506 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



In the lower part of the Chilliwack river valley, from the confluence of 

 Slesse creek to the head of the rocky defile where the river emerges from the 

 mountains, a thick deposit of Glacial clay forms a high bench on the north 

 side of the valley. The cliffed front of the bench opposite Tamihy creek is some 

 300 feet above the river. The surface of the bench rises gradually northward 

 another 350 feet to the rocky slopes of the mountain from 1-5 to 2 miles from 

 the river. 



The clay is generally massive and without evident stratification. Striated 

 boulders of many different rock-species are fairly common in the mass. The 

 writer has concluded that some of the deposit is true boulder-olay. Most of 

 the material probably came down the Chilliwack valley, but some of it may 

 have been carried over the ridge to the north by the huge glacier which moved 

 southwestwardly down the Eraser trough. Some of the more homogeneous 

 clay may have been laid down during a temporary Glacial damming of the lower 

 Chilliwack valley. 



In post-Glacial time the river has cut its gorge through the clay and the 

 debris has gone to form a part of the low grade alluvial fan spread out over 

 the Fraser flat from Sumas lake to Chilliwack village. The radius of this fan 

 averages some seven miles, the apex being about seventy-five feet above the 

 Fraser at average flood-level. 



During the maximum glaciation the Chilliwack and Fraser ice-sheets were 

 confluent at such elevations that they may be regarded as forming part of the 

 Pacific piedmont glacier. On the tops of the ridges south of Cultus lake, 

 erratic boulders of what appeared to be Chilliwack lake granodiorite were found 

 at elevations up to 4,700 feet. The long ridge between Cultus and Sumas lakes 

 (3,000 feet high), and also Sumas mountain were completely submerged by 

 the Piedmont sheet. Over the Fraser flats the latter must have been at least 

 3,000 feet thick and it may have been, at one time, over 4,000 feet thick. The 

 thickness doubtless decreased considerably toward the sea where the main 

 piedmont sheet was moving southward down the Gulf of Georgia, to join the 

 Puget Sound piedmont at the Strait of Juan de Fuca. The Puget Sound 

 sheet, according to Willis, had a maximum general thickness of about 2,500 

 feet* 



There remains to be noted a late-Glacial deposit of large size which is 

 now to be seen, in plateau-remnants, between Westminster and Point Roberts. 

 It consists of a great sheet of gravels and sand which, apparently, was washed 

 out from the Fraser valley and distributed over the floor of the Gulf of 

 Georgia. The general coarseness of the material betokens torrential currents, 

 suggesting that the streams issued from the Fraser glacier as its front long 

 atood near the present head of the river delta. The occurrence of steeply 

 dipping, typical beach-gravels within the mass now exposed well above sea- 

 level, shows that the land then stood lower than now (Plate 61, Fig. B). 

 The deposit seems, thus, to be a coarse-grained delta, built during later 

 Pleistocene time by the waters rushing out of the master ice-sheet which occu- 



• B Willis, Tacoma Folio, U.S. Geol. Survey, 1899. 



