Sediments of late Miocene age along both sides of the southern half of the Cascade Range in Washington contain distinctive hornblende andesite detritus. The lithologic character and fossil flora of these sediments suggest that the Cascade Range did not have present height in late Miocene time, but that a highland probably existed which shed debris derived from the Keechelus andesitic series. Renewed volcanic- activity in this highland provided the hornblende andesite detritus which accumulated locally on both sides of the present range in the form of mudflow and fluvial deposits.

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