North Pennines — Area of Outstanding Natural Beauty and European Geopark: a geodiversity audit. North Pennines Natural Landscape, 2010.
Permo-Triassic rocks
Permo-Triassic rocks formed during the two episodes of Earth history known respectively as the Permian and Triassic periods. The Permian Period is generally assumed to have extended from around 295 to 245 million years ago and the Triassic Period from 245 to 208 million years ago. Despite these being clearly defined periods of Earth history, in many parts of Britain it is difficult to define precisely the boundary between the rocks formed in them. In such places, including the AONB, it is common practice to refer to these rocks collectively as of Permo-Triassic age.
The Permian Period takes its name from the Perm area of Russia, where these rocks were first recognised. The Triassic Period is so called from the threefold division of the unit in Germany where these rocks have been studied in great detail.
Currently protected sites of Permo-Triassic rocks within the AONB SSSIs
Permo-Triassic rocks are exposed within a number of areas scheduled as SSSIs. However there are no sites within the AONB specifically designated for Permian or Trissaic rocks within the Geological Conservation Review.
RIGS
Croglin village Quarry
Frank's Bridge, Kirkby Stephen
Other representative sites in the area
Milburn Beck,
Keisley
Gamblesby
Roman Fell
Hanging Walls of Mark Anthony
Fellside, Ousby
Crowdundle Beck, Blencarn
Croglin
Crowdundle Beck
Permo-Triassic rocks in Great Britain
Sedimentary rocks comprise by far the greatest proportion of Permo-Triassic rocks in Great Britain, though volcanic rocks and intrusive igneous rocks occur in Devon and in parts of southern Scotland.
During Permo-Triassic times, the area which eventually became Great Britain drifted progressively northwards through equatorial latitudes from its position close to the equator at the close of Carboniferous times.
Uplift, folding and tilting of the Carboniferous rocks occurred in late Carboniferous and earliest Permian times, related to the effects of severe earth movements and mountain-building further south in what is now continental Europe. This converted much of this area into a landmass which suffered considerable erosion. This was initially a hot, arid desert, in which coarse broken rock and scree accumulated close to hill and mountain ranges, with widespread wind-blown sand dunes on the intervening plains. During Triassic times, vast thicknesses of sand, silt and mud accumulated over extensive coastal plains. We see these today as thick deposits of siltstones and mudstones, and sandstones.
Permo-Triassic rocks in the AONB
Outcrops of Permo-Triassic rocks comprise 4,688 hectares or 2.4% of the surface area of the AONB at the foot of the Pennine escarpment. They everywhere rest unconformably upon an eroded surface of Carboniferous and older rocks. The rocks beneath the unconformity typically exhibit reddening.
The main divisions of Permo-Triassic rocks (youngest at the top) are as follows:
- St Bees Sandstone (Sherwood Sandstone)
- Eden Shales
- Penrith Sandstone and Brockram
Penrith Sandstone and Brockram
The severe Earth movements, referred to above, at the close of Carboniferous and beginning of Permian times, uplifted the area we know today as northern England, creating mountains and hills in the areas today occupied by the Lake District and North Pennines. Lower land occupied the area of what is now the Solway Plain and Vale of Eden. The area was a hot, arid desert.
Rapid erosion of the newly formed mountains and hills resulted in thick accumulations of angular rock debris in great fans at the foot of the slopes. These are preserved today as the breccias and conglomerates known in Cumbria as the Brockram.
Away from the immediate vicinity of the mountains and hills, and covering much of the lower ground, was a dry desert with huge wind-blown sand dunes, composed of coarse-grained sand. These sands are preserved today as the Penrith Sandstone, which exhibits features typical of desert sands such as rounded 'millet seed' grains and large-scale cross-bedding. Much of the Penrith Sandstone is pale brick red or salmon pink in colour due to small amounts of finely disseminated haematite.
Eden Shales
These comprise beds of mudstone and siltstone, mostly of a dull red, or locally grey, colour.
The Eden Shales were formed as accumulations of mud and silt deposited within desert lakes or on the flood plains of rivers which crossed the desert. A bed of dolomitic limestone, present locally within the Eden Shales, is known as the Belah dolomite. The desert lakes periodically dried up, leaving beds of carbonate-rich mudstone and in places thick beds of rocks known as evaporites, which here comprise mainly gypsum or anhydrite.
