Page:VCH Herefordshire 1.djvu/45

 GEOLOGY HEREFORDSHIRE is 40 miles in length and 35 in breadth. Of its surface-extent of 833 square miles, four-fifths is occupied by the Old Red Sandstone. Obviously then, this is the rock- formation of Herefordshire par excellence. The Old Red Sandstone has a general synclinal arrangement, rocks of greater antiquity rising up from beneath it in places on the east and north- west. This syncline is in plan .^-^-shaped ; the limbs converging to the north (conjoining in southern Shropshire), and diverging to the south. The western one runs obliquely through the north-west part of Herefordshire, Brecknockshire, Carmarthenshire, and Pembrokeshire ; the eastern one, due south through east Herefordshire. The rocks which floor the greater part of Herefordshire north of the escarpment of the Black Mountains and the hills of the Forest of Dean to the south of Ross, belong to the lower division of the Old Red Sandstone, the higher beds having been removed by denudation. The great Herefordshire syncline has a southerly tilt. Now it is easy to see that if the synclinal arrangement and the southerly tilt alone affected the distribution of the Old Red rocks, the higher beds of that system would extend uninterruptedly across the southern end of the county — except, of course, where the rivers had cut gorges. But they do not extend all across the southern portion, because the ,„*^^-shaped syncline has been divided into two by a puckering at Usk in Monmouthshire, where rocks older than the Old Red Sandstone, namely, the Silurian, show through, forming an inlier, thus — ^x^. On the west side of this inlier lies the great South Wales coalfield ; on the east, the smaller, but no less perfect, Forest of Dean coalfield. It is the Carboniferous Limestone of a portion of the north-western fringe of this latter basin that gives rise to the well-known scenery of the neighbourhood of Symond's Yat, where precipitous cliffs overlook the meandering Wye. Not only is the broad southern end of the syncline divided into two by the Usk inlier, but the perfection of the syncline is marred farther to the north by several other inliers of Silurian Beds. Such are Shucknall Hill, Hagley, and the district known to geologists as ' The Woolhope ' — a portion of the outer rim of which is that high ground visible from Hereford to the east. Since all these beds are sedimentary formations, and must therefore have been laid down more or less horizontally, it is evident that at some time subsequent to their formation they must have been much flexured. Crust- pressures were the cause of this flexuring, and Professor T. T. Groom has satisfactorily shown that it took place in that comparatively short interval which intervened between the time of formation of the older and the Upper Coal Measures. In that interval the rocks of Herefordshire, in common with those of most parts of the world, were greatly flexured. Folds, which