167] 



W. T. Thorn 



69 



per cent, of the rocks outcropping within its limits. Five 

 formational subdivisions of the Montana are present, which 

 are, in ascending order, the Eagle sandstone, Claggett shale, 

 Judith Eiver formation, Bearpaw shale, and Lennep sand- 

 stone. The relationship of these units is perhaps best brought 

 out by the accompanying diagram, but a brief description of 

 each formtion may make the details clearer. 



The Eagle sandstone is developed in this area as the marine 

 sandstone apex of a wedge of continental sediments built sea- 

 ward at the close of Colorado time. The strand origin of the 



FIG. 1. Sketch map of an area in Montana. 



upper part of the formation seems clearly indicated by abun- 

 dant impressions of the fossil seaweed Halymenites major, by 

 the coarse grain of the sandstone, and by the almost universal 

 distribution of small, flattened chert pebbles at or near the 

 top of the formation. The Judith Eiver formation is much 

 like the Eagle in mode of origin, but is thicker and shows the 

 development of fresh-water and estuarine phases much farther 

 east than the Eagle. The Lennep sandstone constitutes a 

 third continental wedge, chiefly notable for the predominance 

 of volcanic material among its component sediments. Separ- 

 ating these strand and coastal-plain deposits are the marine 

 shales of the Claggett and Bearpaw formations, which thin 

 westward and disappear at an indeterminate distance beyond 

 the border of the area under consideration. 



