The geology of the Llandovery district: Part II. the Northern Area

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Summary: The area described lies north-east of Llandovery and within about seven miles of that town; it is largely exposed, heather-clad upland. The adjoining area to the south was dealt with by the same author in a paper published in 1925. The three divisions (A) Lower, (B) Middle and (C) Upper Llandovery, established in the south, are recognized in the Northern Area, but here they vary greatly in thickness and some even disappear locally. Along the western boundary the Lower Llandovery rocks are faulted against Upper Bala mudstones with thin grits. Due east of the town only the lower part of the Lower Llandovery occurs between the Western Boundary Fault and the Upper Llandovery, but farther north, in the Crychan valley, the higher parts of the formation appear for a short distance. East of Noethgrug the whole subdivision is overstepped by the Upper Llandovery. The Middle Llandovery first appears in the Crychan valley about three miles northeast of the town, and after trending to the north and then the south disappears under the Upper Llandovery three miles east of the place where it first appears. North of these two points the subdivision is represented probably to its full thickness and in places yields many graptolites. The Upper Llandovery maintains its general characters through Cefn-y-gareg and Noethgrug, but near Craig-wyddon thinning sets in and within about three miles the main development is reduced to only 40 feet of Pentamerus oblongus sandstones. The uppermost part of the formation consists of green mudstones with some thin siliceous bands. The Upper Llandovery below these mudstones near Llandovery consists largely of greenish, sheared-looking mudstones but becomes more sandy to the north-east. It is significant that not only the Upper Llandovery but also the Middle and Lower Llandovery in the Noethgrug area develop coarse sandy and pebbly beds. At the extreme north-east of the district the base of the Upper Llandovery can be seen resting on Upper Bala mudstones. The structure of the area is relatively simple; to the east of the Western Boundary Fault the Lower Llandovery rocks dip with few exceptions very steeply, or are even vertical. In the succeeding Middle and Upper Llandovery of Gefn-y-gareg the dip lessens, and between Cefn-y-gareg and Noethgrug the beds turn over in an impressive pitching syncline. On Noethgrug the rocks dip west at moderate angles, but south of that region they turn over in a broad anticline with a southerly pitch. On the eastern limb of the anticline in the Craig-wyddon ridge they again become vertical. The core of the Noethgrug anticline is occupied by Upper Bala mudstones which are partially surrounded by a strong escarpment of Lower Llandovery rocks. The dips in the folds suggest a push from the north-west.

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