The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . •r. ^.-.:.<-f, ;f!Jl»o^--»lJ GLACIAL EROSION AND TRANSPORTATIOy. 227 two elements enter into the problem : (1) the relative rate ofaction of the two forces, and (2) the relative length of timeduring which they have been in operation. As to time, it is evident that those have a great advantagein the argument who exalt the eroding power of runningwater.
The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . •r. ^.-.:.<-f, ;f!Jl»o^--»lJ GLACIAL EROSION AND TRANSPORTATIOy. 227 two elements enter into the problem : (1) the relative rate ofaction of the two forces, and (2) the relative length of timeduring which they have been in operation. As to time, it is evident that those have a great advantagein the argument who exalt the eroding power of runningwater. However slowly the drops may wear away the stone,ample amends are made in the length of the periods throughM-hich the action has continued. From the earliest ages ofgeological history running water has been at work counter-acting the effect of the forces which have elevated the con-tinents. River - channels are, in fact, more constant thanmountain-chains. Everywhere and at all times the accumu-. Fk;. 71.—Embossed floor of an ancient glacier in the valley of the upper Arkansas River. (Hayden.) latins: waters on a continental area seek and find the lowestpaths to the sea. The sand and gravel which these runningstreams push along over their beds act as the teeth of a sawupon the rising mountain-summits, so that everywhere inmountain-regions of great age we find deep, transverse val- 228 THE ICE AGE IN NORTH AMEUICA. leys of erosion. .Vinong the best-known examples in thecountry are those of the Mohawk and Hudson in New York ;of the Delaware and Susquehanna in Pennsylvania; theOhio and its tributaries on the western flanks of the Alle-ghanies; the Mississippi and all its western branches, togetherwith the Colorado upon tlie eastern flanks of the KockyMountains; and the Columbia, the Fraser, and tlie StickeenRivere, which penetrate in chasms of great depth the rock-bound shore of the Pacific. At the Delaware Water-Gap the river has sawn a verticalchasm more than a thousand fe
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