Like all of the hills in the Black Mountains, Pen Cerrig-calch is formed from mudstones and sandstones of the Old Red Sandstone laid down during the Devonian period but it is unusual in having a band of Carboniferous Limestone outcropping around the summit and giving rise to a few shakeholes. A classic example of an outlier, the name of the hill reflects this situation; it means 'top of the limestone rock' in Welsh. The actual summit itself is formed from pebbly sandstone of the Marros Group (formerly the Millstone Grit Series), which also dates from the Carboniferous period.