It underlies much of south and east Northumberland and the Durham Coalfield. Its maximum known thickness of around 70 metres 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 and the Great Whin. 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 W-SW 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 aureole in a small number of places in Teesdale.
Disused roadstone quarries along the course of the sill offer sections for its interpretation.
During the Tertiary, the British land mass drifted northwards from 40°N to its present latitude. It was also moved eastwards by the widening of the Atlantic Ocean and there was violent volcanic activity over north west Britain. It was in this period that the Cleveland dyke was formed, originating from volcanic activity near the Scottish island of Mull. The highlands and lowlands of Britain assumed their present relative positions by the late Tertiary period, about 2 million years ago.