The Eden Shales are weak, easily eroded rocks and there are typically few natural exposures. As the evaporite beds are very soluble in near-surface groundwaters, they have almost invariably been removed completely by natural dissolution and rarely crop out at the surface.
St Bees Sandstone
Overlying the Eden Shales is a thick group of dull red sandstones, known in Cumbria as the St Bees Sandstone, after its spectacular development at St Bees Head in west Cumbria. The St Bees Sandstone is part of a very much more extensive deposit of sandstone which occupies wide areas of central England, where it is today generally known as the Sherwood Sandstone, taking its name from Sherwood Forest where it crops out widely. The St Bees Sandstone is much finer grained and is a duller, more brownish red colour than the Penrith Sandstone.
The St Bees Sandstone is interpreted as having been deposited on a wide alluvial plain crossed by numerous braided rivers. Fossilised rain-prints and desiccation cracks, preserved in the sandstone, reveal that at times the accumulating sands were exposed to drying out.
Impact on biodiversity
Outcrops are restricted to a very narrow belt of country along the foot of the North Pennine escarpment and comprise the extreme eastern edge of the Vale of Eden, much of which is floored by substantial thicknesses of Permo-Triassic rocks.
Impact on the landscape
The Permo-Triassic deposits of eastern Cumbria are mainly weak rocks, especially when compared with the more resistant rocks which form the main mass of the escarpment and North Pennine uplands. Differential erosion has resulted in the excavation of the Vale of Eden along their outcrop, between the North Pennine escarpment in the east and the mountains and foothills of the Lake District in the west. The harder, and comparatively more resitant Penrith Sandstone forms hilly country around Penrith, a short distance beyond the AONB.
The almost universal red colour of area's Permo-Triassic rocks give rise to the characteristic red soils of the Vale of Eden. The extensive use of red sandstone, particularly the St Bees Sandstone, in vernacular architecture lends a distinctive character to the landscape, particularly in villages such as Croglin.
Impact on biodiversity
Over much of their comparatively restricted outcrop within the AONB, Permo-Triassic rocks are substantially concealed beneath a mantle of superficial, mainly Quaternary, deposits. Only in a few rather limited areas are these rocks exposed at the surface where they exert any direct influence on soil type, supporting very sandy, rather dry soils.
Within the AONB, the Penrith and St Bees sandstones are the most commonly exposed formations. In addition to a few natural exposures of these sandstones, mainly in stream sections and abandoned quarries, more substantial amounts are present in the built environment, providing substrates for a variety of moss and lichen communities.
Economic use
Both the St Bees and Penrith sandstones have been widely employed as building stone in the vernacular architecture of the East Fellside. In most cases stone has been obtained from small pits, sometimes opened to supply stone to the adjoining village or even in some instances to provide materials for a single building or farm.
The coarse-grained breccias and conglomerates of the Brockram have locally served as building stones in parts of the Vale of Eden. In places near Kirkby Stephen, immediately beyond the AONB, limestone-rich parts of the Brockram have, in the past, been quarried for lime burning.
Although gypsum and anhydrite beds within the Eden Shales are known to extend into the AONB, there are no records of any workings of these from the area, though both minerals are still worked in the Kirkby Thore area immediately beyond the AONB.
Wider importance
The Brockram and Penrith Sandstone, and their relationships to each other, offer clear insights into the rapid erosion of the newly formed mountains at the beginning of Permo-Triassic times and the nature of the arid desert conditions which prevailed at that time. The succeeding Eden Shales and St Bees Sandstone record the progressively changing environmental conditions during this period. Studies of the evaporite beds, and features associated with them, have contributed much to the understanding of these rocks elsewhere within Great Britain.
Conservation issues
Exposures of these rocks are mainly in small stream sections and abandoned quarries. Whereas most of these appear reasonably robust, the progressive deterioration of long-abandoned quarry faces, together with risks of quarries being filled, poses some long-term threat. There are not currently perceived to be any specific threats to the integrity of any exposures or features related to these rocks.
Selected references
Arthurton and Wadge, 1981; Burgess and Holliday, 1979; Stone et al, 2010.