Ben Lugmore's geology is what is known as the Mweelrea Formation, and is very different from that of the Twelve Bens, on the other side of Killary Harbour. At a summary level, the Mweelrea Formation consists of Ordovician period sandstones originally deposited on large alluvial fans, and distally‐equivalent alluvial plains and delta fans. Interbedded with these sandstones are tufts, being ash deposits from Ordovician period volcanos.