The development of the Towy and Upper Usk drainage pattern

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Abstract
I. T he G eneral F orm of the T owy D rainage P attern T he Towy, like the Teifi, Tawe, and Neath rivers, its sub-parallel neighbours in South Wales, is markedly asymmetrical, and is fed by tributaries most of which flow southwards and enter on the right or north bank. The great majority of the tributaries, and much of the Towy itself above Llandovery, follow courses which are not geologically controlled but which transgress the caledonid structures of the Palaeozoic rocks in a manner characteristically epigenetic : the associated major land surface is a composite plateau also truncating the strata without relation to the structures. Along one or two reaches, however, the tectonic trend has clearly been instrumental in diverting the streams from a rectilinear course (Dewhirst 1930). This has happened, for example, between Bwlch-v-rhiw and Brechfa (O. T. Jones 1912, p. 355; 1924, p. 583; 1930, p. 79), where the Cothi is obviously a longitudinal stream working along the structures and the softer rocks of the Cothi-Rhiwnant anticline (see Drew and Slater 1910, pi. xxix; Davies 1933, pi. xvii). Such tectonic control attains its greatest expression in the Lower Towy valley, which, from Llandovery to Carmarthen, follows almost exactly the core of the complex Towy anticline, and which manifestly owes its present form to relatively rapid erosion along a belt of structural weakness (Fig. 1). Significantly enough, several of the larger tributaries of the north bank, notably the Cothi and the Llandeilo Dulais, enter the Towy opposite deep cols on the south bank, and there is a strong topographical

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