by vertical stone slabs near the centre of the barrow, and it is currently occupied by the National Trust sign, but will have originally held the ashes or other remnants of a dead person or persons since multiple burials together are common in the British Bronze Age. It also held grave goods left with the human remains, such as flint tools, cinerary urns, or flower tributes. The similar round barrow on Fan Foel was excavated in 2002-4 and revealed such items in the central cist, the flowers being those of meadowsweet (Filipendula ulmaria). Geology Pen y Fan is formed from various units of the Old Red Sandstone which were laid down during the Devonian period. The lower slopes of the mountain are formed from the sandstones and mudstones of the Senni Formation whilst the upper slopes are formed from those of the Brownstones Formation. The very summit, like that of neighbouring Corn Du, is composed of hard-wearing sandstones of the Plateau Beds Formation. The beds of sandstone dip gently southwards towards the South Wales Coalfield basin. The Brecon Beacons nurtured their own glaciers during the last ice age and both Cwm Llwch and Cwm Sere were excavated by glaciers which grew in the lee of the high ground. Llyn Cwm Llwch occupies a glacially excavated rock hollow and the lake is partly dammed by a late-glacial moraine. The mountain is the highest point within the Fforest Fawr Geopark which was established in 2005 to promote economic development, particularly sustainable tourism, based on the area's geological heritage. Tommy Jones' Obelisk Near the route from the Storey Arms (once a coaching inn on the Brecon to Merthyr road) to the summit is a memorial granite obelisk, with an inscription which reads: "This obelisk marks the spot where the body of Tommy Jones aged 5 was found. He lost his way between Cwm Llwch Farm and the Login on the night of August 4, 1900. After an anxious search of 29 days his remains were discovered Sept [2nd]. Erected by voluntary subscriptions. W Powell Price Mayor of Brecon 1901." Tommy was the son of a Rhondda coal miner, and had been visiting his grandparents at the time. He died from exhaustion and hypothermia or exposure. How a small boy managed to climb so high remains a mystery. The obelisk, a useful landmark in misty conditions, was paid for by the proceeds of a