269] W. T. Thorn 71 



persistence of development which lies about 50 feet above the 

 basal sandstone of the formation. 



At Acton, 15 miles northwest of Billings, the formation 

 still retains its characteristic appearance and lithology, but 

 east of the town a very rapid lateral variation of the sedi- 

 ments soon Incomes apparent. The thin carbonaceous sand- 

 stones of the western section rapidly increase in magnitude 

 and in coarseness of grain to the eastward, individual mem- 

 bers increasing from 6 or 8 to as much as 50 feet in thickness 

 within a distance of 5 miles. As a result the formation, as 

 exposed about 6 miles east of Acton, consists of four massive 

 sandstones separated by intervals of shaly sandstone or sandy 

 shale. Abundant impressions of Halymenites major occur 

 throughout these sandstones and indicate the inception of 

 strand conditions, a conclusion confirmed by the discovery 

 of marine fossils a few miles farther east, near Huntley. 



From where the maximum of sandstone development is 

 attained, 6 miles east of Acton, the lower sandstone members 

 begin to taper eastward, merging into shale lithologically 

 indistinguishable from that of the underlying Claggett. 



It is further to be noted that certain surprising features 

 of the Judith River-Bearpaw contact appear southeast of 

 Gibson. Long, narrow ridges capped by hard andesitic sand- 

 stone are developed for considerable distances, especially near 

 Big Lake in the so-called Lake Basin region, their general 

 extension being from east to west or from southeast to north- 

 west. The cap sandstones of these ridges were probably 

 never continuous over the intervening areas, but they lie at 

 practically an identical horizon and are so similar lithologi- 

 cally that they are certainly the products of the same agency. 



Below these upper dark sandstones the section shows 

 great variability; at some localities sandy beds containing 

 more or less carbonaceous shale and lignite occupy the whole 

 of the interval down to the more typical Judith River sedi- 

 ments, while elsewhere, even in the same ridge, the cap 

 sandstone may overlie typical Bearpaw shale with only a 



