110 GliOLOGY OF THE DENVER BASIN. 



STRUCTUBAl DEVELOPMENT OF THE BOULDER REGION. 



In the geological history of this region there again appear the same 

 conditions of post-Cretaceous development that brought about present 

 surface delineations of ancient Mesozoic profiles in the vicinity of Golden. 

 The character of the elevations, the features of stratigraphy, and the nature 

 of the unconformities prior to the general uplift of the range, which would 

 then have required in representation vertical profiles, are now, by reason of 

 this uplift and subsequent reduction of the region to its present configura- 

 tion, clearly delineated in plan upon the prairie. The profiles, therefore, 

 are like those illustrating the unconformities at Golden — simply reduced 

 transfers of surface delineations 



Prior to the movements which brought about the above conditions the 

 succession of events was approximately as follows: For the reception of 

 the Trias the Archean presented the usual uneven floor, with possibly an 

 incipient rise (shown in Profile 1, PI. XII), which later developed into the 

 fold of the region. Deposition of both members of the Trias was carried 

 to completion without interruption. At the close of the Trias compression 

 began to take place in much the same way as at Golden, resulting in the 

 uplift represented in cross-section in Profile II, PI. XII, after the influences 

 of erosion had been at work. On this profile the full rise of the original 

 arch was probably about 750 feet, but the height of the elevation was 

 diminished by denudation until, when closed over by the succeeding 

 formation, it barely reached 150 feet. Deposition of the Jurassic and 

 Dakota followed, and with their completion the first period in the develop- 

 ment of the geological history of the region closed. 



The second period in the developmental history of this region opens 

 with the elevation of the Dakota and underlying beds into the gentle hill 

 indicated in Profile III, PI. XII. The rise of the arch was approximately 

 600 feet, but erosion subsequently reduced the summit of the hill to about 

 350 feet. The axis of the post-Dakota fold on this line of profile was not 

 coincident with that of the earlier fold, being fully a half mile to the north 

 of the latter. Both folds are unsvmmetrical in the section exposed, the 

 earlier fold having the longer slope to the north, the more recent fold to 

 south. Deposition of the Benton upon the eroded sides and crests of this 



