORWAY.
This has been rounded out by glaciers. The moisture in the atmosphere makes it necessary to hang the hay up to dry, as seen in this picture.
The bowlders which are borne along by the ice are deposited irregularly over the surface in all kinds of positions when the ice melts. Some of them are very large and are left perched high up on the hillsides where no other known force besides moving ice could have carried them. These irregularly distributed perched bowlders are called erratics.
170.Glaciated Areas. -Over large areas of what are now the mostthickly populated regions of North America and Europe are found widespread formations similar to those described in the precedingparagraphs. The soil throughout is not like that of the underlying rock; it must have been transported. Careful examination of all the surface formations has led geolo- gists to believe that at a former period in the earth"s history, perhaps not more than a few thousand years ago, the northern part both of North America and Europe was covered with a thick layer of ice, which after several advances and retreats finally disappeared, leaving the country as we nowA HANGING VALLEY.
find it.
Although the border to which the ice extended and many of the changes which the ice made in the surface of the country have been carefully studied and mapped, yet the cause of this extension of the ice and the exact time at which it occurred have not yet been determined. Many theories have been brought forward to account for it, but none of them explains all the facts.
That the ice was here seems to be sure, but exactly when or why is unknown. This period when the ice was of great extent is called the Glacial Period. Probably during the earth"s history there have been several of these periods, but to the last is due the great changes wrought upon the present surface of the country and upon its plants and animals.
A HUGE PERCHED BOWLDER.
171.Glacial Lakes. -In northern countries are found ponds and lakes fill- ing the irregular depressions in the de- posit left by the retreating ice. Lakes of another kind are also sometimes formed in glaciated regions. The advancing or retreating ice may happen to make a barrier to the escape of the drainage, and thus may form a lake with an ice dam at one end. The lake will continue to ex- ist only as long as the ice obstructs the drainage.
The M?rjelen Lake in Switzerland is a
well-known example of this. The Aletsch glacier, the greatest of all the Swiss glaciers, obstructs a lateral valley, forming an ice wall about 150 feet in height, behind which the drainage of the side valley accumulates and forms a lake. Pieces of ice from the glacier fall off into it, forming icebergs which float upon its surface.
Sometimes a crevasse opens in the ice wall, and then the lake quickly drains and floods the valley at the end of the glacier. This formerly caused so much damage that a canal has been constructed across the head of the valley, so that now no great quantity of water can accumulate behind the ice dam. When the lake drains, the bottom is left as a comparatively level,dry plain until the crevasse closes and the lake again forms.
Toward the close of the Glacial Period a vast lake of this kind was formed in the northern part of the United States, the region now drainedby the Red River of the North.
MARJELEN LAKE.
