THE SATSOP FORMATION 451 



through and several hundred feet above the surface of the Satsop 

 fill. The formation is at least 600 feet thick along the Sandy River, 

 with the base below river-level. The material is stream-bedded 

 gravel and sand, indurated in some places to a conglomerate and 

 sandstone. Quartzite is a common constituent for 10 miles south 

 of the Columbia, but has not been found more than 15 miles from 

 the master-stream. Quartzite and basalt are the most important 

 constituents. 



The Satsop formation of the lower Willamette valley is maturely 

 dissected, the dissection adjusted to a base-level 200 feet or more 

 above present flood plains. This level is recojrded in the major 

 valleys by a prominent terrace developed mostly in the Satsop 

 formation but in places cut in the underlying basalt. This is the 

 Cowlitz Terrace already described. 



The uplands of this Satsop plain bear a red clay soil 10-15 ^ ee ^ 

 deep. This grades down into a much-decomposed gravel. At a 

 depth of 30 feet the pebbles are decayed only on the exterior. 

 Below 50 feet most of the material is hard and ringing when struck 

 with the hammer. Near the Columbia the clayey residual soil on 

 the top of the Satsop formation contains scattered quartzite 

 pebbles, hard, bright, polished, and apparently unaffected by the 

 weathering which has reduced the associated basaltic pebbles to a 

 structureless clay. 



The surface of the Satsop formation in the Willamette valley 

 lies at about 500 feet A.T. in mid-valley and rises eastward toward 

 the Cascade Range to 1,200-1,500 feet, at these altitudes passing 

 under the more recent lava-flows of this range. No upward slope 

 of the Satsop surface toward the Coast Range on the western side 

 of the Willamette valley has been found. On this side the forma- 

 tion terminates against hills of older basalt. 



The broadened portion of the Columbia valley between the 

 Cowlitz and the Willamette is really a continuation of the Willa- 

 mette valley northward into Washington. The surface of the 

 Satsop formation constitutes at least 200 square miles of the flat 

 floor of this part of the valley. It is disposed in two levels, approxi- 

 mately 300 and 500 feet A.T., the lower of which is the Cowlitz 



