. Elements of geology : a text-book for colleges and for the general reader. Geology. 58 AQUEOUS AGENCIES. ceivable size, from huge bowlders down to fine earth, mixed together into an heterogeneous mass entirely different from the neatly-sorted de- posits from water. It is, therefore, entirely unsorted and nnstratified, and without organic remains. 2. The mass consists of two parts, viz., that which was carried on the top of the glacier, and that which was forced out beneath (ground moraine). The first consists of loose ma- terial containing angular, unworn fragments; the other of fine compact
. Elements of geology : a text-book for colleges and for the general reader. Geology. 58 AQUEOUS AGENCIES. ceivable size, from huge bowlders down to fine earth, mixed together into an heterogeneous mass entirely different from the neatly-sorted de- posits from water. It is, therefore, entirely unsorted and nnstratified, and without organic remains. 2. The mass consists of two parts, viz., that which was carried on the top of the glacier, and that which was forced out beneath (ground moraine). The first consists of loose ma- terial containing angular, unworn fragments; the other of fine compact material containing fragments worn and polished, and scratched with straight, parallel scratches, but in both cases entirely different from water-worn pebbles. In all respects, therefore, the action of glaciers is characteristic and can not be confounded with that of water. Evidences of Former Extension of Glaciers.—It is by evidence of this kind that the former great extension of glaciers in regions where they now exist, and the former existence of glaciers in regions where they no longer exist, have been proved. We have already stated that during a succession of cool, damp seasons, a glacier may extend far beyond its pre- vious limits. Similar changes take place also in the depth of a glacier. In a word, glaciers are subject to floods like rivers; only these floods, instead of being an- nual, are secular. Now, as rivers after floods leave floating material stranded on the banks, showing the height of the flood- water, so, in the decrease of a glacier, lines of bowlders are left stranded, often delicately balanced, on ledges high up the sides of the valley. These lines of bowl- ders mark the former height of the glacier. Some of these lines have been found in the Alps 2,000 feet above the present level. Fig. 47 is a cross-section of a glacial valley. The dotted lines show the former level. In the same valleys we find old terminal moraines (Fig. 48, a) miles beyond the prese
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
