Concretions are a common occurrence in the Silurian mudstones of central Wales. The concretions in this slab of mudstone were collected from the Waen Trefeglwys, here in mid -Wales. They are of the type known as "cone in cone". A cross section would show cone like structures growing from opposite sides, and interlocking. The inset picture is to illustrate the tops of the cones. These concretions, because of their calcite content, and construction, are very hard, and resistant to weathering, and for this reason are littered on shorelines, and in stream beds, long after the host mudrock has disintegrated. See Tony's article on cocretions in Silurian magazine No. 1 which is on this website.