The summit of Pen Allt-mawr is at the northern tip of a broadly triangular summit surface formed from a slab of southerly tilting upper Old Red Sandstone traditionally known as the Quartz Conglomerate. These rocks which date from the Late Devonian Epoch are continuous with those of Pen Cerrig-calch but other than a tiny outlier forming the summit of Sugar Loaf, are found nowhere else within the Black Mountains. Beneath the Quartz Conglomerate are the Early Devonian sandstones of the Brownstones Formation which form the upper slopes and summits of Pen Twyn Glas and Mynydd Llysiau to the north (though are barely seen). More readily eroded mudstone bands within this formation give rise to the stepped appearance of this hill and many of its neighbours. Sandstones of the Senni Formation (traditionally the 'Senni Beds') form the lower slopes of the mountain.