Near Fort Collins, Colo., the Lower Cretaceous Muddy (or 'J') Sandstone can be subdivided into two distinct lithogenetic units, herein named the Fort Collins and Horsetooth Members. The lower unit, the Fort Collins Member, is interpreted as a nearshore marine sandstone deposited in a late stage of shoaling. The Horsetooth Member overlies the Fort Collins with a knife-edge contact. It is tentatively interpreted as part of a shoal-water delta (or possibly modified alluvial plain) assemblage. The widespread disconformity separating the sandstone members and the scarcity of mudstone in supposed shoal-water deltaic complexes pose serious interpretative problems.

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