CHAPTER 11
ICE AND WIND SCULPTURES
165.Snow in Winter. -When the temperature of the air falls be-low the freezing point， its moisture congeals into little flake-like crys-tals and falls as snow. Where the cold is continuous for a considerable time， the snow may accumulate in deep layers over the ground. If theSNOW CRYSTALS.
heat of the summer is not sufficient
to melt all the snow which falls in the winter， then the layers of snow will increase from year to year.
To have this occur the temperature for the whole year need not be below the freezing point， but the heat of the summer must not be sufficient to melt the snow which fell in the colder season. Lofty mountains， even in the tropics， have their upper parts snow-covered. In the far north and the far south the line of perpetual snow falls to sea level， inclosing the mighty expanse of the Arctic and the Antarctic snow fields.
166.Glaciers. -Wherever there is not enough heat in the warmseason to melt the snow which accumulates during the cold season， a thick covering of snow and ice will in time be formed. The ice is due to the pressure exerted on the lower layers by the weight of the snow above and to the freezing of the percolating water which comes from the summer melting of the upper snow layers.
Although ice in small pieces is brittle， in great masses it acts somewhat like a thick and viscid liquid. It conforms itself to the surface upon which it lies， and under the pull of gravity or pressurefrom an accumulating mass behind， slowly moves forward， resembling in some ways thick tar creeping down an incline or spreading out when heaped into a pile. The exact manner of glacial movement， however， is not fully understood.
In mountain regions where the snow holds over throughSNOW FIELD AT THE HEAD OF A GLACIER.
the summer， the wind-drifts and the snow-slides carry great quantities of snow into the upper valleys， until ever accumulating masses of snow and ice， hundreds of feet thick， are formed. The ice then slowly flows down a valley till a point is reached where the melting at the end is equal to the forward movement. An ice stream of this kind is called a valley glacier or an Alpine glacier， because first studied in the Alps.
THE GORNER GLACIER.
A typical Alpine Glacier.
Although the moving ice conforms to the bed over which it passes， it does not yield itself to the irregularities as easily as does water. When it passes through a narrows or over a steep and rough descent，CREVASSES IN A GLACIER.
The danger points in travel over glaciers.
it is broken into long， deep cracks called crevasses. These make travel along glaciers sometimes very dangerous. The travelers are usually tied together with ropes， so that if one of the party slips into a crevasse， the others will be able to hold him up and pull him out.
A glacier， like a river， is found to flow fastest near the middle and on top， and slowest at the bottom and on the sides. The rate of motion in the Alpine glaciers varies generally somewhere between 50 feet and one third of a mile in a year， being greatest in the summer and least in the winter.
THE COE GLACIER， MOUNT HOOD.
Alpine glaciers are found not only， as the name would indicate， in the Alps， but also in Norway， in the Himalayas， among the higher mountains in the western United States， on Mt. Shasta and in fact wherever the snow accumulates in the mountain valleys year after year.
As glaciers creep down the valleys， dirt and rocks fall upon their edges from the upper valley sides and are borne along upon the ice. If two glaciers unite to form a larger one， the débris upon the two sides which come togetherforms a layer of dirt and rocks along the middle of the larger glacier. At the end of the glacier this material which it has borne along is deposited in irregular piles of rocks and dirt.
The accumulations of débris along the sides are called lateral moraines， those in the middle， medial moraines， and those at the end， terminal moraines. Great boulders may be carried along on the ice for long distances without the edges being worn， since they are carried bodily and not rolledas in streams.
THE DANA GLACIER IN THE HIGH SIERRAS.
On the under surface of the glacier， rocks are dragged along firmly frozen into the ice. The weight of the glacier above presses them with tremendous force upon the surface over which the glacier passes. Inthis way scratches or grooves are made in the bed rock underlying the glacier， as well as upon the boulders themselves. Scratches of this kind are called glacial scratches or striations. They are found abundantly in places that have been glaciated. The rubbing of the rocks upon each other wears them away and grinds them into fine powder called glacial flour， which gives a milky color to the streams flowing from glaciers.
If a glacier extends over a region
THE FIESCH GLACIER.
Notice the medial moraine
where the surface has been weathered
into soil， this fine material may be shoved along under the ice for great distances. When a glacier melts， all the material which it has moved along under it， as well as that which it has carried on its surface or frozen into it， is deposited， forming what is called ground moraine. This is the formation which constitutes the soil of many of our northern states.
The melting of glacial ice， whether by the sun"s heat on top， by friction on the bottom or from whatever cause， produces streams which flow in the ice-cut channels under the glacier and emerge in front， laden with rock， glacial flour， and silt. Where the amount of material these streams carry is great， it is usually deposited in an alluvial plain near the end of the glacier.
The length of a glacier does not always remain the same， but increases and decreases slowly in conformity with the amount of snow which falls in successive years. Like rivers， only more slowly， they are subject to the changing conditions of atmospheric precipitation.
A TERMINAL MORAINE.
A BOULDER BORNE ALONG ON TOP OF A GLACIER.
Notice its size as compared to the umbrella.
A STONE SCRATCHED BY A GLACIER.
