« PreviousContinue »
about thirty-nine degrees above zero, with great force—from beneath the Fahrenheit, it is at its greatest density— glacier. occupies the least space. The higher it is When the water rushes from beneath heated the more it expands; and a piece a glacier it encounters the detritus of ice of ice is much larger at a temperature that has fallen from its front face. The of 60 degrees below zero than is a piece pressure forces the water forward and of the same weight at a temperature of upward between the face of the glacier thirty degrees above zero.
and the detached pieces. Thus is the The bases of all active glaciers have lower front of the glacier altered by a temperature of about thirty-four de- three processes: by the attrition of the grees-or two degrees above the freez- outrushing water; by melting which is ing point. This is why there is always caused by the higher temperature created an underflowing stream. With a tem- by the water, and by the absorption of perature of thirty-four degrees above water which raises the temperature, and zero at the base and sixty degrees below thus contracts the mass—reduces the zero on top, there would be a difference size—of all the ice penetrated by this in temperature of ninety-four degrees- extraneous moisture. So these three sufficient to cause a top expansion of processes augment the force of expanfourteen per cent, or 280 feet in a sion in the work of causing the front glacier 2,000 feet high; 420 feet in a face of a glacier to project forward. glacier 3,000 feet high ; 560 in one 4,000 When a large section of ice falls forfeet high, and 700 feet in a glacier 5,000 ward, as already described, it is quickly feet high. It is evident that a face of submerged—or partly submerged—by ice will not stand at this angle of pro- the outrushing water. The portion jection; so a large, prismatic, or rhom- under water rises in temperature while boidal, section breaks off and falls for the portion in the air remains much ward.
colder. This difference in temperature The underflowing stream is confined causes a secondary breaking and falling by the lateral moraines on the sides, by forward of pieces that constituted the the glacier above and the earth beneath original, large detached section. These it. When the rear end of the glacier is secondary fragments freeze together at, higher than the front this stream works and immediately below, the water line. under hydraulic pressure. But, at all So when a piece drops forward it draws times, the action of hydrostatic pressure or drags the piece immediately behind is persistently driving the water forward it until the frozen bond at the water line
is severed. This constant ORC
dropping forward, 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 WHERE THE PREHISTORIC GLACIERS APPARENTLY STOPPED.
hot stove or sheet of hot Scarboro Cliffs, near Toronto, Canada.
metal. The center of heat is the highest sun also assists in toppling the glacier in point on the metal; but free the ice and that direction. it will start and travel to that high point. Let the diagram represent the front Why? (a) The ice will melt much more of a glacier. A B C will represent the rapidly on the side next the center of face. A B indicates the angle of expanheat, and thus cause the ice to topple sion-about fourteen per cent from a in that direction. (b) There is a current vertical plane. B C represents the angle of air—a regular upward draft—from caused by attrition, melting and absorpthe center of heat. The steam generated tion. Now a fracture occurs along the under the ice "kicks” backward beneath vertical line C D, and the section AB the mass, rebounds against the colder CD drops forward. After a time CD air, rises along the rear side of the ice is altered to D E F. Then comes a and immediately presses forward toward break along the line F G, and D E F G the center of heat. This pressing for- drops forward. So the succeeding secward of the escaped steam assists in tions are modified, broken loose and fall propelling the piece of ice forward. forward.
This first cause—more rapid melting What are the relations of glaciers to on the sunny side—has a tendency to ice-bergs in a glaciated area? topple and propel glaciers to that side. Sometimes glaciers are in series. The
Another cause of motion is capillary first series moves toward the equator attraction. The melting on the equa- in the manner already described. Some, torial side frees much moisture. Capil- or all, of the causes herein mentioned lary attraction carries this moisture into reduce these glaciers to stationary icethe body of the glacier. This raises the fields. A second series, hundreds or temperature of the ice on that side, and thousands of years later, follow over the thus contracts all that portion of it same lines. Finally the secondaries plow through which the moisture percolates. into the ice-fields—the remnants of the This contraction on the side next the first glaciers. They cannot climb over the ice because ice is not plastic enough souri and Mississippi rivers, ran under to form moraines. So the immense the south edge of this ice-field. masses constituting these second glaciers There were numerous ice-streams on are added to the remnants of the first this ice-field. ones. Now the ice-field is an immense One very important stream seems to barrier or dam behind which water ac- have headed in the great inland sea back cumulates and forms a large lake or of the barrier-probably near the locainland sea. A third, fourth, or still later, tion of Indianola, Iowa. This stream series starts for the equator. When flowed southerly to a point near Kansas these reach the margin of the lake their City. sections float in the water and become Along this latter stream were numernewly-born ice-bergs. In the last glacial ous holes through the ice sheet into epoch, or Ice Age, such a barrier seems which water poured. The water, falling to have extended from somewhere in the from such great heights, bored deep vicinity of Sioux Falls, South Dakota, holes in the earth below. One hole at into south central Iowa; thence east Chillicothe, Missouri, is more than 1,100 northeasterly to a point a few miles south feet deep. It reaches from the middle of Chicago, and thence to the Catskill of carboniferous rocks, on the surface, Mountains in New York.
