These fells were formed during the Ordovician period, 488 to 443 million years ago;[73] they form a triangle sheltering the town, reaching a maximum height of 931m on Skiddaw itself.[74] To the west of Portinscale, to the south-west of the village of Thornthwaite, is Whinlatter Forest Park and Grisedale Pike.[3] To the east, beyond Castlerigg stone circle, is St John's in the Vale, at the foot of the Helvellyn range, which is popular with ramblers starting from Keswick.