Page:The Journal of geology (IA journalofgeology11893univ).pdf/281

Returning to the region of the superficial bowlder belts in Illinois, Indiana, and Ohio, we find hillocks of the kame type distributed throughout the same tracts as the bowlders, indeed, practically lying beneath the bowlder belts themselves. Some of these I described nine years ago in the American Journal of Science in an article entitled "Hillocks of Angular Gravel and Disturbed Stratification."^[1] Additional evidence of the same import has been since gathered. Among the materials of these kame-like aggregates, it is not uncommon to find a complete series of gradational forms, ranging from incorporated masses or tongues of typical till of the ground-moraine type, through partially modified masses and layers of half-till, half-gravel, to completely assorted and stratified material, thus showing every stage of the derivation from the common underlying and surrounding till. The attrition of the material shows a like gradation. In some portions the clayey constituents of the till have been simply washed out leaving the rock fragments which show almost no perceptible wear from water. In others, the rounding has been more considerable, and in still others, there has been a reduction to the common rounded gravelly type. Even this is not usually well rounded. The less modified fragments not uncommonly show glacial striation. All these variations occur within the limits of a single hillock, and are often so intimately associated as to compel the conviction that the gravel is but a partially assorted derivative from the till of the region. In some of these hills the stratification is disturbed, not as though the beds had been let down by the removal of ice below, but as though they had been pushed horizontally by glacial pressure. The essentially local derivation of the material is demonstrated by the very notable presence of rock fragments derived from the formations of the neighborhood. More than half the material is not infrequently made up of limestone whose origin must be much nearer at hand than that of the superficial bowlder belt. An analysis representative of the gravel and sand of one of these kames gave as much as 70 per cent. magnesian limestone.