Interfluvial drainage on Siluro-Devonian alluvial plains in Wales and the Welsh Borders

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Abstract
Type A intraformational conglomerates are associated with relatively thick quartzose channel sandstones and include a matrix of quartzose sandstone accompanied at some stratigraphical levels by exotic pebbles and cobbles. They are related to the major streams that entered from distant uplands and drained across the Lower Old Red Sandstone alluvial plains. Type B intraformational conglomerates, frequently interbedded with mudstones and pebbly mudstones, lack a matrix of quartzose sandstone and other exotic elements, and consist wholly of debris (calcrete and mudstone fragments) whose immediate origin was within the bounds of the Old Red Sandstone depositional basin. Erosively overlying calcretes, interpreted as formed on temporarily stabilized geomorphic surfaces, they represent minor systems of ephemeral drainage developed from time to time on broad and probably slightly elevated interfluves situated between the then envalleyed principal rivers.