Garreg Las is formed from the Twrch Sandstone, a coarse sandstone (formerly known as the Basal Grit) of the Marros Group (formerly the Millstone Grit Series) which was laid down during the Carboniferous period. Wrapped around its northern and western flanks is a narrow band of Carboniferous Limestone which gives rise to a karst landscape. The Twrch Fechan Fault defines its eastern margin and brings Old Red Sandstone rocks into contact with the limestone on this flank. A number of major shakeholes in the Basal Grit adorn these eastern flanks, one of which, known as 'Pwll Cynrig' is flooded.