File:Striated cone-in-cone limestone (likely an interbed from the Ohio Shale, Upper Devonian; creek bed of an Alum Creek tributary, northern Delaware County, Ohio, USA) 1 (16986580988).jpg

Striated cone-in-cone limestone (bedding plane view) from the Devonian of Ohio. (sample is ~9.05 centimeters along its base)

Cone-in-cone structures are strange features found as interbeds in fine-grained siliciclastic rocks and sometimes found 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.

The cone-in-cone limestone shown above has fine striations on its bedding plane. These may be Late Pleistocene glacial striations (likely) or fault plane striations (?).

Stratigraphy: likely derived from an interbed in the Ohio Shale, Upper Devonian

Locality: creek bed of a tributary to Alum Creek, between the towns of Kilbourne and Ashley, northern Delaware County, Ohio, USA

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.