This is one of a pair of linked EarthCaches on the ridge of the Blorenge near Abergavenny which combine to give some background understanding of the area’s solid geology. They can be visited along with several physical caches including First in Wales, as well as with an EarthCache on the geological processes that have shaped the area.
The geology of the Blorenge area is relatively complicated, but is a nice example of the Devonian to Carboniferous transition. Rocks covering this transition surround the South Wales coalfield, but here on the Blorenge there are three major rock types close together. The Devonian to Carboniferous transition took place about 360 Million Years Ago (MYA): between the Devonian Period (395-360 MYA) and the Carboniferous Period (360-280 MYA). There are major differences in both terrestrial and marine fossils between these Periods, and several major extinction events marked the transition.
The map below, from the Northern Mine Research Society website, shows the Carboniferous Pennant Sandstone in orange and the Carboniferous Coal Measures rocks in grey, surrounded by the Carboniferous Marros Group in yellow and Carboniferous Limestone in blue, and then the Devonian Old Red Sandstone in pink and brown. The Blorenge is ringed in black.
The rocks that will be examined in the two Blorenge EarthCaches are of three types:
- The Blorenge plateau is formed of the hard Carboniferous sandstone beds known as the Marros Group (more familiarly named Millstone Grit). Much of the plateau is covered with a thin layer of peat: a dark brown soil formed of decaying plant remains, which can hold a lot of water and slowly release it. There are outcrops of exposed rock near the carpark and around the summit, and extensive beds of block scree. The Marros Group sandstones are a sedimentary rock laid down in an ancient river delta and are formed of medium-sized grains of silica-rich sand (0.05 to 2mm in diameter) bound with a cement. Sometimes these medium-sized grains are joined by larger (>2mm diameter) quartz pebbles in what are termed coarse-grained rocks. The Marros Group rocks appear dark grey when weathered, but are yellow-brown when newly exposed.
- The Marros Group rocks lie on top of older Carboniferous Limestone rocks. This limestone is a fine-grained (fragments less than 0.05 mm in diameter) to medium-grained, light grey, sedimentary rock laid down in a shallow sea and formed of the lime-rich shells and hard parts of millions of sea creatures. The Carboniferous Limestone exposure is considerably narrower on the north-east side of the Blorenge than elsewhere to the west, such as on Gilwern Hill or Craig y Cilau. It sits at the top of the impressive slope which overlooks Abergavenny, above a similarly low exposure of Old Red Sandstone conglomerate rocks.
- Below the limestone are outcrops of the Devonian Old Red Sandstone so typical of Monmouthshire and the Black Mountains. The Old Red Sandstone outcrops on the Blorenge as both medium- and coarse-grained, red-brown sandstone and fine-grained, red-brown mudstone. The sandstone is formed of medium-sized grains of silica sand bound by a cement, washed down from ancient mountains into a desert environment. Some beds of Old Red Sandstone here hold abundant coarse Quartz pebbles.
An Unconformity is the term used when there is a gap in a sequence of rocks. This usually happens when older rocks are eroded away before newer rocks are laid down. Carboniferous Limestone rocks formed on top of older Old Red Sandstone rocks, and then Marros Group rocks were laid down on top of the Carboniferous Limestone. However, in the time between the deposition of the Carboniferous Limestone and the Marros Group there were major earth movements including uplift of the Usk Anticline (an arch of stratified rock in which the layers bend downward in opposite directions from a crest) - when rocks in what is now south-east Wales were pushed upwards. Some rocks, including some of the upper strata of the Carboniferous Limestone, were eroded away after those earth movements. When the Marros Group rocks were laid down, some of the rock sequence that is present further west was therefore missing. The strata below the Marros Group are at a different angle to those of the Marros Group, and this different angle combined with the missing rocks indicates that this is an Angular Unconformity, one of the four types of Unconformity.
Limestone has Calcium Carbonate (CaCO3) as its primary mineral whilst sandstone is made of grains of Silica-rich sand (Quartz or Feldspar). Limestone has abundant soluble Calcium that is available to plants and is termed ‘calcareous’, whilst sandstone usually has little available Calcium and is termed ‘non-calcareous’ or ‘siliceous’. Botanists use plants, mosses and lichens to determine whether rocks are calcareous or non-calcareous, because the availability of Calcium ions is a major factor on which plants grow where. Calcareous rocks can be identified by species such as Wild Thyme, Carline Thistle and bright orange Caloplaca lichens; non-calcareous rocks can be identified by the black Rock Moss, the large, grey Woolly Fringe-moss, and the yellow & black Map Lichen.
To claim a find you need to answer the following using the Message Centre or by emailing me. I will reply promptly, but you are welcome to log a find before you hear from me:
At Waypoint 1 N51 48.245 W003 02.957.
1) describe the outcropping bedrock here, including its colour, grain size (if you can see individual grains), variation in appearance, presence of fossils, and whether there are vertical or horizontal cracks and crevices;
2) state whether this is Marros Group, Carboniferous Limestone or Old Red Sandstone and why you think that;
3) if Waypoint 1 marks the bottom of this outcrop and the small building at the top of the slope at 525m altitude marks the top of the outcrop, how tall vertically in metres is the outcrop? Use your GPS to work out the altitude at WP1;
4) the outcrop of the same rock type on the west side of Gilwern Hill is nearly 100m tall. Given the information provided in the text above, suggest why the Blorenge outcrop is so much thinner;
5) identify one plant from the photos above that indicates you are on a calcareous rock - they aren't all there, so you do actually have to look!;
At Waypoint 2 N51 48.265 W003 02.914.
6) describe the outcropping bedrock here, including colour, texture/grain size, whether there is variation in texture and whether any variation forms distinct bands (beds), and also whether there are vertical or horizontal crevices, overhangs etc.;
7) state whether this is Marros Group, Carboniferous Limestone or Old Red Sandstone and why you think that.
Photos to accompany your log would be nice, but please don't give away any of the answers in your images.