The Geology of Pontesford Hill. By PeterToghill, President Shropshire Geological Society


The geological history of Pontesford Hill is essentially the same as its slightly higher neighbour Earl’s Hill.  Both hills lie at the northern end of the geologically famous Pontesford- Linley Fault, a break in the Earth’s crust which runs south from Pontesford towards Linley, Bishops Castle and Clun. The fault line was the site of the famous Bishop’s Castle earthquake of April 1990. In the geological past earthquakes in this area would have been far more intense than those of recent times.


Just north of Habberley the fault line breaks in to two branches widening to about 1km apart and then joining up again at Pontesford. It is between these two branches of the fault that Earl’s Hill and Pontesford Hill, occur formed by an elongated outcrop of ancient rocks. Both hills are formed of ancient volcanic lavas and ashes formed during the late Precambrian period around 570 to 550 million years ago. Apart from a very small area of ancient metamorphic rocks near The Wrekin, these rocks are the oldest occurring in Shropshire. They are called the Uriconian Volcanics, named after the Roman city of Uriconium. South east of the two hills is another area of late Precambrian rocks, part of the Longmyndian sequence of sedimentary rocks, which merges into the larger area of the Long Mynd itself.


The volcanic lavas and ashes, which occupy the northern slopes of Pontesford Hill, belong mainly to a group called rhyolites, although basalt lavas occur, as well as coarse volcanic fragmentary rocks which may have been laid down under water. The rocks have a layered structure, the layers being steeply inclined (dipping) to the south east. The rhyolites, erupted on land, are very fine grained, almost glassy, usually reddish brown or a dark pink colour, and show marked banding where the lava has been streaked out by flowing like a stiff toffee. Some of the rhyolites are nodular and contain tiny glass spheres called spherulites. The lavas which were erupted from ancient volcanoes, were derived from very sticky magma which often blocked up the volcanic vents and caused violent explosions.

The higher parts of the hill are formed of basalts and dolerites which were pushed into the volcanic rocks (intruded) in a molten state just after the volcanic lavas were formed in the late Precambrian.


The ancient volcanoes erupted on arid landscape which in later geological periods was covered by the sea to give marine deposits. The earliest marine deposits in the area belong to the Ordovician period and are around 500 – 450 million years old. No rocks of Cambrian age occur in the area. The early Ordovician rocks called the Shineton Shales, or locally the Habberley Shales, occur on the west side of Pontesford Hill, and the slightly later Ordovician Pontesford Shales occur to the east of the hill, in the deep valley of the Habberley Brook. It is thought that the Pontesford-Linley Fault was actually the shoreline between a deep ocean to the West and a landmass to the east during the early Ordovician period in Shropshire, because the great sequence of Ordovician rocks found further south around the Stiperstones-Shelve area is not found east of the fault.

Rocks of the famous Silurian period, as found on Wenlock Edge, are not found in this area nor are those belonging to the Devonian period. But just to the north of the Hill are important exposures of the Upper Coal Measures of the Carboniferous period formed around 300 million years ago. Coal seams in these rocks were mined for coal around Pontesford. The coal was mainly used for smelting lead ore brought down from the lead mines further south around Snailbeach.


Although these hills are made of very ancient rocks the actual shape of the hills themselves did not start to appear until more recent geological times, maybe in the last 30 million years when erosion of the British Isles stripped away many of the younger rocks to expose the older rocks underneath.  The Ice Age which started about 2 million years ago, produced  a number of advances of ice sheets over the Shropshire area from around 300,000 years ago, and these had important effects on the Shropshire landscape. A glacier came down the Rea Valley about 25,000 years ago and covered the lowland areas as an ice sheet, but did not cover the hills themselves. The landscape was not actually shaped by ice action but melt waters from the ice sheets eroded glacial overflow channels and deposited great thicknesses of sand, clay and gravel to the north of Pontesford. Ice sheets also blocked off river channels and caused diversions. The Habberley Brook used to flow to the west of Pontesford Hill but masses of ice coming down the Rea valley caused a lake to form south of Earl’s Hill, which then overflowed north east to cut the deep valley to the east of Pontesford Hill in which the river now flows.