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GNOSALL PARISH NEWS

Produced by publishers of The Popular Village Monthly

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The Solid Geology of Gnosall

h2.jpgSolid geology refers to the underlying rocks. At Gnosall these are sandstones and mudstones, up to 250 million years old. Superficial geology refers to the unconsolidated surface deposits left behind by ice, glaciers and rivers. At Gnosall they are sands, gravels and silts. They were formed in the last 2.6 million years and mostly in the last 18,000 years.

In the Carboniferous period, the area now recognised as Britain lay close to the equator. Rocks formed in the earlier Cambrian period stretched across what is now mid-Wales, the Midlands and East Anglia. Gnosall was close to its northern shore.In the later Carboniferous period, swampy forests covered most of Britain including the Midlands. They were periodically inundated by sea water forming Coal Measures. So, deep beneath Gnosall there are Coal Measures sitting on top of lower Palaeozoic rocks.

In the succeeding Permian period, the area that was to become the Midlands was part of a large continental basin. Its latitude was between 15o and 20o north of the equator, similar to the Sahara Desert today. Few sediments were deposited in this environment and the Permian period is not represented in the sedimentary sequence of Gnosall.During the Permian period, crustal tensions formed a down-faulted rift valley system extending north from Worcestershire and opening out into sedimentary ‘troughs’ in Staffordshire and Cheshire.

By the beginning of the Triassic period, mountains which had formed to the south were in a climatic zone that attracted seasonal rainfall. Rivers started to flow north through the Worcestershire rift valley and periodic floods washed large quantities of pebbles, sands and silts into the Staffordshire basin. Subsidence in the basin allowed sediments to accumulate to great thicknesses, formerly known as the Bunter Pebble Beds, now the Kidderminster Formation. At Gnosall they are buried beneath later sediments, but outcrop to the east at Stafford and to the west at Newport.

A quieter episode followed of temporary or seasonal lakes and rivers, flowing north into the Staffordshire basin. Fine, well-sorted sandstones were deposited with some bands of mudstone, left behind by overbank floods. There were occasional dry periods when the rivers dried out and were overwhelmed by wind-blown sand. These sediments are the Wildmoor Formation and outcrop on the west side of Gnosall. They are poorly cemented and therefore easily eroded.

The subsequent Bromsgrove sandstones, which were deposited on top of the Wildmoor Sandstones, were formed in a similar environment. Bromsgrove sandstone is well-cemented, making it resistant to erosion. Figure 1 shows the outcrop of the sandstones.

 

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