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The Vashon drift is ordinarily an unsorted till, with 

 numerous undecomposed granitic boulders. In some 

 places the finer materials of the drift are rudely stratified. 

 Near the surface it is usually oxidized to dark brown, 

 and passes into a dark, sandy and gravelly loam, which 

 usually covers it. The drift seldom forms distinct and 

 characteristic topographic features such as moraines, 

 but is merely a mantle covering the crystalline rocks and 

 the inter-glacial deposits. Below elevations of 250 feet 

 (80 m.) except in restricted localities, the mantle is thin, 

 seldom more than 3 or 4 feet (-9 to 1-2 m.) thick. Fre- 

 quently it thins out so completely that over large areas it is 

 absent or is represented only by glacial boulders, which 

 are strewn over the surface of the inter-glacial deposits. 

 Above elevations of 250 feet, (80 m.) the larger part of 

 the entire drift mantle is the Vashon till, although it is 

 probably mixed with more or less of the Admiralty till. 



In the western part of the region is a deposit of sand 

 and gravel about 200 feet (60 m.) thick, which forms a 

 plain, the smooth Colwood plain, two to three miles (3 to 

 5 km.) wide and from 200 to 250 feet (60 to 80 m.) above 

 sea level. On it are well defined terraces up to 20 feet 

 (6 m.) high, and near its inner border are several kettle 

 holes, the largest of which are 100 to 800 feet (30 to 250 m.) 

 across and 10 to 80 feet (3 to 25 m,) deep. The deposit 

 consists chiefly of coarse sands and gravels, which are well 

 stratified and have a pronounced delta structure, the 

 larger part of the deposit consisting of fore-set beds with 

 dips of 15 to 25 degrees to the southeast. These are capped 

 with 10 to 15 feet (3 to 5 m.) of top-set beds of horizontally 

 stratified coarse gravels. 



Since the superficial deposits described above are of 

 glacial origin a discussion of their origin together with a 

 description of the glaciation of the region is necessary. 

 The lowland portion of the region was overridden by the 

 southward flowing, piedmont, Strait of Georgia glacier. 

 The results of the severe abrasion of the hard rocks by the 

 glacier are most striking [7]. The rocks are not only 

 smoothed, but are striated and grooved, the grooves even 

 in the crystalline rocks attaining a width of 3 to 5 feet 

 (•9 to 1-5 m.) and a depth of 1 to 5 feet (-3 to 1-5 m.). 

 The striations and grooves are not confined to the flat 

 surfaces, but occur also on the sloping and vertical ones, 

 in some instances the rocks being actually undercut. 

 35069—3^ 



