SMITH, PHYSIOGRAPHY OF SKYK02IISH BASIN 209 



thickness measured perpendicular to the valley Avail is variable but never 

 great. Five miles farther west, near the confluence of Tve and Foss 

 Eivers, glacial till occurs to an estimated height of 350 feet above the 

 water. It is filled with boulders from the size of an egg up to great, 

 rounded individuals weighing a ton or more. This till shows little sign 

 of weathering and probably belongs to the last glacial advance. Bretz 

 has described similar valley till on the Middle Fork of the Snoqualmie.^ 

 There is no evidence of any earlier till to be seen in the Skykomish Basin, 

 it probably having been entirely removed by the last glacier (Vashon) 

 that occupied the valley. At Berlin, ten miles below Martin Creek, there 

 is blue glacial clay which occurs in lenses in the glacial till on the valley 

 sides to a height of 500 feet above water level. Bretz has estimated the 

 thickness of the Puget Sound Glacier, which was of the Piedmont type, 

 originating in ice coming not alone from the Cascade Alpine glaciers, 

 but also southward from British Columbia, at 4000 feet.^° My own ob- 

 servations tend to show that the surface of the ice was about 3500 feet 

 above sea level at Berlin. About 1000 feet of the Puget Sound ice was 

 below present sea level. Inasmuch as considerable gradient would be 

 necessary for ice to flow down to a confluence with the Piedmont Glacier, 

 only something less than 3500 feet could be allowed for the thickness of 

 this Piedmont tongue, an amount in discrepancy with Bretz's figure. 



The drainage modifications due to glacial moraines were not specially 

 studied. From observations near Berlin, however, it would appear that 

 material deposited from the Miller Eiver Glacier has crowded the Sk}^- 

 komish Eiver to the north wall of the latter stream's valley. There are 

 many other minor modifications, but they are of little importance. 



We have seen thus far the forms of topography due to glacial erosion. 

 Let us now consider the remnants of the mountain ridges between which 

 the valley glacier so effectively worked. These may be considered under 

 three divisions: first, serrate ridges; second, alpine peaks; third, pre- 

 glacial uplands. 



With few exceptions all the ridges are serrated. A serrate ridge or 

 arete is produced by headward development of cirques^^ or^^ by trough 

 widening. These leave the "fishbone" edges so characteristic of the 

 northern Cascades. The best example in the Sk5^komish Basin is prob- 

 ably the ridge separating the AVest Fork of Foss Eiver and the East Fork 



s) .7. H. Beetz : "Glaciation of the Puget Sound Region," Wash. G. S. BuH. No. 8, p. 224. 

 1913. 



10 .7. II. BuETZ : "Ghichition of the Puget Sound Region," Wash. G. S. Bull. 8, id. .36. 

 1913. 



11 W. H. HoBBS : Characteristics of Existing Glaciers, p. 33 et seq. 



12 E. DE Martonne : Traite de Geographie Physique, p. 622. 



