Geology and Geomorphology at Badock’s Wood

The landscape at Badock’s Wood is quite surprising as within the gently sloping area of Southmead and Westbury on Trym there is a deep gorge with a small tributary gorge coming in from the south. The River Trym and the smaller tributary have cut down through the softer overlying rocks into the much harder limestone. It is puzzling to know where the rivers, which are really small streams, obtained enough energy to cut these deep valleys. They are not unique. Many similar gorges can be found in the Bristol district and the River Trym with another tributary, the Hazel Brook, form even bigger and deeper gorges downstream.

There are various theories but, to date, no proof as to the origin of these gorges. We do know that the climate has changed many times from cold ice age climates to temperate climates during the last two million years. It could be that when periods of rapid melting occurred, so much water was around that the streams were much bigger and so were able to cut these deep gorges. This may have all happened a very long time ago, about 700,000 years in fact, as recent work has shown evidence of a glaciation in the area of Kenn in North Somerset about this time. Today, there is only a small stream running in the bottom of these big valleys.

The rock into which the valleys are cut is a limestone formed some 350 million years ago when this area was not only under the sea, but it was located in the tropics. The fossils contained in the rocks give evidence that the waters were warm and fairly shallow. Corals grew in the waters and sea lilies or crinoids, which were attached to the seabed by long segmented stems, are now found as broken pieces within the rock. The waters were rich in lime, hence the formation of limestone when the weight of overlying sediments caused the water to be squeezed out and the soft sediments on the seabed turned into rock. This process would take some millions of years.

The earth’s crust is not stable and gradually the rocks were folded into great arches and troughs called anticlines and synclines. These extended for hundreds of miles across what is now Europe. One of these anticlines arched up from the area of Kingsweston Ridge and Blaise to the Downs and Leigh Woods. Badock’s Wood is situated on one side of this fold and the rocks can be seen to be dipping steeply in a southeasterly direction. During the 300 or so million years which have passed since these great upheavals of the earth’s crust much of the overlying rock has been eroded away leaving a limestone plateau into which the river has cut the gorge. Look for the bedding planes along the valley side. Each of these was once the sea floor. You may also be able to see some fossil corals or crinoid stems.

Eileen Stonebridge

October 2007