assigned to the Old Red Sandstone and dating from the Devonian period. This thick sedimentary sequence comprises sandstones, mudstones, siltstones and numerous thin limestones. The exception is the summit area of Pen Cerrig-calch where a thin sequence of Carboniferous rocks occur, an outlier of the more extensive outcrop to the south of the Usk valley. The lower slopes of these hills are formed from the mudstone-rich St Maughans beds at the top of which lies a calcrete – a discontinuous limestone band known as the Ffynnon Limestone. Above this are the sandstone-dominated Senni Beds which form the upper reaches of much of the range. Higher again are the Brownstones which form the summit areas of the central and southern parts of the range. The Old Red Sandstone extends back into the late Silurian period and forward into the earliest part of the Carboniferous period. The body of rock, or facies, is dominated by alluvial sediments and conglomerates at its base, and progresses to a combination of dunes, lakes and river sediments. The familiar red colour of these rocks arises from the presence of iron oxide but not all the Old Red Sandstone is red or sandstone — the sequence also includes conglomerates, mudstones, siltstones and thin limestones and colours can range from grey and green through red to purple. Glacial legacy The area lay at the margins of the British ice-sheet during the ice ages; these hills were shaped by ice from a source in mid-Wales rather than generating any major glaciers of their own. Non-local rock fragments within the glacial till show that Wye valley ice penetrated the Rhiangoll valley from the north, moving over the low col at Pengenffordd. No such evidence has been found in the Vale of Ewyas though the profile of this valley strongly suggests the presence of a major glacier. The valleys of the Grwyne Fawr and Grwyne Fechan were probably ice-free during the last ice age. One result of the over-steepening of valley sides by glacial action is the suite of landslips affecting the range, notably in the Vale of Ewyas. The most impressive of all is that at Darren and Cwmyoy. Another impressive set of landslip forms can be seen at Black Darren and Red Darren ('Darren' signifies 'edge' in Welsh) on the eastern side of the Hatterrall ridge west of Longtown. Another, at the northern end of the Skirrid just to