Page:The Journal of geology (IA journalofgeology181910univ).pdf/496

 GLACIAL LAKES OF PUGET SOUND PRELIMINARY PAPER J. HARLEN BRETZ

On the western margin of the North American continent an uninterrupted fjord coast extends from Cross Sound in the Alaskan panhandle southward to Puget Sound in the state of Washington. The topographic expression of the whole extent of coast line is glacial, but the Strait of Juan de Fuca appears to separate this coast into (1) a region of predominant glacial erosion and (2) a region where glacial deposition much exceeded the erosion of the ice. Puget Sound is this latter region to a unit.

The complexly fingered arm of the sea known as Puget Sound lies in meridionally oriented troughs whose walls sometimes rise in sea cliffs 300 feet A.T. and whose maximum depths are approximately 1,000 feet below sea-level. To sea-level, at least, these fjord- like troughs are largely drift-walled. The larger stream valleys, lake valleys, and divides of the Puget Sound region are likewise fashioned in glacially transported material, and all share the roughly meridional orientation.

It is established that the ice of the last glaciation of Puget Sound was derived largely from snowfields to the north. The axial lines of the grooved topography parallel the striae, and the ridges and valleys are at least veneered and often deeply covered by the youngest till of the region. The Vashon ice which deposited that till sheet must have been related to the genesis of this peculiar and persistent topography in one of three ways: (1) the ice conformed closely to the ridges and grooves which were fashioned before its advent; (2) the last glaciation produced the present topography by erosion of an older and more nearly uniform drift surface; or (3) the Vashon