The area’s oldest rocks formed in the Silurian Period around 420 million years ago when the part of the Earth’s crust that would eventually become northern England lay around 30° south of the Equator. These Silurian rocks date from the last stages of collision between two continents and the final disappearance of the Iapetus Ocean that once separated them. The continents are known as Avalonia (which included England and Wales) and Laurentia (which contained Scotland and much of North America).
A reconstruction of the world nearly 500 million years ago, showing the positions of the pieces of Earth’s crust that would eventually become Scotland and England. © E. Pickett
As the continents came together and the ocean disappeared, Laurentia overrode and pressed down on Avalonia causing subsidence and the creation of a sea. A thick sequence of mud, silt and sand built up, deposited by underwater avalanches of muddy sediment. These sediments hardened to become mudstones, siltstones and sandstones. They were later compressed and crumpled during millions of years of Earth movements related to continental collision and the creation of the Caledonian mountain range. The eroded core of this ancient mountain chain can be traced today through Scotland, Scandinavia and North America.
We see these rocks today in the Howgills as slaty rocks whose layering, once horizontal, is now vertical or tilted. Deformation has also created a ‘cleavage’ along which the rocks tend to split.
The northern Howgills, which are underlain by Silurian rocks, viewed across the upper Lune Valley from Orton Scar. © E. Pickett
The Silurian rocks of the Howgills are part of a thick sequence known as the Windermere Supergroup, which also forms much of the southern Lake District. This large group is subdivided into a series of smaller units. Along the northern edge of the Howgills the rocks are mainly mudstones, siltstones and sandstones of the Bannisdale Formation.
A small quarried exposure of steeply dipping slaty mudstones and siltstones of the Bannisdale Formation, on the lower slopes of the Howgills near Intake, south of Gaisgill. © E. Pickett
Steeply dipping siltstones and mudstones of the Bannisdale Formation in Pinskey Gill, near the unconformity with overlying Carboniferous rocks. © S. Woodhead
Slaty siltstones and mudstones of the Bannisdale Formation in Pinskey Gill near Weasdale. © N. Pearson