KELLOGG SYSTEM OF RIVER TERRACES 521 



slate rises 40 feet above the Modern floodplaiii aiid the original surface 

 of the terrace at the north edge was about 20 feet higlier. The uneroded 

 surface rises toward the south at first 2 degrees and then steeper on the 

 slope leading up to the next terrace. The deposit seems to comprise 

 several feet of coarse cobbly and even bouldery gravel and over this ordi- 

 nary gravelly alluvium. Tliis is overlaid, near the foot of the slope 

 leading to the next terrace on the south, by a bed of non-pebbly silt 

 several feet thick. 



This is the youngest of a series of gravel-capped rock benches and has 

 no particular significance, as it merely marks a local vicissitude in the 

 down-cutting of the valley. There are traces of the same terrace farther 

 down the valley, but this is not a prominent terrace horizon and will not 

 be considered in the further discussion of the subject. 



TWO-HUNDRED-FOOT TERRACE 



This terrace is well developed along the south side of the valley from 

 Kellogg to Grouse Creek as a series of flats separated by the broad 

 canyon-shaped valleys of Deadwood and Government creeks and by sev- 

 eral narrow gulches. Bedrock generally rises in the terrace till within 

 30 to 40 feet of its surface. In the most easterly flat (back of the Kel- 

 logg hospital) the first bed over the bedrock appears to abound in water- 

 worn boulders, many of which are 18 inches in longest diameter. This 

 is a much coarser river deposit than any seen lower. Above this there is 

 a light brown sand and silt with scattered Cobbles, but not much fine 

 gravel. The uppermost part of the deposit is largely a light brown 

 sandy silt. The remarkable feature of this terrace is the boulders scat- 

 tered over its surface, apparently weathered out from the upper part of 

 the deposit. They are not numerous, but widely distributed. 



Besides the local quartzites there is an unusually large percentage of 

 igneous rocks, including greenish crystallines that might have been de- 

 rived from dikes in the drainage basin of the river, a granite of doubtful 

 occurrence in the basin, and a pink quartzite not known from the Belt 

 rocks of the region. The largest boulder, which has a maximum diame- 

 ter of 30 inches, consists of a porphyritic granite, resembling the monzo- 

 nite near Gem. These boulders reach an altitude of 250 feet above the 

 river or 2,500 feet above the sea. No boulders were observed on tlie next 

 flat west (which is 200 yards wide), probably because of tlie tliick brush, 

 and near Deadwood Gulch the horizon of the granite boulders is buried 

 under an old alluvial fan of Deadwood Creek. Government Creek has 

 trenched across the terrace a steep-walled canyon about ITo yards wide 



