. Report of the geological exploration of the fortieth parallel . OF TEE :;Til IK 1 (■ !■: AC. K .. ^^twwm QUATERNARY. 487 would have been produced by the erosion or deepening of the canonalone. A glacial or U canon carries bowlder-accumulations throughout muchof its length in the form of terminal moraines. The bottom rock, whennot covered with soil or bowlders, shows glacial scorings as far down thecanon course as the U shape extends. There is no modification since the last melting out of the glaciers. The U part opens downward into the V part of the gorges. The Great Siena
. Report of the geological exploration of the fortieth parallel . OF TEE :;Til IK 1 (■ !■: AC. K .. ^^twwm QUATERNARY. 487 would have been produced by the erosion or deepening of the canonalone. A glacial or U canon carries bowlder-accumulations throughout muchof its length in the form of terminal moraines. The bottom rock, whennot covered with soil or bowlders, shows glacial scorings as far down thecanon course as the U shape extends. There is no modification since the last melting out of the glaciers. The U part opens downward into the V part of the gorges. The Great Siena or Uinta canons may have the U form for forty milesfrom the summit and then suddenly give way to the V form. It is evident either that there was an original V formed canon wliichthe subsequent glacier occupied and modified as far as it descended, or elsethat the whole canon was simultaneously cut, the U by glaciers and theV by floods below the point where the ice gave out. Since the caiSons are post-Pliocene, i. e., wholly within the Quaternary,the question naturally suggests itself. If the deep U gorges, often
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