SHROPSHIRE GEOLOGY DOWN THE AGES
The Anglian glaciation, around 450,000 ya, was the most extensive in the area. The last was the Devensian which began 100,000 ya.
The ice fields covered the whole county with features like the Berwyns, the Longmynd and the Clee Hills sticking up above the ice as ‘Nunataks’.
Not much detail is known about the Anglian glaciation as it was largely covered and obliterated by the Devensian ice. The Devensian ice moved eastwards from Wales meeting Irish Sea ice from the North.
Before the Devensian glaciation the river Severn flowed northwards from Welshpool to the Dee estuary.
The glacial sediments produce a complex layering through a ‘Cold Base Supraglacial System’ where the glacier slides over the ground and a ‘Warm Base System’ which scours and erodes.
The Devensian ice finally retreated about 20,000 ya.
The gorge was originally cut by a sub-glacial river which was much larger than the current river Severn.
The gorge has particularly steep sides (20deg North side and 30 deg South Side). This has resulted in the inherent instability of the gorge as the clay has slowly drained over 1000’s of years and lost strength. Slippage will continue along the whole length of the gorge.
The development of the landscape (prior to man’s influence) is due to several factors:
The development of gorges is not solely due to river erosion as this would tend to produce near vertical sides. Weathering, soil erosion and landslides are equally important.
There is also a controversial idea which affects the overall landscape of the general area of Shropshire and Wales. Although the idea is not generally accepted there is some evidence to support it. The Welsh geologist O T Jones observed in 1951 that the river systems in Wales are radially centred on a ‘high point’ in Anglesey. Brown in 1960 observed that there is a wide elevated plateau covering Central and Southern Wales and the Marches. Other factors include:
The combination of all these factors has led to the suggestion that the region was once over an ancient mantle plume or ‘hot spot’ (now inactive).
Palaeocene Period
Warm, Mediterranean climatic conditions prevailed during the Palaeocene Period; this lasted for about 10 million years [65.5±0.3 to 55.8±0.2 Mya].
Following uplift due to magmatic underplating, the central Wales peneplain as we now know it began to evolve.
Palaeozoic Era
The Late Palaeozoic (~250 Mya) was well understood by the beginning of the 19th century, down to the Carboniferous Coal Measures underlain by Old Red Sandstone.
In 1832 Murchison came to the area looking for evidence of the earlier history and discovered the now famous Ludlow Bone Beds. This was a thin ‘ginger’ layer sandwiched between two quite different types of rock. As well as sea life this band yielded fossils of the earliest signs of life on the land. The fossils were a mix of primitive freshwater fish, centipede type animals and the ‘burnt cornflake’ which was the fossilised remains of a land based arachnid burnt in a fire. This was evidence that plant life must have then existed on land. The bone beds also marked the boundary between the Upper Silurian and Devonian periods.
The bone beds were deduced to have been formed on the shore line of an ancient sea lapping against the Malvern Hills and other high ground in Shropshire like the Long Mynd. This was named the ‘Silurian Sea’ or later the ‘Iapetus Ocean’ and through study of Brachiopod communities was shown to get deeper to the West with Wales being on a continental shelf. The land to the East had the first land based plant and animal life being established some 420 Mya.
Shropshire life in the Lower Palaeozoic sea was full of predators and hard shelled creatures as well as algae. It was a shallow sea on the edge of a major continent to the East, with the Iapetus Ocean to the West rather like today’s Red Sea.
The oldest known British Trilobite (Olenellus) found in Comely Quarry in 1878 dating from the early Cambrian (~540 Mya)
This area of Shropshire had attracted many geological studies over the last two centuries. It is now internationally recognised as being an important area for both teaching and research.
Recent Research
Recent work on the Longmynd has revealed evidence of even earlier forms of life dating back to the Precambrian. These are body fossils of primitive multicellular organisms (Beltanelliformis brunsae) which are very difficult to see and have been mistaken for imprints of raindrops!
Shropshire’s oldest rock outcrop is of Rushton Schist on the bridleway between Rushton and Uppington. It was exposed through vehicular erosion and is thought to have originally been subducted mudstone subsequently regionally metamorphosed to a medium/high grade about 670 Mya.
Dick Harris