The bedrock is Old Red Sandstone (often referred to as the 'ORS') consisting of Upper Silurian strata overlain by the Lower Devonian. In the upper reaches of Cusop is a notable geological horizon known as the Townsend Tuff Bed, which is a volcanic air-fall ash band. Today this is a marker used in the Anglo-Welsh ORS area to divide the Silurian from the Devonian. Previously the calcrete zone "often quarried for limestone" was considered as the boundary between the Silurian and Devonian. These inorganically formed calcrete limestones were formerly known as the Psammosteus Limestones but now known as the Bishops Frome Limestone.