In the area to the north of Sticks Pass the Birker Fell andesites are overlain by the Lincomb Tarns Tuff Formation. This formation is one of the most widespread of the volcanic rocks of the Lake District; it seems that the whole district was buried beneath at least 150 m of densely welded ignimbrite, a rock formed from a pyroclastic flow of very hot gas and rock. This formation must represent a series of eruptions of truly exceptional magnitude, accompanying the formation of a volcanic caldera probably in the area around what is now Helvellyn.
