Article: The Llandovery transgression of the Welsh Borderland
The study of the evolution of several brachiopod genera has enabled the early Silurian shelf sequences of the Welsh Borderland to be correlated with the type area of Llandovery, and, to some extent, with the graptolite zonal sequence. The transgression across the Borderland started at the beginning of Llandovery time, when areas in Montgomeryshire became inundated after a short break in deposition at the end of the Ashgill. The sea reached Shropshire, and possibly as far east as the Malverns and May Hill, by Middle Llandovery times. By late Upper Llandovery times, much of the English Midlands were covered.Fossil communities indicate the relative depths in which the Llandovery sediments were deposited. With the advance of the sea, most sequences show a progressive increase of depth with time, although minor reversals are known. The many gaps in the local sequences are probably due to submarine erosion or non-deposition, rather than to uplift and sub-aerial erosion, because they are characteristically followed by progressively deeper-water communities. The community distribution indicates that a continuous gradient was maintained from the coast to the shelf margin, once the topographic relief of the original surface had been filled in.