Area in Louisiana and Eastern Texas. 69 



the more prominent surface feature of the two. The 

 sandstone beds are lenticular and thin out to the north, 

 whereas the intervening beds of shale do not appear to 

 thin out in this direction. The absence of the sandstone 

 to the north therefore can not be explained by the hypo- 

 thesis that it extended farther north and that it was subse- 

 quently eroded. 



Blaylock sandstone of Ouachita Mountains. 



Although the Blaylock sandstone, which at places 

 reaches a thickness of 1,500 feet and is of Silurian age, 

 is of wide extent from east to west, its outcrops stretch- 

 ing from a point near Malvern, Ark., nearly to Bismarck, 

 Okla., it is present in the Ouachita Mountains only on 

 their south side. The northward thinning of the sand- 

 stone may be due partly to erosion, as is indicated by the 

 local occurrence of a conglomerate at the base of the over- 

 lapping Missouri Mountain slate. If the thinning out of 

 the sandstone is due entirely to erosion this would mean 

 that at least 1,500 feet of material has been removed from 

 the northern part of the present Ouachita region, and it 

 would be expected that the underlying Polk Creek shale 

 would also have been removed from large areas at the 

 same time the Blaylock was being removed. But the 

 Polk Creek shale is generally present in the region north 

 of that in which the Blaylock sandstone occurs, and its 

 thickness there is much the same as it is in places where 

 it underlies the Blaylock. The conclusions regarding 

 the Blaylock are that it was deposited in a minor east- 

 west trough on the south side of the Ouachita geosyn- 

 cline, that the northward thinning of the formation can 

 be attributed in only a very small part to erosion, arid 

 that the land-derived sediments for it came from the 

 south. 



Stanley shale and Jack fork sandstone of Ouachita Mountains. 



The Stanley shale, 5,000 to 6,000 feet thick, and the 

 Jackfork sandstone, 5,000 to 6,600 feet thick, both of 

 Mississippian age, are exposed through the entire length 

 of the Ouachita region, and the Jackfork sandstone is 

 exposed at places in the Arkansas Valley in Arkansas 

 but both formations thin out to the north and west. They 

 are absent in the Arbuckle Mountains, and at a locality on 



