STRUCTURE 339 



under discussion, it is probable that the Dakota-Benton sea did not ex- 

 tend many miles west of the present limit of the Plains in this latitude. 

 A shore established on a flat surface and remaining nearly fixed during 

 an episode of deposition in an adjacent zone is a line between an area of 

 subsidence and one of uplift. The case is one of markedly unequal 

 subsidence, the movement being on the one side plus, on the other 

 minus. The sequence of changes is shown in sections 1 and 2, plate 

 53. Section 1 represents the condition during the Dakota invasion. 

 Section 2 shows the effects of subsidence and uplift incident to deposi- 

 tion of 4,000 feet of Dakota and Benton sediments. The drawings are 

 to natural scale. 



The surface on which Dakota sediments gathered probably consisted 

 of Carboniferous and Algonkian rocks. The observed angular uncon- 

 formity beneath the Carboniferous is slight. The present deformation of 

 the Algonkian is apparently attributable altogether to post-Cretaceous 

 movements. It is assumed that the Algonkian strata were essentially 

 flat in Dakota time and were bent in consequence of the subsidence 

 which occurred during Benton time. These assumptions also are shown 

 in sections 1 and 2, plate 53. 



Whatever later stages of deposition followed may have been accom- 

 panied by further subsidence and curvature, but that which is attributed 

 to the Benton epoch is sufficient to occasion the development of an anti- 

 cline (section 3, plate 53) whenever compression should occur. 



The preceding statement is made to show reason why an anticline 

 should have developed near the present site of the Front range. Its 

 essential principle is that of initial dips, as a condition which determines 

 the loci of anticlines and synclines* and the special case is based on the 

 probable position of the Dakota-Benton shore. In what follows the epi- 

 sodes of deformation and erosion leading to the present relations are 

 discussed. 



Antecedents by folding and erosion. — Along the eastern base of the Rocky 

 mountains in general the facts of structure express the action of a com- 

 pressive stress, the Cretaceous and older strata being folded. The post- 

 Cretaceous effects are commonly attributed to a single episode of com- 

 pression; in what follows they are assigned to two episodes, at least for 

 the particular district under discussion. 



The first episode of compression began at some date not closely deter- 

 minable, but which may be placed not earlier than Laramie time, nor 

 later than early Tertiary. It is possible that flexure went on during 

 Laramie deposition. It is also possible that it did not begin till after 

 that deposition was completed. The distinction is not important to the 



* Mechanics of Appalachian structure, op. cit., plates showing models A to El. 



