450 / HARLEN BRETZ 



The Satsop formation in the Columbia valley west of the Cascade 

 Range. — The Satsop formation in the lower Columbia valley does 

 not differ in any essential from that in the Chehalis and Willapa 

 valleys. It contains a surprisingly large amount of quartzite 

 gravel. In some strata more than 50 per cent of the pebbles are of 

 quartzite, all of which undoubtedly have come from east of the 

 Cascade Range. Basalt, a common country rock, is also a leading, 

 constituent of the gravel. The basalt pebbles are decayed, except 

 in the deeper portions of the deposit. 



There are three large areas in the drainage of the lower Columbia 

 where the Satsop formation covers many square miles, instead of 

 being limited to narrow terraces. One of these areas is in the valley 

 of the Cowlitz River, a tributary of the Columbia from the north; 

 a second is in the valley of the Willamette River, a tributary from 

 the south; and a third lies in a broad portion of the Columbia 

 valley between the two areas just mentioned. 



The Satsop formation in the Cowlitz valley is at least 150 feet 

 thick. It here constitutes a broad, terraced plain and rises north- 

 ward to a summit level tract about 500 feet AT. This tract is a 

 portion of the divide between the Cowlitz and Chehalis rivers. It 

 bears a residual soil and with little doubt is part of the original 

 upper surface of the formation. The pebbles in the upper 30-50 

 feet are softened by decay, those immediately below the soil being 

 spaded through in excavating. At depths greater than 50 feet the 

 pebbles are hard, but the reddish to yellowish stain penetrates as 

 far as excavations have gone. No quartzite pebbles have been 

 found in this part of the Satsop formation. The dissection of the 

 tract is adjusted to a base-level recorded by a broad terrace 100 

 to 150 feet lower than the summit plain and about 250 feet above 

 the present flood plain of the Cowlitz River. This terrace has 

 been found in most of the major valleys of the region studied. 

 From its notable development in the Cowlitz valley it is here 

 named the Cowlitz Terrace. 



Only the lower 25 miles of the Willamette valley of Oregon have 

 been examined in the study of the Satsop formation. Most of this 

 portion is covered by the Satsop. Numerous hills of basalt rise 



