THE SATSOP FORMATION 447 



terraces and given a dull red or orange color to the upper 50 feet of 

 gravel. 



The Satsop formation along the Pacific Coast of Washington. — 

 The terrace gravels which constitute the Satsop formation of the 

 Chehalis valley are traceable almost continuously in the cliffs along 

 the lower part of this valley and in the bluffs of Grays Harbor to the 

 sea-cliffs of the narrow coastal plain. 



North of Grays Harbor the formation differs from that in the 

 Chehalis valley only in containing much clay and sand, with frag- 

 ments of driftwood. In places there are strata of peat or lignite 

 several feet thick. The gravel in some exposures is a beach shingle 

 and lies on wave-worn and mollusk-drilled Tertiary sandstone. 

 The formation is horizontal for the most part, and such warping as 

 does exist is very slight. The formation rests unconformably on 

 Tertiary and older rocks. A few marine shells record the presence 

 of the sea, and the interbedded peat tells of tidal marsh condi- 

 tions in places during accumulation of the deposit. The thickness 

 of the formation as shown in the cliff sections doe's not exceed 

 200 feet. 



The shore line of Willapa Bay, south of Grays Harbor, is 

 largely cliff ed, and all of the cliffs are cut in the Satsop formation. 

 Clay and sand predominate. Peaty strata record the presence 

 of fresh- or brackish- water swamps during aggradation. Shells of 

 marine mollusks, cross-bedding due to tidal currents, and beach 

 shingle in the gravelly strata tell of deposition in marine water. 

 One stratum of highly fossiliferous clay is traceable for several 

 miles along the bluffs. Most of the shells in it are of oysters, many 

 of the valves yet attached in pairs. The shell-bearing stratum rests 

 on blue clay, which is full of molluskan borings but contains no 

 shells. Above the shell bed is a peaty clay containing much drift- 

 wood. Stumps in situ and upright stems in this layer record suc- 

 cession of the oyster bed by a coastal marsh. In the gravelly 

 strata are pebbles of granite, gneiss, schist, and quartzite, all but 

 the quartzite considerably decayed. None of these materials occur 

 in the drainage area of either the Chehalis or the Willapa River, 

 while all of them are common in the Satsop formation of the 

 Columbia valley. This gravel undoubtedly was brought over into 



