The Higher Ludlovian Rocks of the Built District

The rocks to be described in this paper occupy an area of about 22 square miles in the counties of Radnorshire and Breconshire, which includes the Wye valley between Builth and Erwood station and the hilly ground south of Builth. It is represented on the following six-inch Ordnance Survey maps: Brecknock XI N.E. (Radnor XXXI N.E.), XI S.E., XII N.W. (Radnor XXXII N.W.), XII S.W. (Radnor XXXII S.W.), XV N.E., and XVI N.W. (Radnor XXXV N.W.). The latest geological maps are those of the Geological Survey, Old Series, No. 56 S.W. (1850) and 42 N.W. (1845, with additional Silurian hues in 1855 and 1856). The strata form an apparently continuous succession, mainly of shelly mudstones and shales, between the graptolitic shales of the zones of Monograptus nilssoni and M. scanicus and the unfossiliferous green marls at the base of the Old Red Sandstone. The faunal succession remains constant throughout the limited area under consideration, but the stratigraphical distribution of many of the common species differs from that of the classical region of Shropshire. The absence of the Aymestry Limestone and such characteristic members of its fauna as Conchidium knighti and Strophonella euglypha is a further point of difference. In view of such differences it has not been possible to adopt in its entirety the well-known classification proposed for rocks of the Ludlow district (Elles and Slater 1906, p. 198). There is in central Wales, however, such an extensive development of Ludlow sediments of the type displayed to good advantage in