Triangulating from the cairns…
This study deals with the stratigraphy, structure, and economic deposits of the Grand Hogback area south of Glenwood Springs, Colorado. The sequence and distribution of the rock units that crop out are shown on the accompanying geologic map and in the columnar section. Sedimentary rocks range in age from Pennsylvanian Minturn Formation to lower Eocene Ruby Formation. The Maroon, State Bridge, Entrada, Morrison, Burro Canyon, lower and middle Dakota, Mesaverde, Ohio Creek(?), and Ruby formations consist primarily of conglomerates, sandstones, and shales and are mainly continental deposits, whereas the gypsiferous Minturn Formation probably accumulated in restricted basins. The Mancos Group, which overlies the Dakota, is dominated by marine shale and correlates with the Benton, Niobrara, and lower Pierre formations of eastern Colorado. A light-colored petroliferous sandstone unit at the top of the Maroon (restricted) Formation thins southeastward and is correlated with the Schoolhouse Tongue of the Weber Formation. The Schoolhouse Tongue divides into two lobes near Glenwood Springs, with the southern lobe extending southward into the map area. Middle or late Eocene igneous rocks intrude the folded and faulted Permo-Pennsylvanian sediments. Miocene(?) lavas overlie Mesozoic and Cenozoic rocks with angular unconformity. Intense middle to late Eocene compressional forces from the east and northeast developed northwest-trending folds and faults that characterize the southern Grand Hogback area. Late Eocene uplift to the north in the White River Plateau resulted in north- to northeast-trending faults that truncate the earlier features, indicating two major crustal movements during late Laramide time. Redefinition of the Grand Hogback is deemed necessary, as it is genetically related to regional tectonics and therefore should be considered a structural feature rather than strictly topographic, as usually implied. The southern Grand Hogback fold, which is the northern continuation of the Elk Mountain thrust zone, is believed to be draped over the thrust fault. Economic deposits are discussed to some extent, with emphasis placed on the Mesaverde coal deposits, which constitute the most important economic product in the area.
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