Abstract Driving north from Denver to Fort Collins, the road skirts the eastern edge of the Front Range of the Southern Rocky Mountains, primarily on lower Tertiary and then upper Cretaceous sedimentary formation of the Denver Basin. To the east lie the Great Plains. The basement rocks exposed in the Front Range are buried by up to 12,000 feet locally along the front. The ranges are comprised of Proterozoic crystalline rocks ranging in age from the 1800Ma Idaho Springs metasediments and metavolcanics, the 1700MaSilverplume granite and the 1400Ma Log Cabin and Sherman granites of the Sherman Batholith. North from Fort Collins toward Laramie the route traverses hogbacks of uplifted Permian Pennsylvanian through lower Cretaceous sedimentary beds, locally traveling down the strike valleys. Road cuts provide excellent exposures of the Jurassic Morrison. Formation, and the Cretaceous Dakota and Ingleside Formations. Quarries developed in these formations produce sandstone for building materials and glass sand, limestone for agricultural and construction use and shales for cement manufacture. The oldest sedimentary rocks of the Permian-Pennsylvanian Fountain Formation unconformably overlie the paleo-erosion surface of the Proterozoic crystalline rocks. This route also follows the old Overland Trail stagecoach route.

Abstract Driving north from Denver to Fort Collins, the road skirts the eastern edge of the Front Range of the Southern Rocky Mountains, primarily on lower Tertiary and then upper Cretaceous sedimentary formation of the Denver Basin. To the east lie the Great Plains. The basement rocks exposed in the Front Range are buried by up to 12,000 feet locally along the front. The ranges are comprised of Proterozoic crystalline rocks ranging in age from the 1800Ma Idaho Springs metasediments and metavolcanics, the 1700MaSilverplume granite and the 1400Ma Log Cabin and Sherman granites of the Sherman Batholith. North from Fort Collins toward Laramie the route traverses hogbacks of uplifted Permian Pennsylvanian through lower Cretaceous sedimentary beds, locally traveling down the strike valleys. Road cuts provide excellent exposures of the Jurassic Morrison. Formation, and the Cretaceous Dakota and Ingleside Formations. Quarries developed in these formations produce sandstone for building materials and glass sand, limestone for agricultural and construction use and shales for cement manufacture. The oldest sedimentary rocks of the Permian-Pennsylvanian Fountain Formation unconformably overlie the paleo-erosion surface of the Proterozoic crystalline rocks. This route also follows the old Overland Trail stagecoach route. In the Livermore area a large open plain named the Livermore Embayment is formed by a graben. The bounding faults, seen on the south, west and north, place the sedimentary units up against the Proterozoic rocks. North of Livermore, the route enters the 1400Ma Sherman Batholith. The principal rock types are the coarse grained pink hornblende granite termed the Sherman Granite. The Sherman exhibits widely spaced joints and rounded weathering surfaces. The tightly jointed blocky white granite is termed the Log Cabin Granite. The route bisects the Virginia Dale Ring Complex, a striking circular feature on aerial photographs mapped by D.H. Eggler (1967). The core is largely Sherman Batholith, while more mafic metamorphic units are common locally in the outer rings. The route follows a long curved valley formed in one of the outer rings. The State Line Kimberlite District consists of Devonian age kimberlite intrusions into the Proterozoic crystalline rocks. Kimberlite emplacement is not restricted or controlled by any specific structural or age unit. M.E. McCallum of Colorado State University identified the first kimberlite in 1965, at the Sloan Ranch in the southern portion of the District. In collaboration with Eggler, several other occurrences were then identified. McCallum and Eggler continue their kimberlite, mantle and diamond

Abstract Currently accepted terminology for Pennsylvanian units in the area includes the Tensleep, Amsden, Quadrant, Minnelusa, Hartville, Casper, Ingleside, and Fountain formations. A review of nomenclatural history, lithologies, and lateral continuity of these units results in elimination of the terms Quadrant, Hartville, Casper, and Ingleside. The Minnelusa is raised to group status and is extended to include the strata formerly designated as Hartville. The Amsden is restricted vertically to the carbonate upper portion of the original formation, and Branson’s term Sacajawea is applied to the clastic formation below. The Ten-sleep, Amsden, and Sacajawea formations are extended laterally to include lithogenetically equivalent strata. Age variations within the units suggest a southern Wyoming land mass connecting with the Black Hills until Morrowan time. Amsden-Sacajawea seas, initiated in Chesterian time to the north and southeast of this land bridge, were later joined. Tensleep sands advanced southward from Montana forcing the Amsden seas to retreat southeastward until overwhelmed by the youngest Tensleep deposits in the Hartville uplift area during Wolfcampian or Leonardian time. Isopach variations are relatively constant from one unit to another, and show tectonically negative areas in the Lusk embayment and southwestern corner of the map area. The northern zero isopach is interpreted as an erosion edge, but areas of thinning on the Montana-Wyoming border, in the Black Hills, and in southern Wyoming are thought to be related to tectonic instability or position of former land masses. A superjacent strata map illustrates post-depositional erosion intervals by showing the age of the overlying formations. Lithofacies analyses reveal a southern and southwestern source for the Tensleep, Amsden, and Sacajawea sediments. Reef-structures are postulated in the Amsden on the basis of lithofacies patterns, and coincide with the margins of evaporite distribution. A lithofacies map of the entire sequence emphasizes the masking effect introduced when this technique is used with large units involving long depositional intervals.