THE GENEEAL PLAINS STRUCTURE. 143 



which the strata hold between Coal Creek and the Platte River, that to this 

 may be added at least 150 feet more, affording thus the thickness of 900 

 feet given: The maximum thickness of the Laramie at the line of section 

 is probably about 1,100 feet. This, however, does not represent the 

 original depth of Laramie deposits in this vicinity, for on the western face 

 of the ridge east of the Platte River a considerable degree of erosion prior 

 to the deposition of the Arapahoe is clearly discernible in the gradual rise 

 to the north of the line of union between the two formations. 



The di-posits of Quaternary crossed by Section I are chiefly river 

 gravels and sands, with some loess. In the valley of the Platte the gravels 

 underlie both the present bottom and the broad benches to the east — the 

 loci of the river's early channels — attaining a depth beneath the latter of 

 30 to 40 feet. Heavy sand deposits occur in the highlands east of the 

 Platte, and in the valley east of these they show considerable irregularity 

 of outline and position, having locally the appearance of dunes. 



THE DAVIDSON SECTION. 



The western half of this section also involves the geology of the 

 Boulder Valley and the foothills to the west, and has been discussed in the 

 preceding pages. The eastern half closely resembles in structure the 

 eastern half of the Boulder section (1). There is a general easterly dip of 

 about 1° for the greater part of its length, steepening to 2° or 3° in the 

 ridge dividing the valleys of Coal and Dry creeks. The main body of the 

 Arapahoe formation extends much farther west than in the Boulder section, 

 and there is, besides, a. prominent outlier of this formation on the divide 

 between the creeks just mentioned. An especial feature of the section 

 and the region adjoining is the illustration afforded of the unconformitv 

 by erosion between the Arapahoe and Laramie formations. In the area 

 immediately west of the Platte River the Arapahoe forms the merest cover 

 upon the subjacent Laramie, and in almost any of the shallow vet sharply- 

 cut ravines the uneven line of contact between the two formations clearly 

 appears; within a distance of ■_'(>!) yards the difference in the level of this 

 line sometimes amounts to 50 feet. A still more pronounced difference in 

 level occurs midway between Sections I and II, at the head of a short gulch 



