The sandstones and evaporitic rocks of the Lake District:

An Introduction

 

Around 300 million years ago the continental landmass that was to become the Lake District lay near the latitude of the present day Sahara Desert. In place of the tropical shallow seas and lush swamps of the earlier Carboniferous period, hot, arid climate conditions prevailed and sand dunes slowly moved across the desert. Today, these dunes are preserved as the distinctive red sandstones of Penrith.

The Penrith sandstones formed from wind-blown or Aeolian sands deposited in the Permian period. The fine layering in the rock dips in a variety of directions, reflecting shifting sand on the original dune slopes. The sand grains were originally very rounded because they were polished by the action of the wind, but after they were buried, small amounts of quartz were precipitated over them from groundwater and serve as a cement, holding the rock together.

Later in Permian times, sea level rose and the sea sometimes invaded the land forming shallow lagoons in the arid environment. When these dried out, deposits ofevaporites developed. Evaporites are essentially the salts that are left when seawater is evaporated. In the Vale of Eden they can be more than 100 metres thick and are mainly made up of calcium sulphate minerals, gypsum and anhydrite. These are used in the manufacture of plaster and cement and as a source of sulphur. Flash-floods washed large amounts of sand, silt and mud into these transient lagoons and today we see mudstones with dessication cracks (cracks made when the mud dried out).

More desert sandstones were deposited above the evaporites during the Triassic period and now form the conspicuous red cliffs of St. Bees Head. Similar sandstones of this age occur over much of England and are a major source of groundwater. Probably the red sandstones, like the limestones, were once continuous over the top of the Lake District as we know it.

For more information follow the link below:

The Triassic Sandstones of the Lake District

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Created by Ruskin Rocks Team, August 2010
Last updated: Rebecca Hildyard, 25 August 2010