The Old Red Sandstone describes a suite of sedimentary rocks deposited in a variety of environments during the Devonian but extending back into the late Silurian and on into the earliest part of the Carboniferous. The body of rock, or facies, is dominated by alluvial sediments and conglomerates at its base, and progresses to a combination of dunes, lakes and river sediments.
The familiar red color of these rocks arises from the presence of iron oxide but not all the Old Red Sandstone is red or sandstone - the sequence also includes conglomerates, mudstones, siltstones and thin limestones, and colours can range from grey and green through red to purple. These deposits are closely associated with the erosion of the Caledonian Mountain chain which was thrown up by the collision of the former continents of Avalonia, Baltica and Laurentia to form the Old Red Sandstone Continent - an event known as the Caledonian Orogeny.
Many fossils are found within the rocks, including early fishes, arthropods and plants. The rocks may appear paleontologically barren to amateur geologists but careful study, particularly with an accomplished fossil hunter, can uncover pockets of fossils. Rocks of this age were also laid down in southwest England (hence the name 'Devonian') though these are of true marine origin and are not included within the Old Red Sandstone.
Geology
The sheer cliffs are actually undercut in some places by erosive force of the North Sea wave action and associated strong marine winds, giving rise to cliff overhangs in numerous stretches of the blufftop trail. (Off shore winds commonly attain mean velocities of 80 kilometres per hour here, especially in winter months.) The underlying rock formation is known as Old Red Sandstone, which occurs from DunnottarCastle five kilometres north to the town of Catterline seven kilometres south. This sandstone formation may be as thick as 2700 metres.
In places the fissured red and green coloured sandstone is replaced by picturesque conglomerate with roundish stones varying in diameter from two to thirty centimetres(historically known as pudding stone in this region of Kincardineshire). In other places more greenish volcanic extrusions are evident as harder veins within the sandstone bluffs.
The Formation at Dunnottar Castle
The Dunnottar Castle Conglomerate Formation consists primarily of massive and weakly-bedded, clast-supported conglomerate with lenses of horizontally-bedded, medium-grained sandstone. The conglomerate is of essentially ‘Highland’ type in that its coarse fraction is dominated by andesitic lavas, quartzite and psammite.
The basal beds in the lower part are of red, massive volcaniclastic sandstone, containing angular clasts of felsite and andesitic lava with rarer, rounded clasts of quartzite. Sequences of red, coarse-grained, medium bedded,volcaniclastic sandstone up to 30m thick are present in the upper part of the formation which are composed largely of coarse-grained, angular clasts of fine-grained ‘felsitic’ material.
Thickness:
The redefined formation is 2385–2482 m thick.
Log conditions:
At the listing coordinates you have an overview to some Dunnottar Castle Conglomerate Formations. Answer the following question:
1. At this location describe the sandstone, its colour, grain size, etc. Is it made of one or many colours? If many, which is the dominant colour?
2. Describe the Old-Red-Sandstone you can see here. Is there only one kind of stratum ('layer'), or are there various types of layers? What makes the rocks here so stable? Is the "Old Red Sandstone" only red or do you see various colors?
3. In the North Sea in front of you, you see some sea stacks rocks. What has shaped these rocks?
4. Take a photograph of the Formation, your GPS and the North Sea in the background and insert it within your log.
Send the answers of the questions to Fam.Weiss with your GC-name and the Earthcache name in the subject-line. We contact you if something is wrong.
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Log entries without answering the questions will be deleted by us directly without any further enquiry!
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