Northern
England as a desert. Permian and Triassic times. 296 to 208 Ma.
By the early
Permian, all the world's continents had converged, becoming one huge
landmass called Pangaea. During late Carboniferous-early Permian
times uplift coupled with a worldwide fall in sea-level led to
erosion of up to 1.300 km. of Carboniferous sediments. Early Permian
deposits in Northern England, therefore, rest unconformably on
Carboniferous rocks.
As the British
Isles drifted northwards from the equator the climate became hot and
arid. During the early Permian higher ground, located in the present
day North Sea and the Pennines, underwent continued erosion while low
lying areas became deserts of wind-blown dune sands and
conglomerates deposited by flash floods.
Later, parts of
Britain were periodically invaded by a sea known as the Zechstein
which extended to Germany. The Zechstein Sea flooded in and
evaporated in four major cycles. It was this sea which laid down the
Marl Slate, famous for its fossil fish, followed by beds of
Magnesian Limestone (dolomite) which are quarried in many parts of
Durham. High rates of evaporation at various levels of the upper
Permian resulted in deposits of gypsum, halite (salt) and potash
(carnallite KMgCl3.6H2O). Some of these minerals are mined at Boulby
and also at Kirkby Thore near Penrith.
During the
Triassic period the Zechstein sea had retreated and the climate had
become a little wetter. Lack of fossils makes the
Permian-Triassic boundary uncertain in Northern England. The horizon
is characterised by a succession of red marls (calcareous mudstones)
deposited on coastal flats, followed by the Sherwood Sandstone
(formerly Bunter Sandstone). The sandstones were fed by rivers mainly
from the south.
Towards the end
of the Triassic, high rates of evaporation returned and low lying
areas such as Cheshire and north east Yorkshire became sabkha
environments. (A sabka is a wide area of coastal flats bordering the
sea.) Periodic flooding caused by spring tides and strong on-shore
winds followed by intense evaporation results in the precipitation of
carbonate-sulphate and halite deposits. The name comes from such an
area on certain parts of the coast of Arabia.) It was in this type of
environment that the Mercia Mudstone Group (formerly Keuper Marl) was deposited.
A major marine
incursion which deposited sediments of the Penarth Group
(formerly the Rhaetic beds) marked the end of the Triassic period and
heralded the beginning of the Jurassic.
Desert dune
sands containing large scale cross-bedding are well displayed in
Quarrington Quarry, Durham, where the Yellow Sands underlie the
"Marl Slate" famous for its fossil fish as well as on the
coast around Cullercoats harbour and below Tynemouth Priory.
Between Frenchman's Bay (where there is a geological information
board) and Marsden Bay (on the coast near South Shields) Permian
rocks viz. the Yellow Sands, Marl Slate and overlying Magnesian
Limestone are well displayed. The cliffs on the coast between
Hartlepool and Tynemouth consist mainly of Magnesian Limestone. Reef
limestone is exposed on the coast at Black Halls rocks and on the
Tunstall Hills (SSSI) south of Sunderland.
Exposures of
Triassic rocks are sparse. Some can be seen by the River Leven west
of Hutton Rudby and also on the beach at Seaton Carew.
