A couple of weeks ago, I found myself avidly staring into a freshly-dug, multi-coloured trench. It was one of several located in a recently-harvested farmer’s field west of Martley.
The site – known as Martley Rock – has an extraordinarily diverse range of geology located within a small area. Rarely can one find rock from such a variety of ages within such close proximity: in a short stretch, there’s half a billion years’ worth of geological history represented by five different bands of rock!
In the image here, the bands in the trench are, from bottom to top:
- Bromsgrove Sandstone from the Triassic (240 million years ago);
- A narrow band of Raglan Mudstone, which has a slightly pinker tinge, and is from the Silurian (420 million years ago);
- The greyish band is from the Halesowen Formation, which is mudstone and clay from the Carboniferous (310 million years ago);
- The Malverns Complex from the PreCambrian (about 700 million years ago – that is, substantially before advanced life-forms appeared on Earth), which is the dominant rock type in the Malvern Hills;
- The pale whitish-yellow band at the top is Malvern quartzite from the Cambrian era (520 million years ago).
At this point the trench goes left and downhill.
One of the reasons for the complexity here is that there are two dominant fault-lines running through the site: one is the East Malverns fault, which runs roughly north-south and then down along the eastern side of the Malverns; the other is the much shorter Martley Rock fault. In both cases, major crustal movements have brought together rock types that were formed over a hundred million years apart (the Triassic sandstone and the Silurian mudstone, and the two Malvern rock types).
However, there are also other important geological boundaries represented here: in the trench shown, there’s an obvious one on the near side of the grey band. The Raglan mudstone from about 420 million years ago is the narrow strip of pinkish-brown rock, which would have been formed when England was about 20 degrees south of the equator: the Martley area was probably covered by a shallow sea. Next to it is the grey-green Halesowen formation from about 310 million years ago, when the area was probably a low-lying river delta system, with dense forests of ferns on land: this also gave rise to the coal beds further north. Both eras were times when rock was predominantly being formed; in between was an era in which erosion was the more important process, which is why there is such a large time gap between the two rock layers. (See here for a fuller description of some of the geology on the site)
John Nicklin – the secretary of the Teme Valley Geological Society – flew a radio-controlled helicopter over the site and took some spectacular photos, including the one below. The different rock types are clear not just in the spoil heaps by the trenches but also in the changing soil colour as well.