File:Annual report of the United States Geological Survey to the Secretary of the Interior (1898) (14779541961).jpg

Identifier: annualreportofu2021geol_2 (find matches)
Title: Annual report of the United States Geological Survey to the Secretary of the Interior
Year: 1880 (1880s)
Authors: Geological Survey (U.S.)
Subjects: Geological Survey (U.S.) Geology Water-supply Forest reserves Mines and mineral resources
Publisher: Washington : G.P.O.
Contributing Library: University of Illinois Urbana-Champaign
Digitizing Sponsor: University of Illinois Urbana-Champaign

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e CascadePlateau. Rising above the general level of the Cascade Plateau there are twoclasses of peaks. First, volcanic mountains, of which Glacier Peakis the only known representative in the region considered in this paper;and, second, granitic mountains, such as the Wenache Mountains andthe lofty peaks about Lake Chelan. The volcanic mountains standon the Cascade Plateau and were formed after the period of base-level-ing referred to above, and need not claim further attention at thistime. Some of the granite peaks have an elevation of over 9,000 feet,and hence rise some 2,000 feet above the general level of the CascadePlateau. These are the mountains which, in my opinion, could nothave been in existence as topographic elevations at the time the maindrainage lines were established. Possibly the granitic mountains referred to are of the nature oftnonadnocks, r remnants left standing on the Cascade peneplain. Ifthis is true, the river courses which cross them may be explained as
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OF IHfc UNIVERSITY of ILUNOlb. RrssF.l.l..) (iK()I,<)(H(AL STKUCTUKK. 141 an inheritance from an curlier time of erosion which preceded thegeneral base-leveling. It may also be suggested in this same connection that th Cascadepeneplain was developed above the present general summit elevation ofthe large majority of peaks and ridges now remaining, and has beenlowered by erosion, leaving the more resistant rocks in the boldestrelief. Under this supposition the Cascade Plateau would now havea general surface level of about 10,000 feet, having been raisedfrom near sea level. In favor of this hypothesis it is to be noted thatthe peaks and ridges of the Cascade Mountains are nearly all sharp.No recognizable flat-topped remnants of the original plateau remainin the more elevated portion of the region under review. As soon asa region has been so deeply dissected by streams that the ridges arcsharp crested, any further erosion will tend to a general lowering oftheir summits, and for

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