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 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 The regolith supports alpine/Arctic flora which includes spring gentian (Gentiana verna), bird's-eye primrose (Primula farinosa), mountain avens (Dryas octopetala) and Teesdale violet (Viola rupestris). This is an example of the intimate interaction and dependence of geology and ecology. The resistance to erosion of the dolerite gives rise to a number of striking geomorphological features such as High Force, Low Force, Cauldron Snout, High Cup Nick, Scordale and Holwick Scars. See also Allolee to Walltown SSSI Bamburgh Coast and Hills SSSI Burnfoot River Shingle and Wydon Nabb SSSI References Geology of Northumberland Paleozoic England Sills (geology)