A large region in northeastern Montana has never been thoroughly explored by geologists, owing to the fact that it is a part of the Great Plains and the belief that it is too monotonous and uninteresting to tempt anyone to turn aside from the pronounced geologic features a little farther west, for which Montana is noted. This region includes parts of Sheridan, Valley, Phillips, and Blaine counties. Its investigation was begun by Smith in 1908, when he made a geologic survey of the Fort Peck Indian Reservation. Beekly explored a strip of land along the Montana-North Dakota line from Missouri River to the international boundary, and Bauer examined the townships in which Plentywood and Scobey are situated. Their results are here included with those of the writer, who during the field seasons of 1915 and 1916 was engaged in an investigation of the lignite resources of the remainder of this region, which extends from a line within 12 miles of the Montana-North Dakota boundary westward about 200 miles.