Feet of Clay ?

Clay is a fine-grained natural rock or soil material

Clay is a fine-grained natural rock or soil material that combines one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure. Clays are plastic due to that water content and become hard, brittle and non–plastic upon drying or firing.[1][2][3] Depending on the soil's content in which it is found, clay can appear in various colours from white to dull grey or brown to deep orange-red.