The Valentian succession around Llanidloes, Montgomeryshire

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Abstract
Summary: Llanidloes is situated in the Severn Valley, in the western part of Montgomeryshire, and is surrounded by areas which have been described by other authors. The area of Tarannon (Wood 1906) lies to the north, Ponterwyd (O. T. Jones 1909) to the west, Rhayader (H. Lapworth 1906) to the south, and Abbey Cwmhir (R. O. Roberts 1929) to the south-east. Most of the ground between these areas has now been mapped on the scale of 6 inches to the mile. Very little detailed work has previously been done in this area. Sedgwick amde journeys across it and Murchison makes occasional references to it; Professor O. T. Jones (1922), however, has discussed in detail its ore occurences. The greater part of the country is dissected by the Severn drainage system, and the main sequence is exposed in these streams. The Wye system is confined to the south-west and reveals a few good sections in rock-groups near the top of the Valentian. The average height of the higher land is between 1000 and 1500 feet, but the ground rises to nearly 2000 feet in the extreme east. All the higher ground is pasture, while the lower country of the valleys is usually cultivated. The strata range from Upper Bala to Salopian in age, but only the Valentian rocks are described in detail. Mudstone is the dominant lithological type; intercalations of thin shales occasionally yield graptolites. These organisms are the sole fossils found in the area and are preserved either in full or in half relief; they afford an interesting comparison with the forms preserved in the flattened condition. The fundamental structure of the district'is a dome which has been named the Clywedog Dome, from the river of that name which joins the Severn at Llanidloes. The Central Wales Syncline passes through the area near its western margin and exhibits a culmination of pitch near tne northern margin. The Towy anticlinal axis does not enter the area but runs some three miles to the south-east, near Abbey Cwmhir (R. O. Roberts 1929). Faulting" is an important feature; several of the major faults have been identified as belonging to the group of faults and lodes occurring in the Ystwyth Valley, 4 miles to the west. Strike-faulting is not common, but locally the major faults may behave as strike-faults.

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