down through the lower carboniferous, North of this barrier floated thousands the Devonian and probably reaches the of ice-bergs. Many of them ran aground. Niagara limestone-a member of the When the water subsided the final rest- upper Silurian. This hole is, of course, ing place of each ice-berg became a now filled with glacial materials. Deep lake. It is a well-known fact that there borings around the town prove that the are hundreds of such lakes in Minnesota, strata are elsewhere undisturbed. Wisconsin and northern Iowa. These Such holes are not uncommon in what lakes have moraines around their mar- were once glaciated fields. gins except on the side where the ice A stream of water falling through a berg plowed in—usually on the north- hole in a moving glacier cuts a slotted west side.
or elongated hole, or pit, that may be Extending southward from this bar scores of miles long. But if the icerier was, for thousands of years, an ice field is stationary the water will fall in field which was formed from the rem one spot and make a very deep hole in nants of older glaciers—i. e., older than the earth. I saw a stream that flowed those that formed the ice barrier.
about 40,000 gallons per minute falling The Missouri River, from the mouth through a hole in the Valdez glacier in of the Platte to the junction of the Mis- Alaska, where the ice is about 4,000 feet
thick. This waterfall would generate There has been much discussion on á out 41,000 horse-power! And this the subject of loess. Some authors tell ergy was expended on a few square us that loess deposits were formed by yards at the base of the glacier ! Would the agency of wind—as are sand dunes. it not bore a hole at a terrific rate? But there are several objections to the
How is loess or bluff formation wind theory. One is that shells, pieces formed and deposited ?
of wood, large fragments of rock and Probably you have seen the thick de- other large substances are found in posits of loess along the bluffs of the them. These could not have been deupper Mississippi, the entire length of posited by the wind. Another objection the Ohio and Missouri rivers, as well as is the manner in which the grains and in other parts of the United States north particles that compose loess are placedof the thirty-eighth parallel of north laid down. The particles that compose latitude. In Manchuria and other parts a wind deposit are arranged in vortices of north China this formation is said to each large group forms a vortex or be, in sonte localities, 2,000 feet thick! spiral. Now the particles that compose
To the ordinary observer these de- loess are not so arranged; but their posits appear to be of sandy clay of a large ends are usually all pointing in yellowish or brownish yellow color. But one direction. This condition indicates they are not of clay. A face or escarp- the agency of water. But the objections ment of loess will stand for generations to the water theory have been (a) that at an angle of from five to eight degrees they show no regular lines of deposition, from a vertical plane, while clay will and (b) deposits in one place vary much weather down to an angle of about sixty in elevation from other deposits in the degrees from that plane. Another pecu- same vicinity. It is evident that loess liarity of loess is its manner of weather- deposits were not laid down in water ing. Its exposed faces weather into exactly as were ordinary flood plains. semi-cylindrical buttresses that simulate Loess forms at the mouths of streams. the pipes of an immense pipe organ. that flow across large glaciers or iceThese deposits in the United States vary fields. Take the Valdez glacier in from a few feet to 300 or 400 feet in Alaska. There are several streams on thickness. It is well known that these top, and rushing down the sides of it. deposits were laid down at about the As these streams pour off the sides of close of the last Ice Age or Glacial the glacier they cut deep canyons in the Epoch. But what are their relations to ice. At the mouth of each long stream glaciers ?
is a deposit of loess. Sometimes the
mouths of these canyons and streams The Valdez glacier—near Cook's Invary several hundred feet in elevation. let, Alaska-is probably the largest This explains why loess is found at so glacier in the world. It is seventeen many different elevations in the same miles wide, one mile high and of unvicinity. But in order to grind the ma- known length. terials—the earthy and organic matter- The writer spent seventeen days at in an ice stream to a sufficient fineness one time, and four days at another, on to form loess the stream must be long. this glacier. Short streams that flow down steep ice A study of the Valdez, Muir, Taku and canyons form deposits of gravel, sand other glaciers affords data for the foland bowlders—such as are found lowing conclusions, with reference to throughout the United States north of formation, movement, etc.: the thirty-eighth parallel. The irregular 1. A body of ice must be very thick distribution of bowlders and other glacial before it can become a glacier. It must deposits is caused by the irregularity of be so thick, or high, that the pressure at the occurrence of ice streams and crev the base of it will generate enough heat asses that extended through the ice to melt ice. Probably not less than 1,500 sheets to the earth below them.
feet in height would be necessary to But why are bowlders sometimes more create the required pressure. abundant along certain lines ? Let us 2. When a glacier, from melting or imagine an immense ice sheet scores or other causes, becomes too thin to genhundreds of miles in extent. It is not erate sufficient heat at its base to melt in motion; but it has an occasional ice it ceases to move, and becomes a stacrevasse that reaches to the earth below. tionary ice-field. It is melting on top—i. e., in the summer 3. A gravity glacier may be an excepseasons. Many streams flow across it tion to the two foregoing conclusions. and carry bowlders, gravel, sand and 4. A stream of considerable length on other detritus into these crevasses. When a glacier or ice-field, deposits earthy these materials fall to the earth the matter at its mouth. If the earthy mawater beneath the ice carries all of the terials gathered by the stream are of precipitated matter away—all except the suitable composition, the matter deheavy bowlders. If the ice above never posited at the mouth of the stream will moves enough to disturb them, but be loess. finally melts away, the bowlders will lie 5 . The elevation of a bed of loess coron the surface in such lines, or zones, as responds to the elevation of the mouth will describe the location of the crevasse of the stream or canyon around which it that once existed above them.