Towards the close of
Carboniferous Period and in the early
Permian, some 295 million years ago (Ma),
crustal extension caused by movement of the Earth's
tectonic plates during the
Variscan orogeny allowed the emplacement of
igneous intrusions of
magma across much of northern England of a suite of
tholeiitic dolerite intrusions. These were largely concordant with the
strata of the existing
country rock. On cooling, these crystallised and solidified to form the Great Whin Sill. It is dated at around 301- 294 million years old, thus spanning the Carboniferous/Permian boundary at 299 Ma. It underlies much of south and east Northumberland and the
Durham Coalfield. Its maximum known thickness of around occurs in the North Pennines. Surface and subsurface records of the Great Whin reveal it is not always concordant over wide areas and often rises and falls in the stratigraphical succession in marked leaps and gentle transgressions different levels. Studies of the
petrology of the dolerites of the Whin Sill complex have revealed significant differences between the
Little Whin Sill and the
Great Whin Sill. The Little Whin Sill is
olivine-bearing and believed to be composed of an early
differentiate of the Whin dolerite magma. On the other hand, the Great Whin, non-olivine-bearing and slightly density graded, is a later differentiate of the Whin magma. Two separate periods of Whin dolerite injection are confirmed by studies of
vitrinite reflectance over the
Alston Block where two periods of Whin
contact metamorphism have been recognized. The two periods of Whin dolerite emplacement form part of the end-Carboniferous earth movements in northern England. They can be shown to have occurred between a period of compression from a west-south west direction and later gentle doming of the Alston Block near the Westphalian-Stephanian boundary, dated about 300–295 Ma. The
lithology is notable in many respects, including well developed
pegmatite segregations which can be found in Upper Teesdale. Late stage
hydrothermal mineralisation has filled the joints with
pectolite. The dolerite has been carbonated as
metasomatic selvages into veins and joints and other flaws to produce 'White Whin'.
Thermal metamorphism of the country rock in the contact zone produced coarse grained
marbles within the
contact aureole in a small number of places in Teesdale. Disused roadstone
quarries along the course of the sill offer sections for its interpretation. ==Effects on landscape and geography==