pos>n. (Geol.) An extensive series of strata, principally developed in the Rocky Mountain region, as in the Laramie Mountains, and formerly supposed to be of the Tertiary age, but now generally regarded as Cretaceous, or of intermediate and transitional character. It contains beds of lignite, often valuable for coal, and is hence also called the lignitic group. See Chart of Geology. [ 1913 Webster ]