THE SPECIAL ACTION OF ICE.
Ice as an Eroding and Transporting Agent.—§ 29. In the preceding discussion we have for the sake of simplicity omitted the part which ice in the shape of glaciers has played in the formation of clays. At the present time, especially in temperate regions, this agent cuts so small a figure that it might almost be left out of consideration entirely, but there have been times when this was not true, and during one of these periods a large portion of our own country was covered with thick de posits of clay through its action.
During the geological period immediately preceding the one in which we live, immense fields of ice accumulated over Labrador and over the Keewatin country which lies just south and west of Hudson's Bay. These ice fields increased in thickness until it is believed they reached 10,000 feet or more. This great thickness induced large pressures which caused the lower layers to behave as if they were plastic and move out from under the mass, increasing the area of the fields until they united and covered all that portion of the United States which lies roughly north of the 37th parallel and east of the 100th meridian.
For a very long period preceding the action of the glacier this area had been exposed to the action of ground-water and had accumulated vast beds of clay and other residual material produced by the decom position of the underlying rocks. As the ice sheet moved away from the centeis in which it accumulated and passed over these areas cohered with earthy accumulations, it gradually picked up the loose material over which it was passing and as the motion of the ice particles was one of flowage in consequence of which they moved upward and downward as well as on onward, the debris came to be distributed throughout the entire mass.
Chamberlin's articles on the glaciers of .Greenland, published some years ago in the Journal of Geology, contain excellent illustrations show ing the distribution of rock waste through the entire thickness of the ice sheet, and give a clear conception of the vast amount of such mate rial which was carried by these glaciers.
In an earlier section (§ 28) it is shown that rock decomposition pro ceeds unequally, and that in consequence fragments of the original rock, less readily decomposed than the rest, are imbedded in the earthy mass. These fragments vary from minute pebbles to boulders the size of a 6-room house, so that the debris carried by the glacier contains not only clay and other results of decomposition but quantities of rock, sand, gravel, and boulders as well. In addition to this each rock fragment which is dragged along by the lower layer of the glacier becomes a graver's tool which cuts away the bed over which the ice moves, wearing away both itself and the surface over which it is dragged. Hundreds of these glacial tools with their smoothed and striated surfaces may be found in any bank of glacial gravel, and occasionally large boulders carry facets which show that they have been used for this purpose. Two boulders lying near this university are excellent illustrations. One is of granite and weighs 10. or 12 tons, and the other of limestone weighing probably not less than 20 tons. Both carry several facets beautifully striated. When, as in the digging of the Chicago Drainage Canal, the glacial debris is cleared from large areas of underlying rock, these sur faces are found to be smoothed, striated and grooved by the glacial mill.
Through the agency of this grinding action great quantities of rock flour or ground but undecomposed rock debris (silt) are added to the load carried by the glacier, and the finer particles are distributed among and deposited with the clay and other products of decomposition, pro foundly affecting their properties. It should be noted that each kind of rock over which the glacier passes adds its quota to the grist of rock flour which the ice is carrying. There is no other known agency which brings together such heterogeneous masses of material as, glaciers.