File:Concretion in the Dakota Sandstone (Lower Cretaceous) (Dinosaur Ridge, Colorado, USA).jpg

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English: Concretion in structurally tilted Dakota Sandstone in the Cretaceous of Colorado, USA.

The outcrop shown above is part of Dinosaur Ridge (a section of the Dakota Hogback), which is a north-south trending ridge of eastward-dipping Mesozoic sedimentary rocks. Triassic, Jurassic, and Cretaceous rocks are present, tilted by Laramide Orogeny uplift during the Cenozoic (the Front Range of the Colorado Rocky Mountains is immediately west of here).

This exposure consists of structurally tilted beds of the Dakota Sandstone, a Lower Cretaceous succession of nearshore terrestrial to intertidal to shallow marine quartz sandstone deposits. Dinosaur tracks are common on some Dakota Sandstone bedding planes in this area.

The rounded structure shown above is a concretion, which is a post-depositional structure found in some sedimentary successions. Concretions vary in mineralogy, size, and shape, but often have slightly flattened spheroidal shapes. They typically form by post-depositional mineralization around some nucleus (a rock fragment or fossil fragment).

Stratigraphy: Dakota Sandstone, Albian Stage, upper Lower Cretaceous

Locality: crest of Dinosaur Ridge, between Interstate 70 and the town of Morrison, west of Denver, north-central Colorado, USA
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Author James St. John

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