34:0 B. WILLIS LEWIS AND LIVINGSTON 



present thesis. Flexure in its early stages was an effect involving rela- 

 tively great stress, as Ihe nearly flat Algonkian strata were exceedingly 

 inflexible. It is probable that folds developed slowly. As the Laramie 

 sea was shallow and was succeeded by emergence of the area, the anti- 

 clines were subject to erosion, whether they developed earlier or later, 

 and the synclines received their waste either as sediments beneath ma- 

 rine waters or in estuaries or in lakes or as valley deposits. 



The effect which for a time satisfied the compressive stress was one of 

 moderate folding (see section 3, plate 53). The succeeding condition 

 was one of quiescence and it endured long enough for the planation of 

 Cretaceous rocks to the Blackfoot peneplain. The name Blackfoot may 

 be extended to the topographic cycle ending in the development of the 

 plain. The Blackfoot cycle can not be accurately dated by any evidence 

 now available. It was post-Laramie and probably earlier than the oro- 

 genic movements which, in Montana, gave rise to ranges and lake basins. 

 The latter having yielded Miocene vertebrates, the movement may be 

 placed in mid-Tertiary. That it was preceded by the Blackfoot cycle is 

 an inference based on general observations of an extensive peneplain 

 over the summits of the Rockies of western Montana and Idaho, obser- 

 vations which leave no doubt in the writer's mind of the existence of 

 such a peneplain, but which do not suffice positively to identify it as the 

 Blackfoot plain. On the probability of that identification the Blackfoot 

 cycle may be placed in early Tertiary time. 



At the close of the Blackfoot cycle the topographic features of the re- 

 gion under discussion were the peneplain on Cretaceous rocks and low 

 hilly, past-mature relief on Algonkian rocks, such as is now presented 

 by the summit hills of eastern Flattop. To illustrate them would re- 

 quire a profile differing from that of section 3, plate 53, only in degree 

 of relief. 



Conditions of over thrusting. — Among the effects of folding and erosion, 

 at the close of the Blackfoot cycle was the exposure of the edges of some 

 Algonkian strata as outcrops; being gently inclined westward, these 

 beds had probably wide extent underground. They were relatively stiff 

 and lay with one edge free. Under these conditions, supposing that a 

 compressive stress again became effective, a part at least of the Algon- 

 kian beds were so placed that they met but slight resistance in their 

 tendency to yield by moving forward. So far as they were unopposed, 

 or not sufficiently opposed to check and fold them, they did ride for- 

 ward (see section 4, plate 53). That part which was thus overthrust 

 separated from that which was not in general along bedding planes near 

 the base of a particularly rigid stratum, such as the Altyn limestone. 



