PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



THE GEOLOGICAL SOCIETY OF WASHINGTON 



The 273d meeting of the Society Avas held in the Cosmos Club on 

 October 29, 1913. 



REGULAR PROGRAM 



Physiography of the Southern Andes (illustrated.): Bailey Willis. 

 No abstract. 



Use of physiography in the study of Rocky Mountain stratigraphy: W. 

 T. Lee. Reasons are here presented for abandoning the belief that 

 extensive land masses persisted in the southern Rocky Mountain region 

 thruout the Cretaceous period. Radical differences of opinion exist as 

 to the significance of certain continental deposits that rest unconform- 

 able^ on the Cretaceous rocks in the southern Rocky Mountain region. 

 In many places on either side of the mountains the base of these con- 

 tinental deposits is conglomeratic and contains pebbles of older rocks, 

 mainly the pre-Cambrian crystallines which constitute the core of the 

 range. Those who beheve that this conglomerate marks an unconform- 

 ity of considerable magnitude appeal to the evidence of these pebbles 

 as proof of a long interval of erosion following a notable upheaval of 

 the mountains. Others who doubt the magnitude of the unconformity 

 are of the opinion that the pebbles may have been derived from nearby 

 lands that had not been previously submerged, and argue that they 

 might have found their wa}^ to their present resting place without any 

 notable upheaval or long interval of erosion. It seems necessary, there- 

 fore, to inquire whether the evidence warrants the belief that any con- 

 siderable part of the southern Rocky Mountain region remained above 

 sea-level thruout Cretaceous time or whether the Cretaceous sea ex- 

 tended uninterruptedly over this region. The determination of this 

 question would materially aid in the solution of the problem whether 

 the unconformity in the southern Rocky Mountain region marking the 

 separation of the conformable Cretaceous beds, mainly of marine origin, 

 from the lowest conglomerate of the younger rocks of continental origin, 

 denotes an orogenic movement and erosion of sufficient magnitude to 

 make it the appropriate line of separation between the Cretaceous and 

 the Tertiary systems. 



It seems evident that no notable crustal movement affected the 

 southern Rocky Mountain region for a long time prior to the beginning 

 of the Upper Cretaceous period, and that there was ample time for the 

 formation of a peneplain over this region. 



