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about thirty-nine degrees above zero, Fahrenheit, it is at its greatest density— occupies the least space. The higher it is heated the more it expands; and a piece of ice is much larger at a temperature of 60 degrees below zero than is a piece of the same weight at a temperature of thirty degrees above zero.
The bases of all active glaciers have a temperature of about thirty-four degrees—or two degrees above the freezing point. This is why there is always an underflowing stream. With a temperature of thirty-four degrees above zero at the base and sixty degrees below zero on top, there would be a difference in temperature of ninety-four degrees— sufficient to cause a top expansion of fourteen per cent, or 280 feet in a glacier 2,000 feet high; 420 feet in a glacier 3,000 feet high; 560 in one 4.000 feet high, and 700 feet in a glacier 5,000 feet high. It is evident that a face of ice will not stand at this angle of projection; so a large, prismatic, or rhomboidal, section breaks off and falls forward.
The underflowing stream Is confined by the lateral moraines on the sides, by the glacier above and the earth beneath it. When the rear end of the glacier is higher than the front this stream works under hydraulic pressure. Rut, at all times, the action of hydrostatic pressure is persistently driving the water forward
with great force—from beneath the glacier.
When the water rushes from beneath a glacier it encounters the detritus of ice that has fallen from its front face. The pressure forces the water forward and upward between the face of the glacier and the detached pieces. Thus is the lower front of the glacier altered by three processes: by the attrition of the outrushing water; by melting which is caused by the higher temperature created by the water, and by the absorption of water which raises the temperature, and thus contracts the mass—reduces the size—of all the ice penetrated by this extraneous moisture. So these three processes augment the force of expansion in the work of causing the front face of a glacier to project forward.
When a large section of ice falls forward, as already described, it is quickly submerged—or partly submerged—by the outrushing water. The portion under water rises in temperature while the portion in the air remains much colder. This difference in temperature causes a secondary breaking and falling forward of pieces that constituted the original, large detached section. These secondary fragments freeze together at, and immediately below, the water line. So when a piece drops forward it draws or drags the piece immediately behind it until the frozen bond at the water line is severed. This constant dropping f o r w a r d, and dragging of pieces from behind, enables a glacier to move piecemeal up an incline. As soon as a glacier, that has thus reached a summit, starts down an incline again the immense pressure from behind soon unites the parts into a comparatively solid mass once more.
Of course the element of expansion is indirectly attributable to the sun—the source of all terrestrial heat. But a second cause of motion in glaciers is the direct force of the sun. Place a piece of ice on a hot stove or sheet of hot metal. The center of heat is the highest point on the metal; but free the ice and it wilLstart and travel to that high point. Why? (a) The ice will melt much more rapidly on the side next the center of heat, and thus cause the ice to topple in that direction, (b) There is a current of air—a regular upward draft—from the center of heat. The steam generated under the ice "kicks" backward beneath the mass, rebounds against the colder air, rises along the rear side of the ice and immediately presses forward toward the center of heat. This pressing forward of the escaped steam assists in propelling the piece of ice forward.
This first cause—more rapid melting on the sunny side—has a tendency to topple and propel glaciers to that side.,
Another cause of motion is capillary attraction. The melting on the equatorial side frees much moisture. Capillary attraction carries this moisture into the body of the glacier. This raises the temperature of the ice on that side, and thus contracts all that portion of it through which the moisture percolates. This contraction on the side next the
sun also assists in toppling the glacier in that direction.
Let the diagram represent the front of a glacier. ABC will represent the face. A 15 indicates the angle of expansion—about fourteen per cent from a vertical plane. B C represents the angle caused by attrition, melting and absorption. Now a fracture occurs along the vertical line C D, and the section A B C D drops forward. After a time C D is altered to D E F. Then comes a break along the line F G, and D E F G drops forward. So the succeeding sections are modified, broken loose and fall forward.
What are the relations of glaciers to ice-bergs in a glaciated area?
Sometimes glaciers are in series. The first series moves toward the equator in the manner already described. Some, or all, of the causes herein mentioned reduce these glaciers to stationary icefields. A second series, hundreds or thousands of years later, follow over the same lines. Finally the secondaries plow into the ice-fields—the remnants of the first glaciers. They cannot climb over the ice because ice is not plastic enough to form moraines. So the immense masses constituting these second glaciers are added to the remnants of the first ones. Now the ice-field is an immense barrier or dam behind which water accumulates and forms a large lake or inland sea. A third, fourth, or still later, series starts for the equator. When these reach the margin of the lake their sections float in the water and become newly-born ice-bergs. In the last glacial epoch, or Ice Age, such a barrier seems to have extended from somewhere in the vicinity of Sioux Falls, South Dakota, into south central Iowa; thence east northeasterly to a point a few miles south of Chicago, and thence to the Catskill Mountains in New York.
North of this barrier floated thousands of ice-bergs. Many of them ran aground. When the water subsided the final resting place of each ice-berg became a lake. It is a well-known fact that there are hundreds of such lakes in Minnesota, Wisconsin and northern Iowa. These lakes have moraines around their margins except on the side where the iceberg plowed in—usually on the northwest side.
Extending southward from this barrier was, for thousands of years, an icefield which was formed from the remnants of older glaciers—i. e., older than those that formed the ice barrier.
The Missouri River, from the mouth of the Platte to the junction of the Mis
souri and Mississippi rivers, ran under the south edge of this ice-field.
