milTH, PHYSIOGRAPHY OF SKYKOJillSH BASIN 213 



the divide above mentioned are carved from the granodiorite batholith, 

 with the ]30ssible exception of the ver}^ highest of all, which was imex- 

 plored by the writer. 



There are apparently stronger grounds for stating a relation between 

 drainage and rock type. The long, nearly straight, north and south valley, 

 occupied in turn by Beckler Eiver and by the Foss, is developed in Eocene 

 sedimentaries and was very probably determined by the lesser resistance 

 to corrosion offered by these terranes. The trend of the valley corre- 

 sponds with the strike of the rock strata. It will be seen at a glance that 

 a stream like Miller Eiver, which is quite analogous in nearness to base 

 level and in size of drainage basin and hence in volume, has succeeded 

 far less strikingly in cutting its valley. The position of the master 

 streams has no known relation to rock type. 



Summary 



The Cascade peneplain was up-arched in Pliocene time with a north- 

 south axial trend. Orogenic processes resulted in unsymmetrical surface 

 warping with an axial trend oblique to that of the main Cascade uplift. 

 In the pre-glacial stage this was maturel}'' dissected and in the Pleisto- 

 cene, due to changed climatic conditions, was burdened with alpine gla- 

 ciers. These produced a mature topography characterized by splendid 

 examples of peak, cirque and valley glacial corrasion, leaving the surface 

 excessively rugged. The post-glacial stage has been of short duration. 



The local climate favors rapid destruction of land forms. Precipita- 

 tion, largely in the form of snow, is unusually high, due to the rising of 

 warm, moisture laden southwest winds in passing the high Cascades. 

 This is accompanied by a high run-off, with the result that streams of 

 mapable size and continuous flow are very numerous. The drainage pat- 

 tern is dendritic and the land surface is maturely dissected by both 

 streams and sflaciers. 



