Igneous intrusions outcropping at the surface in South Shropshire are few and small, but much larger bodies are believed to exist at shallow depths, evidenced by geophysical anomalies and radon-bearing groundwater. Several sills of dolerite intrude the Clee Hills - Brown Clee, Titterstone Clee and Clee Hill itself. These are some of the highest points in Shropshire, and serve as outcrops for Old Red Sandstone, and also various Coal Measures and limestone. Indeed, Brown Clee is considered to be one of the best exposures of Old Red Sandstone that exists. South Shropshire was on the border of the Old Red Sandstone continent, explaining why it appears so prominently in the Clee area.