There were numerous ice-streams on this ice-field.
One very important stream seems to have headed in the great inland sea back of the barrier—probably near the location of Indianola, Iowa. This stream flowed southerly to a point near Kansas City.
Along this latter stream were numerous holes through the ice sheet into which water poured. The water, falling from such great heights, bored deep holes in the earth below. One hole at Chillicothe, Missouri, is more than 1.100 feet deep. It reaches from the middle of carboniferous rocks, on the surface, down through the lower carboniferous, the Devonian and probably reaches the Niagara limestone—a member of the upper Silurian. This hole is, of course, now filled with glacial materials. Deep borings around the town prove that the strata are elsewhere undisturbed.
Such holes are not uncommon in what were once glaciated fields.
A stream of water falling through a hole in a moving glacier cuts a slotted or elongated hole, or pit, that may be scores of miles long. But if the icefield is stationary the water will fall in one spot and make a very deep hole in the earth. I saw a stream that flowed about 40,000 gallons per minute falling through a hole in the Valdez glacier in Alaska, where the ice is about 4,000 feet thick. This waterfall would generate a out 41,000 horse-power! And this e-'crgy was expended on a few square yards at the base of the glacier! Would it not bore a hole at a terrific rate?
How is loess or bluff formation formed and deposited?
Probably you have seen the thick deposits of loess along the bluffs of the upper Mississippi, the entire length of the Ohio and Missouri rivers, as well as in other parts of the United States north of the thirty-eighth parallel of north latitude. In Manchuria and other parts of north China this formation is said to be, in some localities, 2,000 feet thick!
To the ordinary observer these deposits appear to be of sandy clay of a yellowish or brownish yellow color. But they are not of clay. A face or escarpment of loess will stand for generations at an angle of from five to eight degrees from a vertical plane, while clay will weather down to an angle of about sixty degrees from that plane. Another peculiarity of loess is its manner of weathering. Its exposed faces weather into semi-cylindrical buttresses that simulate the pipes of an immense pipe organ. These deposits in the United States vary from a few feet to 300 or 400 feet in thickness. It is well known that these deposits were laid clown at about the close of the last Ice Age or Glacial Epoch. But what are their relations to glaciers?
There has been much discussion on the subject of loess. Some authors tell us that loess deposits were formed by the agency of wind—as are sand dunes. But there are several objections to the wind theory. One is that shells, pieces of wood, large fragments of rock and other large substances are found in them. These could not have been deposited by the wind. Another objection is the manner in which the grains and particles that compose loess are placed— laid down. The particles that compose a wind deposit are arranged in vortices —each large group forms a vortex or spiral. Now the particles that compose loess are not so arranged; but their large ends are usually all pointing in one direction. This condition indicates the agency of water. But the objections to the water theory have been (a) that they show no regular lines of deposition, and (b) deposits in one place vary much in elevation from other deposits in the same vicinity. It is evident that loess deposits were not laid down in water exactly as were ordinary flood plains.
Loess forms at the mouths of streams that flow across- large glaciers or icefields. Take the Valdez glacier in Alaska. There are several streams on top, and rushing down the sides of it. As these streams pour off the sides of the glacier they cut deep canyons in the ice. At the mouth of each long stream is a deposit of loess. Sometimes the mouths of these canyons and streams vary several hundred feet in elevation. This explains why loess is found at so many different elevations in the same vicinity. But in order to grind the materials—the earthy and organic matter— in an ice stream to a sufficient fineness to form loess the stream must be long. Short streams that flow down steep ice canyons form deposits of gravel, sand and bowlders—such as are found throughout the United States north of the thirty-eighth parallel. The irregular distribution of bowlders and other glacial deposits is caused by the irregularity of the occurrence of ice streams and crevasses that extended through the ice sheets to the earth below them.
But why are bowlders sometimes more abundant along certain lines? Let us imagine an immense ice sheet scores or hundreds of miles in extent. It is not in motion; but it has an occasional crevasse that reaches to the earth below. It is melting on top—i. e., in the summer seasons. Many streams flow across it and carry bowlders, gravel, sand and other detritus into these crevasses. When these materials fall to the earth the water beneath the ice carries all of the precipitated matter away—all except the heavy bowlders. If the ice above never moves enough to disturb them, but finally melts away, the bowlders will lie on the surface in such lines, or zones, as will describe the location of the crevasse that once existed above them.
The Valdez glacier—near Cook's Inlet, Alaska—is probably the largest glacier in the world. It is seventeen miles wide, one mile high and of unknown length.
The writer spent seventeen days at one time, and four days at another, on this glacier.
A study of the Valdez, Muir, Taku and other glaciers affords data for the following conclusions, with reference to formation, movement, etc.:
1. A body of ice must be very thick before it can become a glacier. It must be so thick, or high, that the pressure at the base of it will generate enough heat to melt ice. Probably not less than 1,500 feet in height would be necessary to create the required pressure.
2. When a glacier, from melting or other causes, becomes too thin to generate sufficient heat at its base to melt ice it ceases to move, and becomes a stationary ice-field.
3. A gravity glacier may be an exception to the two foregoing conclusions.
4. A stream of considerable length on a glacier or ice-field, deposits earthy matter at its mouth. If the earthy materials gathered by the stream are of suitable composition, the matter deposited at the mouth of the stream will be loess.
5. The elevation of a bed of loess corresponds to the elevation of the mouth of the stream or canyon around which it was deposited.