Ice action in the Brecon Beacons:
Craig cerrig gleisiad Nature Reserve.
1. General view of Craig cerrig gleisiad cirque (corrie) from the north, in late afternoon. Contrast the steep rocky glacial slopes in the cirque with the smooth solifluction slope in the foreground, and note the deep shadows. It dates from the Mountain Readvance (Windermere Stadial) 12 ooo yrs BP.
2. Vertical aerial image of the upper Tarell valley. Craig cerrig gleisiad is the horseshoe on the left, Y Gyrn is the J-shaped hollow in the centre, and Llyn Cwm Llwch glacial lake the spot on the right. The Afon Tarell meanders its way northwards from the Storey Arms (just off the bottom of the picture), with the A47o main road parallel to it. Note the contrasts in land use between the Tarell trough and the surrounding moorland. (For scale, the area covered is approximately 5km by 3km.)
3. Craig cerrig backwall, with gulleys and a post glacial debris fan. Ravens, ring ouzels, red kites, and peregrine falcons have been seen here, and rare arctic saxifrages grow on ledges inaccessible to grazing sheep (and collectors!) - descendents of the plants that colonised the cirque as the ice melted. Freeze-thaw is very active on these north-facing slopes. Note the southerly dip of the Old Red Sandstone strata, bounded by fault lines that helped initiate the gullies.
4 a and b. The cirque backwall and moraine, (a) in spring, and (b) in summer. Snow is swept over the 630 m high summit ridge by westerly winds, and collects in the sheltered lee hollows (some 200 m lower). The steep slope here faces north and north-east, so there is little sun, and winter snow patches sometimes accumulate and linger through spring and even into early summer. In the Pleistocene sufficient thickness of snow accumumulated to become ice, which pulled away from the face, moved downslope, and deposited the stones it had quarried, plucked and scraped from the back of the steepening hollow to form the prominent moraine in the foreground. Note the rock outcrops scraped bare originally by ice, and nowadays reactivated by freeze-thaw weathering; also the bouldery and hummocky surface of the moraine.
5. Upper backwall and post-glacial debris slide (from the north west limb). The debris slope probably began as a rocky scree in glacial times, but it has been subsequently modified by weathering and mass movements, and recent plant colonisation. Note the three different angles on the slope profile nearest us: a steep free face at the top, the constant (talus) slope below it, and the lower creep and wash slope bottom left.
6. Sketch map showing principal glacial landforms. P = a former glacial lake, contained within small arcuate morainic ridges Y; it has been colonised by marsh vegetation which has turned to peat over many centuries. X is the main cirque moraine. W = a lobe of ground moraine forming a slightly hummocky low plateau; it is bounded by small lateral moraines which are given emphasis by two incised streams alongside them. Z = a steep irregular slope consisting of part moraine, part freeze-thaw debris, and part landslip. The 'backward tilted blocks' are partial landslips, and the 'tension cracks' are large incipient slip planes resulting from the weight of ice formerly frozen to the cirque backwall.
7. North-facing backwall and moraines overlooking the Tarell valley, with Y gyrn and Pen y fan beyond. The ridge in the foreground is the main cirque moraine; the hummocky ground beyond it to the left is ground moraine left by the glacier as it advanced into the main valley. It is bounded along the far edge by an elongated ridge of lateral moraine, becoming a terminal moraine on the left where it spilled over into the valley. On the far side of the Tarell valley there is a hollow on the flank of Y gyrn enlarged by nivation - a combination of weathering, freeze-thaw and meltwater action under a large snow patch - a preliminary stage of cirque formation (see also photo 8 below).
8. Looking east to Y gyrn nivation hollow in snow (across the Tarell Valley, with Pen y fan on the skyline). Y gyrn never became a fully-fledged cirque because it faced west and north-west across an open valley, so the prevailing wind tended to blow snow up over the rounded summit and down into Cwm llwch on the other side, rather than allowing it to build up year by year in the hollow. It is also at a lower altitude than Craig cerrig gleisiad, and therefore slightly less cold. Note that the hollow has a thinner covering of snow than the slopes around it in this recent picture.
9. Composite view north-east down the Craig cerrig cirque valley, from the backwall to its junction with the Tarell. At its peak the cirque glacier was joined by ice from Craig y fro and Storey Arms (up valley to the right). The augmented valley glacier then carved a typical U-shaped trough on its way to meet the main Usk glacier moving west to east in the background. In the immediate foreground is the main cirque moraine; just right of centre is the hummocky ground moraine plateau, bounded on either side by ridges of lateral moraine; where the Craig cerrig glacier descended into the Tarell trough you may be able to make out a terminal moraine on the lip. On the far side of the Tarell trough, on the right as we look, a shadow marks a former valley spur truncated by the ice at its maximum thickness of about 300 metres.