The crag, just like the neighbouring Peckforton Hills, is part of a thicker sequence known as the New Red Sandstone. It was formed from easterly dipping layers of Triassic sandstone. The lower slopes are formed from Wilmslow Sandstone Formation while the upper strata are part of the Helsby Sandstone Formation which is around 245 million years old. The hillock is capped by a small outcrop of sandstones assigned to the Tarporley Siltstone Formation (and formerly known as the Keuper Waterstones). Both types of sandstones were once quarried at multiple sites within the castle grounds.