8 A. G. LEONARD 



from 200 to 600 feet or more. In northeastern South Dakota wells 

 have encountered 400 to 480 feet of Benton. 



While the Cretaceous formations have been largely removed 

 by erosioii from the Red River Valley, a dark-colored shale overlying 

 a soft sandstone has been encountered in a number of wells in 

 different parts of the valley, and it seems not improbable that this is 

 the Benton shale. In that case this shale underlies the drift over a 

 portion of the valley. 



Niobrara formation. — Overlying the Benton, the upper member 

 of the Colorado group is present in the wooded escarpment known 

 as the Pembina Mountains, which border the Red River Valley on 

 the west, in eastern Cavalier County. This escarpment extends 

 far to the north in Canada, and the formation appears in the Tiger 

 Hills, Riding and Duck mountains, and the Pasquia Hills. In 

 North Dakota the Niobrara occupies a narrow belt extending 30 

 miles south of the international boundary, the outcrops being 

 found along the Pembina, Tongue, north fork of Park River, and 

 tributaries of these streams, which have cut deep valleys in the 

 escarpment and exposed the Niobrara beneath the Pierre shale. 

 A highly calcareous shale exposed in the valley of the Cheyenne 

 River at Valley City is also probably to be referred to the Niobrara. 

 It contains 45 per cent of carbonate of lime, and lies just below 

 the black and white bands forming the base of the Pierre, as 

 described on a later page. 



The Niobrara is a light to dark gray, moderately hard calcareous 

 shale. It contains numerous small white specks of lime which give 

 it a finely mottled or speckled appearance, plainly seen on fresh 

 fractures. Where the rock has been weathered it becomes almost 

 white and has a chalky appearance; in fact, in many localities 

 outside the state the formation is a nearly pure chalk. Its lime 

 content, which is the most marked characteristic of the Niobraraj 

 is due almost entirely to the presence of minute Foraminif era which 

 are readily seen under the microscope. The most abundant forms 

 are the two species so common in chalk, Globigerina cretacea and 

 Textularia glohulosa. The fact that these Foraminif era are mingled 

 with clay is evidence that the sea in which they lived was not clear, 

 but contained more or less fine sediment and the shells settled to 



