SMITH, PHYSIOGRAPHY OF SKYKOMISH BASIN 207 ' 



dence to identify the Skykomish Uplift as a separate warping of the 

 Cascade peneplain (Fig. 3). 



Willis has shown mathematically'^ that the cause of uplift of the Cas- 

 cade peneplain, of whatever nature it may be, was the result of deep-seated 

 strain. The strain was due to a disturbance of isostatic equilibrium. 

 This disturbance may have been due to the transfer of material eroded in 

 the formation of the j^eneplain, thus decreasing the load on the pene- 

 ]3lained block and increasing it on the adjacent block. Another possible 

 cause of the disturbance might be found in magmatic movements result- 

 ing in the vulcanism and batholithic intrusion of the Miocene. Both of 

 these may have acted. In the Skykomish Basin, Miocene igneous rocks 

 are affected by the planation and are included in the uplift. Therefore 

 the upward movement must he post-Miocene. On the other liand, the 

 movement was pre-Pleistocene because the latter is the period of glacial 

 occupancy. Thus by delimitation the age of the peneplain dates to the 

 Pliocene. The fact that the plateau is maturely dissected by valleys 

 reaching 5000 feet in depth is evidence that a considerable part of the 

 Pliocene must have elapsed since the orogenic process began. The begin- 

 ning of the uplift may then be confidently stated as early Pliocene, pos- 

 sibly extending to, or even into. Pleistocene. 



The nature of the uplift was a regional warping of a block of the 

 earth's crust by deep-seated forces acting principally in Pliocene time. 



GLACIATION- 



The efficacy of glaciers as agents of erosion seems too well established 

 to need comment. For one interested in the forms due to such erosion 

 few localities in the United States are more favorable to their study than 

 the Skykomish Basin. Eight small glaciers in the area, lying in two 

 groups of four each, are lone descendants of mighty alpine glaciers that 

 formerly crowded the valleys with sloAV-moving ice. These may be classi- 

 fied as transitional between cliff glaciers^ and alpine glaciers. The lowest 

 elevation to which they extend is at present about 5000 feet. 



Comparatively straight, glaciated or "U" shaped valley troughs with 

 hanging valleys from which Avater falls in cascades and which are often 

 occupied by deep, narrow bodies of water are typical features of the 

 Skykomish Basin. Truncated spurs are seen on either side of these val- 

 leys and till is foimd on the valley walls of the largest streams. Cirques, 

 often with tarn lakes, abound. Serrate ridges or aretes lead to alpine 

 peaks of the Matterhorn type and only a little rolling, pre-glacial upland 



^ Bailey Willis : Prof. Paper 19, p. 97. 1903. 



8 CiiAMBEnLAix & Salisbury: Geology, Vol. 1, p. 256. 1904. 



