Whin Sill

[2] 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.

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.

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.

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).