The top diagram illustrates the elevation of the area, Worcestershire plain of today, by Variscan orogenic crustal shortening. The whole area covered by vast Bahada and undergoing extensive erosion of higher terrain. Cambrian, Silurian, Devonian and earlier Carboniferous rocks would have capped the elevated area. By the time depicted in the diagram these rocks have already been largely eroded away.
The lower diagram depicts a later time when crustal relaxation has set-in, resulting in the formation of the Worcester graben, inverting the topography of the earlier time.
In the later Carboniferous the compressive forces of the Variscan Orogeny uplifted an extensive horst block. The western side of this feature was related to multiple westerly thrusts in the Malverns to Martley area. A second set of thrusts with eastward directed motion was present in the Alcester to Stratford upon Avon region, The horst block would probably have included Cambrian, Silurian, Devonian and earlier Carboniferous rocks, all of which underwent severe erosion in the harsh climate of the late Carboniferous into early Permian.
By Permian, the crust of Britain ‘relaxes’ from the Variscan orogenic compression and extensional (pulling apart) forces dominate. In response, the thrust faults that mark the western and eastern boundaries of the horst block, reverse direction of movement and the area switches from being one of uplift to one of depression, forming the so called Worcestershire graben. A graben is opposite to a horst.
Rocks of the Permian (e,g. the Bridgnorth Sandstone) and rocks of the Triassic (e,g, Helby Sandstone) are deposited in the graben.