As the ice sheet moved forward into warmer and warmer latitudes it was continually wasted by the heat of the sun, but in spite of this melting its front would continue to advance so long as the rate of motion exceeded the rate of melting, and as the amount of heat to which it was subjected varied from day to day, month to .month, and year to year, the ice front would continually change its position, now advancing now retreating, as the balance between rate of motion and of melting varied.
Ice as an Agent of 30. Over most of the area covered by the ice it was an agent of destruction and transportation, picking up any loose material with which it came in contact, and tearing away the surface over which it moved, but near its lower end the whole character of its work was changed. Here rapid melting caused it to deposit great quantities of unassorted material, and whether its front moved backward or forward in consequence of varying rapidity in melting, it was continually dropping its load of debris. Near its front the thick ness of the sheet became so reduced and the rate of melting so rapid that it no longer picked up the loose material which it found in its path, but overrode it and so as it moved backward and forward it added layer upon layer to the mass accumulating on its bed. If the excursions were short and repeated many times over the same area, the successive layers would build up one of those ridges, which we call terminal mor aines, because they mark positions in which the glacier was nearly stationary for a long period, but if the excursions were long, these layers would build up those broad level plains which we call ground moraines (prairies). It was in this way that nearly the entire surface of IIlinois, but especially its north-eastern portion, was covered with a thick coat of glacial debris aggregating from 5 to 10 feet in its thinnest portions' and 250 to 300 feet in its thickest. The broad level prairies of our state are in general its ground moraines which the retreat of the ice and the obstruction of drainage by the terminal moraines converted into broad lakes in which vegetation grew and, decomposing, imparted its black color to the accumulating lake sediments, while the relatively narrow, hummocky highland ridges are its terminal moraines.
Characteristics of Glacial Clays.—§ 31. Most, but not all, of this thick coating of glacial debris is made up of unassorted material. In it the clays are mixed in all proportions with undecomposed rock particles and with rock flour (silt) both derived from all kinds of rocks. This is our boulder clay, and is probably more variable in its composition and prop erties and more difficult to use than any other variety with which we have to deal. Occasionally we find a deposit of this material whose prop erties fit it admirably for the manufacture of a certain kind of ware and sufficiently uniform in composition and texture to furnish an abundant supply, but this usually is not true. The conditions under which they
are deposited usually make the boulder clays exceedingly variable, occur ring in pockets rather than in large deposits. Let us understand that the finding of heavy deposits of boulder clays suitable for use in the man ufacture of any grade of ware, even the best, is not to be considered as impossible, but rather as unusual.
The melting of the ice not only caused the deposition of great quan tities of debris but produced large amounts of water as well, and as this water moved away from the points where it originated, it picked up more or less of the boulder clay, and after carrying it for some distance as sorted and redeposited it. It is to this assorting action that we owe the pockets of sand and gravel which we find in glacial deposits, and it is to the same force that we owe many of the hills and ridges of sand or gravel which rise above them. It is to this action, too, that we owe the deposits, often small, but sometimes very large, of exceedingly uniform clay, free from pebbles or from any considerable irregularities in structure, compo sition or properties. We believe that these masses were laid down in lakes or ponds fed by streams which, before entering the pond, had so far lost their velocity that they had dropped all the gravel or sand which they were carrying and so brought only the fine clay-like materials which settled in, the quiet waters.
When the ice front retreated it left a generally level but more or less billowy surface behind it. This character is given to the surface partly by the unequal distribution of debris in the ice mass, and partly by the fact that large pieces of ice often break from the retreating cliff and are buried in debris before they melt. This cover causes the melting to go on very slowly and as it progresses the covering gradually settles and forms a depression which subsequently becomes a pond. Another factor which tended to produce these ponds or lakes was the formation of small ridges at points where the ice front became practically stationary for a short time during its retreat. These ridges ran across the natural depressions which ordinarily furnish surface drainage and imponded the water. These ponds were generally small and the amount of ma terial deposited in each was small also, but occasionally large and deep depressions were formed and in these, extensive masses of peculiarly uniform clays were laid down, so while heterogeneity is the most prom inept characteristic of the glacial clays, considerable deposits having just the opposite character are not very unusual.