142 GEOLOGY OF THE DENVER BASIN. 



THE BOULDER SECTION. 



The structure of the western half of this. section has been considered 

 in the discussion of the geology of the Boulder Valley and the uncon- 

 formity in the foothills near the town of Boulder. The eastern half will 

 be described in the following. 



The region east of Coal Creek Valley in the vicinity of this section is 

 almost wholly occupied by the clays of the upper division of the Laramie 

 formation; the eastern 2 or 3 miles only are underlain with Arapahoe beds, 

 covered with considerable deposits of Quaternary sand. The homogeneity 

 of the Laramie beds renders it quite impossible to determine the particular 

 horizon outcropping in a designated locality, but observations of dip in 

 connection with surface profile and the aid afforded by well borings have 

 enabled the construction of a section sufficiently correct for the consider- 

 ation of economic questions. 



The section shows that in the high ridge east of Coal Creek the strata 

 have a general easterly dip, although on the western slope of the ridge a 

 number of rolls of minor importance occur. In the valley of Little Dry 

 Creek slight local rolls also exist, but for long distances the strata lie 

 horizontal or with a maximum dip of from 0° 30' to 1° 0' to the east. In 

 the valley of the Platte the Quaternary deposits are so widely distributed 

 that only occasional outcrops of the underlying formations occur; these 

 outcrops, however, with the wells and borings to the south of the section, 

 indicate for the strata a horizontal position, or one of slight easterly dip. 

 For the country east of Coal Creek, therefore, the strata have a general 

 easterly dip, with longer or shorter horizontal reaches and occasional 

 gentle rolls. The thickness of the Laramie measures beneath the Platte, 

 on the line of Section I, is probably about 900 feet. 'Phis estimate is 

 based on artesian borings in the Platte Valley less than 1 mile south of 

 the section, which reach depths of 300 and 530 feet without striking coal 

 or the heavy sandstones at the base of the formation, or even encountering 

 a change in the composition of the strata. Increasing the greater depth, as 

 that nearest the line of section, by 220 feet — the thickness of the lower 

 division of the Laramie — the accounted thickness of the formation becomes 

 750 feet. It is believed, however, from considerations of dip and the position 



