. Bulletin of the Geological Society of America. Geology. ERODED RIVER BEDS. 65 surfaces in the valleys or ravines were very little above those now exposed. It was the high exposures, peaks and cliffs, which furnished the moraines to be fouud at lower altitudes. Thus the relation between the ancient and modern stream beds is that rep- resented in the following Figure &—Erosion by modern rivers. Erosion influenced by Ice Cap.—The climatic conditions affecting erosion during the Pliocene were very different from those which influenced it during the period of glaciation, and for thi
. Bulletin of the Geological Society of America. Geology. ERODED RIVER BEDS. 65 surfaces in the valleys or ravines were very little above those now exposed. It was the high exposures, peaks and cliffs, which furnished the moraines to be fouud at lower altitudes. Thus the relation between the ancient and modern stream beds is that rep- resented in the following Figure &—Erosion by modern rivers. Erosion influenced by Ice Cap.—The climatic conditions affecting erosion during the Pliocene were very different from those which influenced it during the period of glaciation, and for this difference an allowance must be made. During the Pliocene there were no glaciers and probably little snow, if any. The rain must therefore have flowed down the mountain sides very rapidly, eroding them as it went. During the glacial period, on the other hand, the flats and valleys from 5,000 feet or less to the summits of the passes were deeply buried in ice and snow. The upper half of the range was thus pro- tected from direct rain erosion, though subjected to the action of ice. Some few geologists still believe that glaciers not only sweep their beds clear of loose rocks and soil and polish the solid underlying mass, but that they vigorously erode solid rock. In my opinion this theory is maintained in opposition to overwhelming evidence. Reference has already been made to some of the many facts indicating a trifling amount of erosion since a preglacial date in the higher part of the Sierra, and long before my exam- inations Professor Whitney reached the conclusion that the solid rock had been scoured rather than eroded by glaciers. It is more satisfactory, how- ever, to observe what erosion modern glaciers actually effect than to infer what ancient ones accomplished. On this question Professor Albert Heim has no equal as an authority. A few of his conclusions, based upon his own fully described observations and confirmed by those of others, are as follows: "Advanci
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