The landscape was carved into the shape we see today by a series of ice ages during the last two million years.  Geologists refer to the most recent one as the Devensian Ice Age.  It has left the freshest evidence of its passing in the landscape.

-  map showing distribution of Quaternary deposits within Geopark appearing here soon -

An ice age starts . . .

Snow falling on the flat tops of the hills was blown by the prevailing winds onto their sheltered northern and eastern slopes where it gradually accumulated.  Turning to ice, it eventually began to move downhill under its own weight as a glacier.  At the height of each ice age, the many small glaciers originating in the highest cwms would merge and flow down the main valleys as broad rivers of ice. The Usk Glacier flowed east over Brecon and Abergavenny before heading south as far as the village of Usk.  Ice from the northwestern part of the Geopark flowed southwestwards as the Towy Glacier towards Carmarthen whilst other glaciers flowed down the Tawe and Neath valleys.

. . . then melts away

The glaciers left behind a legacy of till (boulder clay) comprising clay, sand, gravel and boulders of the local rocks eroded from beneath them.  Meltwater from the glaciers carved out channels underneath and downstream from them and formed extensive spreads of sand and gravel across valley floors.

More recently . . .

Geologists refer to the period of time since the last Ice Age as the Holocene, Flandrian or post-glacial.  During this time, rivers such as the Usk and the Towy have cut flood-plains into the glacial debris and deposited alluvium across them.  Peat has accumulated in hollows left in the till as at Traeth Mawr near Libanus and steeply eroded slopes have collapsed en masse leaving rocky scars on hillsides eg the eastern slopes of Fan Frynych.

- - aerial shot of Craig Cerrig-gleisiad appearing here soon - -

What's in a name?

The name of this period belongs to a time in the history of the science when geological time was divided into four great 'orders': the Primary, Secondary, Tertiary and Quaternary.  We no longer use the first two terms but 'Tertiary' and 'Quaternary' are still commonly used.

The geological world is split on the question of when the Quaternary Period started.  Some define it as 1.8 million years ago, others put it at 2.6 million.  It is by far the shortest of all the geological periods but arguably the most important - if only for the fact that we are living in it!

The name 'Devensian' comes indirectly from the Roman name for Chester - 'Dewa' - for it is Cheshire where especially important deposits and landforms dating from this time have been studied in detail.

Further reading

Campbell, S. & Bowen, D.Q. (1989) Quaternary of Wales, Geological Conservation Review Series, No. 1, Nature Conservancy Council, Peterborough, 238pp.
ISBN 0-86139-570-0
Together with an overview of the Welsh Quaternary, the book contains brief articles on four locations within the Geopark: Cwm Llwch, Mynydd Du, Traeth Mawr and Craig Cerrig-gleisiad.

Carr, S.J., Coleman, C.G., Humpage, A.J. & Shakesby, R.A. (eds), (2007) Quaternary of the Brecon Beacons Field Guide, Quaternary Research Association, London 277pp
ISBN 0907-780-725
Published for a 5-day field meet by QRA, this guide covers a host of glacial and post-glacial sites within the Geopark.

Shakesby, R.A. (2002) Glacial landforms of the Brecon Beacons Geographical Association, 48pp

This slim but information-packed book (no 13 in the GA's 'Landform Guides' series) describes the geomorphology of glacial landforms in Y Mynydd Du, Fforest Fawr & the central Beacons.