English: Shale & concretionary cone-in-cone structures in the Cretaceous of Colorado, USA. (cross-section view)

Cone-in-cone structures are strange features found as interbeds in fine-grained siliciclastic rocks or in the outer layers of concretions. They consist of sets of conical structures stacked up within each other. Mineralogically, they are often composed of finely-crystalline fibrous calcite. Numerous hypotheses have been proposed over the years that try to explain cone-in-cone structures. None has emerged as the most popular or most likely (see Lugli et al., 2005 for a long list of proposed formation mechanisms). These structures remain a mystery.

Stratigraphy: concretion in lower Benton Shale, Upper Cretaceous

Locality: slope exposure along the Lower Hogback Trail, Red Rock Canyon Open Space, western side of the town of Colorado Springs, western El Paso County, central Colorado, USA (vicinity of 38° 50’ 59.79” North latitude, 104° 52’ 30.08” West longitude)

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For more info. on cone-in-cone structures, see (as examples):

Melichar & Shkovira (2001) - Case study of the cone-in-cone structure based on Czech and Crimean samples. GeoLines 13.

Lugli et al. (2005) - Silicified cone-in-cone structures from Erfoud (Morocco): a comparison with impact-generated shatter cones. in Impact tectonics. Impact Studies 6: 81-110.