566 PRE- WISCONSIN GLACIAL DRIFT IN MONTANA 



of the name "Albertan" will lead only to confusion. We do not wish to 

 propose any substitute at this time, preferring to designate this earliest 

 known stage of Cordilleran glaciation as pre-Wisconsin until more ex- 

 tended detailed observations are made. 



The question of correlation with the deposits recognized in the Missis- 

 sippi Valley must still be left open. It seems probable on a priori grounds 

 that this early stage of glaciation in the Cordillera corresponded with 

 either the earliest known stage of continental glaciation, that to which 

 the names pre-Kansan, sub-Aftonian, and Xebraskan are now variously 

 applied, or to the stage following the Aftonian interglacial stage to that 

 now known as Kansan stage of glaciation. For better grounds of classi- 

 fication we must await detailed study of the pre-Wisconsin deposits in the 

 area between this region and western Iowa. 



A POSSIBLE INTERMEDIATE STAGE OF COEDILLERAN GLACIATION 



On pages 534 and 535 attention is called to the presence of remnants 

 of a second and lower plain, or set of plains, on the high levels of the 

 Browning and Blackfoot quadrangles. The best representative of this 

 second set of plains comprises the larger part of the top of Milk River 

 Ridge. On pages 539 and 540 brief consideration is given to the hy- 

 pothesis that the earliest extension of the mountain glaciers may have 

 occurred after the second set of high-level plains had been developed, but 

 the statement is made that we think there is some ground for the opinion 

 that the earliest glaciation occurred prior to the development of the second 

 set of plains. If this opinion is tenable, what is the significance of "the 

 glaciated material on the second set of plains ? 



For purposes of record we present here a somewhat detailed statement 

 of the glacial deposits observed on the remnants of the second set of high 

 plains. 



In the former paper by the senior author 28 the following statement was 

 made concerning the deposits on that part of the top of Milk River Ridge 

 which was traversed in 1911 : 



■ "Underlying this extensive high level plain is a yellowish to brownish 

 oxidized gravelly deposit like that described above, consisting principally of 

 quartzite with much green and red argillite. diorite. and some limestone. The 

 stones are subangular to rounded, and it seemed to me there was no notable 

 decrease in size with increasing distance from the mountains. Numerous 

 boulders 1 to 1% feet in diameter are found 2 to 3 miles from the head of 

 this plain and 8 to 10 miles from the mountains, the source of all the material. 

 That exposed in a ravine in the northeast quarter of section 12. township 33 

 north, range 13 west, consisted of clay, pebbles, and boulders mixed as in 



29 Bull. Geol. Soc. Am., vol. 23, 1912, pp. 696. 



