3 66 



F. H. KNOWLTON 



"That the unconformity at this critical horizon is of more than 

 local significance is borne out in large measure by observations 

 made in the course of an examination of the lignite region in east- 

 ern Montana in 1910. Trending southeast from Yellowstone 

 River, 10 miles southwest of Glendive, is an anticline which extends 

 to the South Dakota line and along which the Lance formation is 

 exposed in a zone on either side. Along the axis of this anticline 

 Pierre shale is at the surface in a band several miles in width with 

 a sandstone formation appearing as a zone of outcrop between it 

 and the Lance. Although fossils have not been found in this 

 sandstone it is believed to be the equivalent of the Fox Hills in 



Fig. i. — South bank Moreau River, near Go vert P.O., South Dakota, showing 

 angular unconformity between Lance and underlying beds identified as Fox Hills. 

 Photograph by Barnett. 



its type locality. Transition into it from the Pierre shale is perfect, 

 and as in the Dakotas it is overlain by the markedly dissimilar 

 strata composed of alternating lenticular sandstone, somber clays, 

 and carbonaceous zones. In at least two localities in this region 

 the upper surface of the sandstone referred to the Fox Hills is 

 irregular and with every appearance that sedimentation between 

 that formation and the overlying Lance formation was interrupted. 

 These were noted and mapped by Pishel and Bowen and are in 

 Sec. 22, T6N, R60E, Sec. 32, T7N, R61R. The amount of 

 erosion is not great in either case, but in the opinion of the 

 writer the occurrence of an unconformity in this region at appar- 



