FOOTHILLS STRUCTURE IN NORTHERN COLORADO 727 



10. There are many minor strike thrust faults clearly shown, as 

 well as dip faults with steep fault planes and a hingelike displace- 

 ment toward the east — all of which are most pronounced in the 

 immediate vicinity of Golden. 



11. Objections based on interpretation: 



11. The arch hypothesis presupposes the existence of a shore 

 line immediately to the west. The work of Lee 1 has proved the 

 former extension of the Cretaceous formations in a continuous 

 sheet across the entire area of the Front Range. Hence this whole 

 area must have been an epicontinental sea, and consequently a 

 " headland" similar to the arch could not have been present at 

 Golden. 



12. Too much oscillation required of a small local area. Such 

 rapid alternations of up and down movements are a strain on 

 credulity. 



13. It seems incredible that an elevation sufficient to prevent 

 the deposition of any Pierre shale could have taken place at Golden, 

 while a few miles to the north and south the true thickness of the 

 Pierre (7,700 feet according to Eldridge) was deposited. 



14. Another weak point in the arch hypothesis is the present 

 attitude of the strata. These if considered in the plane of their 

 bedding form a syncline with east-west axis over the crest of the 

 supposed arch. The limbs on each side are dipping inward as much 

 as 35 . None of the strata show any evidence of the original anti- 

 clinal folding to which they must have been subjected in order 

 to form the "arch." A simple calculation will show that the arch 

 requires an anticline with average dips of at least io° in the older 

 rocks for 10 miles north and south of Golden, while in the crest 

 of the arch the dips must have risen as high as 30 . Eldridge, 

 Emmons, and Fenneman are therefore driven to the conclusion 

 that this arch was flattened out probably in Denver time and 

 distorted into its present shape. This requires the sudden conver- 

 sion of a persistently and rapidly rising area into a remarkably 

 rapidly subsiding one, and requires a bending in the strata similar 

 to that of a card bent back and forth between the fingers. This 

 whole reasoning is not only laborious but also illogical. 



1 Op. cit., p. 32. 



