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 light orange and the Carboniferous Coal Measures rocks in grey, surrounded by the Carboniferous Millstone Grit 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 released 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.
By the Devonian Period, plants were abundant on land. They colonised in the Ordovician Period (500-435 MYA) but remained relatively small until the Devonian. During the Devonian Period forests of giant Tree-ferns and Club-mosses covered much of the land. These changed the composition of gases in the atmosphere and some palaeontologists have suggested that it was these atmospheric changes that caused the mass extinctions which mark the Devonian to Carboniferous transition. Giant Tree-ferns and Club-mosses were also present in early Carboniferous rocks, and their fossils can sometimes be found in Marros Group rocks.
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 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 email. I will reply promptly, but you are welcome to log a find before you hear from me:
At Waypoint 1 N51 47.405 W003 04.223.
1) describe the bedrock under your feet, including its colour, texture, grain sizes (if you can make out individual grains), cracking etc.;
2) thinking about the location and position of the exposed rock you are standing on, explain why you think much of the rock here is bare rather than being covered with peat, heather or mosses. Do any stains on the rocks suggest what natural process occurs here?;
3) identify one moss or lichen that indicates you are on a non-calcareous rock, remembering that not all of those in the photos above will be growing at this location so you do have to look!;
4) state what kind of rock this is and why you think that.
At Waypoint 2 N51 47.956 W003 03.731.
5) describe the boulders here and the structure of the rock exposure they are associated with. Are they of uniform appearance and if they are not then explain what may have caused the differences? What rock type do you think this is?;
6) if the rocks at WP1 and WP2 are different then why is that, and if they are the same then why - explain their similarities and differences;
7) locate the rock photographed in the gallery (and below) and suggest what the horizontal depression approximately 2m long and 25cm tall that crosses its south-west face might be.
Photos to accompany your log would be nice, but please don't give away any of the answers in your images.
