Geological History of English Bicknor by Dave Green

The Geological History of the area around English Bicknor: development of its rocks and landscape

Dave Green

Meeting of 11 May 2017

In geological terms, our area does not go back very far – only 360 million years. It sounds a long time, but Earth is 4567 million years old. The last geological episode to shape our landscape was 300 million years ago; changes in the way it looks since then are historical (and mostly manmade). Humans did not arrive until around 750,000 years ago.

Bicknor stands on a ridge above the Wye Valley. The area to the northwest is on old red sandstone (like the building stones in Ross) and because this erodes easily the landscape is very gentle (as you see in the view from Symonds Yat Rock over the Hereford Plain, in contrast to out rockier and steeper landscape). Above the sandstone is a layer of quartz conglomerate; outcrops can be seen on Huntsham Hill. This is very hard and in very thick layers with a few cracks, so gives the steep slopes which mark the boundary into our area. This rock is also known as pudding stone, and was used to make cider mill stones.

Above the Quartz Conglomerate lies the Tintern sandstone, which weathers easily and forms fairly gentle slopes. Above this is the Lower Limestone Shales, towards Eastbach and Stowfield, overlain by the Carboniferous Limestone which outcrops at Yat Rock, Rosemary Topping and Bicknor church. On top of these we have the Coal Measures, comprising the Trenchard Measures – heavy clays on the gentle slope leading south from the church to Dryslade Farm. Then we have the Coleford High Delf coal seam and resistant Pennant sandstone layers forming Chapel Hill where there were several mines all around its outcrop, finishing at the outlier of the Coleford High Delf seam near Holly Barn; The Carboniferous Limestone (Lower Dolomite division) was quarried and burnt for quicklime around Rosemary Topping, as were limestones in the Limestone Shale towards Eastbach – which was extensively quarried for building stone. The Coleford High Delf is too deep in the centre of the Forest to be reached – so it is mined in our area where it is closer to the surface and reached by mine shafts and levels (which go sideways into hillsides). Whereabouts these rocks appear at the surface depends on how the layers lie – horizontal, folded, faulted and tilted

The Earth changed dramatically over time; we are currently in an ice age, but for most of Earth’s history there has been no ice at the poles. The earliest of our rocks (the Old Red Sandstone) were laid down by rivers in arid, semi desert conditions south of the Equator; the limestones at the bottom of a shallow tropical sea; the sandstone was then formed in a muddy swampy river delta as world water levels changed due to an ice age 320 million years ago. The coal measures formed from swamped tropical forests as England was then part of the large land mass on the equator.

We can tell how the land masses broke up and moved by examining the magnetite (lodestone) present in some rocks – as the rock is formed, these tiny metallic particles line up with the earth’s magnetic field; these can be matched up with magnetite patterns elsewhere. (Magnetite is seen as the tiny black particles that form lines on sand). Fossils also are used – they show what the environment was like at the time the rock was laid down. Collisions between the moving land masses pushed up mountains. These then erode – so we can roughly date how long mountain ranges have been there and when collisions took place (the Himalayas will probably only last another 50 million years!).

It is believed that the strange meanderings of the River Wye are actually caused by an uplift of the land caused by uprising hot material below the crust – but without volcanic action at the surface. Before this the river flowed and meandered across a flat plain close to sea level with all the rock layers smoothed off; then the uplift of about 200m meant the river cut its way back down through all the different rock types, maintaining its original course – ignoring their different hardnesses – a case of “superimposed drainage.” CS