page_title: Mynydd Illtud
section_title: Geology
hierarchical_section_title: Mynydd Illtud / Geology
caption_attribution_description: English: Mynydd Illtud, Powys, Wales: Twyn y Gaer triangulation point
context_page_description: Mynydd Illtud is an extensive area of common land near Libanus, Powys, Wales, located in the Brecon Beacons National Park and some three miles south-west of Brecon. The common is an undulating plateau lying between 330 and 370 metres above sea level. Its highest points are 381 metres at Allt Lom and 367 metres at Twyn y Gaer trig point overlooking the valley of the River Usk. Twyn y Gaer is the site of an Iron Age hill fort.
context_section_description: Like Cefn Llechid and Fforest Fach to the west, Mynydd Illtud is formed from the mudstones and sandstones of the St. Maughans Formation of the Old Red Sandstone laid down during the Devonian period. The sandstones have been quarried in places such as at Allt Lom.
  
  The area was overrun by ice during the last ice age and evidence of this is seen in the glacial till which is spread across this undulating plateau and the occasional erratic block of sandstone. Traeth Mawr and Traeth Bach, two rock hollows towards the south-western end of the common are now filled with peat, core sections of which have proved valuable in unravelling the climatic and vegetational history of this area over the last 10,000 years.
caption: The trig point at the Twyn y Gaer hill fort, Mynydd Illtud
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