Cone-in-cone structure forms in sedimentary rocks composed of limy clay.
In my opinion, they form as a result of movement but that an uneducated
guess. There is no consensus amoung geologist as to how they form.
The first example is from a clay layer below the Dokota Sandstone east of
Horsetooth reservoir west of Fort Collins, CO. It was associated with deformed
Inoceramus and Ostea shells.
The second example was associated with alunite from near Briggsdale, CO.
There are two views, from the side and fromt he top. The second, top view,
shows the typical cone shaped holes that form.
