Sut ffurfiwyd tirwedd Mynyddoedd Cambria
Mid-Wales was a pioneer region in 19th century geology as it was here that geologists first worked out how to disentangle the history of large thicknesses of folded and faulted sedimentary rocks with few internal markers and sparse fossil content. Fossils from Mid Wales dating from the Silurian period helped more than a century ago to unravel the time sequence of rocks in Wales and to establish principles now applied globally. Given this historical foundation, the area is still of great importance for ongoing geological research, as well as for teaching geology.
The rocks of the Cambrian Mountains were laid down 500-400 million years ago, and consist of marine sedimentary rocks, which were later deformed by earth movements, resulting in the folding of the rock. This folding can be clearly appreciated on the ground at Pumlumon, for example. Older Ordovician rocks outcrop along the south-eastern edge of the Cambrian Mountains, whilst the rest of the area is composed mainly of Silurian rocks. (The geological era ‘Ordovician’ takes its names from a tribe who lived in what is now North Powys, whilst the ‘Silurian’ is named after a tribe of mid-Wales).
Features of particular geological interest which were identified in the Cambrian Mountains include:
- the importance of graptolites in the faunal subdivision and correlation of these very largely deep water Palaeozoic strata (eg classic section in Rheidol gorge at Pont Erwyd);
- one of the earliest recognitions of a ‘fossil’ submarine channel (the Caban Conglomerate at Elan Valley)
- the importance of tectonic processes (now known to be plate driven) in the accommodation of huge thicknesses of sediment (the concept of ‘geosynclines’) in what is now termed the Welsh Basin;
- recognition of the importance of ‘turbidite’ sandstones (gravity driven sediment flows, seen for example at the Teifi Lakes, Moel Bentwrch and the Pysgotwr country) in the deep water facies of the Basin fill;
- the late Ordovician sediments (especially at Punlumon) contain the record of the Hirnantian ice age in S Britain; and
- the variably oxic / anoxic facies of the Silurian have become of importance for the study of ancient climate and sea level change.
The mineral veins (lead, zinc, copper and silver) of North Ceredigion and North-West Powys were formed 390 to 220 million years ago, during phases of hydrothermal activity.
The most significant events to affect the landscape were a series of ice ages. The first of these started around 2.4 million years ago, but the most recent ended only 12,000 years ago. During these periods, the Cambrian Mountains were covered by a deep sheet of ice, several thousand feet thick, large ice-caps formed on the Welsh mountains, and glaciers occupied the valleys. The glaciers carved deeply into the rocks to give the landscape its now familiar appearance. Pumlumon is a fine example of a landscape sculpted by glacial ice; Llyn Llygad Rheidol on the north slope of Pumlumon is a moraine-dammed lake occupying a corrie gouged out by ice. The Elan & Claerwen valleys also owe their origins to this period.