I20 JOSEPH E. POGUE 



Cantwell formation/ In 1913 MojHit and Pogue, during a reconnais- 

 sance survey of the Broad Pass region, encountered this belt at 

 the mouth of Jack River and studied in some detail its eastward 

 development up to the West Fork Glacier of Susitna River. The 

 present paper deals primarily with this eastward extension of the 

 Cantwell, although it is believed that the conclusions are appli- 

 cable to the formation in its entirety. 



Character. — -In its westward extent and along its northern 

 border, the Cantwell formation is characterized by Brooks^ as 

 follows : 



The Cantwell formation includes a series of heavy conglomerates inter- 

 bedded with a few shale layers and succeeded by finer conglomerates and red 



sandstones interbedded with gray and black clay shales The basal 



conglomerate of this formation is made up chiefly of well-rounded white quartz 

 and chert pebbles, the largest of which are 2 inches in diameter. The basal 

 character of the conglomerate is well illustrated in several localities where it 

 rests uncomformably on the older rocks and also contains rounded fragments 

 of them. That part of the formation seen by Elridge contained only the basal 

 beds of the series. On being traced to the north and west these are found to be 

 succeeded by reddish sandstone and gray, drab, and black shales. The sand- 

 stones are in some places bright red, but grade from this into a reddish-brown 

 to medium-brown color. The shales are both argillaceous and arenaceous, 

 the latter phase grading into a shaly sandstone. Some of them carry a large 

 amount of carbonaceous matter, and coal seams are interbedded with these 

 rocks, but those seen by the writer appear not to have any commercial impor- 

 tance 



In the area traversed by the present writer, the Cantwell forma- 

 tion attains its most representative development along the north- 

 ward course of Nenana River below the mouth of Jack River. The 

 stream here cuts through mountains of conglomerate, which shows 

 in heavy massive beds up to 100 feet and more in thickness, of 

 gentle dip and somber aspect, alternating with subordinate beds 

 of sandstone and argillite. Eastward from the mouth of Jack 

 River the formation changes progressively in character: The con- 

 glomerate members become less conspicuous, with increasing promi- 

 nence of graywacke, sandstone, and carbonaceous shale; while the 



" Alfred H. Brooks, "The Mount McKinley Region, Alaska," United States Geol. 

 Survey, Prof. Paper 70 (1911), Plate 9. 

 ^Op. cit., p. 78. 



