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.
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.
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; the name comes from such an area on certain parts of
the coast of Arabia). 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. It was in this type of environment
that the Mercia Mudstone Group (formerly Keuper
Marl) was deposited.
Desert dune sands containing large scale cross-bedding
are well displayed in Field House Farm and 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.