Geology of the Ullswater Valley

by John Rodgers

All images © John Rodgers

500 million years and a journey across the globe

From Kirkstone Pass to Pooley Bridge, the Ullswater Valley contains a record of changing environments that began nearly 500 million years ago. The earlier parts of this story are preserved in the various rock formations while recent events are portrayed in the majestic landscape of fells and valleys. Our geological history is also a journey across the globe from deep in the Southern Hemisphere to our present northern latitude.

Oldest rocks laid down in deep oceans

The oldest (Ordovician) formations are mainly concealed below the fields of the lower lying ground below Barton Fell and the Watermillock area including Swinburn’s Park. They can be found in the beds of a few streams that have cut through the glacial surface layers to expose black slaty rocks of the Skiddaw Group. These were laid down in a deep ocean at a time when no part of Cumbria existed as land.

Skiddaw GroupBlack slaty rocks are exposed in the stream bed of Aik Beck, which has cut through overlying glacial deposits

Volcanic activity forms the rocks of the rugged fells

This was to change as moving tectonic plates generated rising molten magma which raised the thick pile above sea level allowing erosion to strip away much of the younger upper layers. There followed a period of intense volcanism with the eruption of lava flows which began to build volcanic islands. This activity grew more explosive and thousands of metres of ash, lava and volcanic debris accumulated in about 5 million years to form the Borrowdale Volcanic Group. These harder rocks form the rugged fells which dominate the landscape around most of Ullswater and right across the central part of the Lake District. As eruptions ceased the whole region subsided and a return to marine conditions led to thousands of metres of sandstones and mudstones which are now seen as the Silurian age Windermere Group of the southern Lakes but are no longer to be seen in this area.

Borrowdale Volcanic GroupLava flows exposed in the crags of Barton Fell overlie the softer Skiddaw Group sediments which lie below the fields on either side of the lake

Rocks of the Borrowdale Volcanic Group form the rugged landscape around the southern end of Ullswater

Colliding continents build mountains; debris from their erosion forms rounded hills

Continuing motion of tectonic plates resulted in continental collisions and the compression and uplift of the great thickness of sediments and volcanic rocks into a mountain range of at least Alpine elevation. Extensive erosion of the high ground transported debris flows of gravels into the lower valleys during the Devonian Period; remains of these being the coarse-grained beds of the Mell Fell Conglomerate. This formation of consolidated boulders, cobbles and pebbles lies below the region of rounded hills to the northwest of Ullswater from Great Mell Fell to Dunmallard.

Mell Fell ConglomerateThis coarse consolidated gravel is the result of debris flows carried by flash floods from surrounding mountains. Outcrops can best be seen in roadside cuttings on either side of the junction between the B5320 and the A592 (the lakeside road west of Pooley Bridge)

The conglomerate outcrop area of the MFC forms the rounded hills of Great and Little Mell Fell and extends across to the wooded slopes above the northern end of the lake.

Warm tropical seas contribute limestone beds

By the end of the Devonian most of the mountains had been worn down and, as Britain drifted further north into equatorial latitudes, our area became submerged under a warm tropical sea. Coral reefs grew and sheltered a variety of organisms whose hard parts eventually contributed to the development of the Carboniferous Limestone beds. These can be seen as grey exposed scars on the high ground of Heughscar Hill, overlooking the north-east end of the lake. It is quite likely that these warm waters covered most if not all of Cumbria. However, subsequent earth movements and the slow re-emergence of the old Lake District formations (largely influenced by the rise of large masses of buoyant granite from below) have resulted in another elevated region – the Cumbrian Mountains.

Carboniferous LimestoneThis outcrop of limestone on Heughscar Hill can be seen looking southeast from pooley Bridge. It extends eastwards to form most of the ground of Askham Fell.

Ice Age glaciers sculpt the landscape we see today

There is no direct evidence of the prevailing environments for the next 300 million years in the Ullswater area because no formations produced in this time are preserved (although in the nearby Eden Valley a sedimentary record for the next 80 million years does exist). That brings us to around 2-3 million years ago, by which time our continued northern passage brought us into colder climates, and eventually the onset of the Ice Age. Glaciers grew in the high ends of radial valleys that had been incised by river erosion into the dome-like mountain region. Successive glaciations saw the growth of a Lake District ice cap and substantial glaciers that carved out wider and deeper valleys that today remain as the many dales in the Ullswater catchment area, all feeding into a great single glacier in the Ullswater Valley. With de-glaciation, the deeply-eroded trough acts as a linear basin for the lake while the smaller hollows of the high fells are now occupied by tarns.

Glacial FeaturesThese named coves are the eroded hollows from which glaciers grew to then join together and scour out the Grisedale valley. There may have been as many as 18 separate starting points for tributary glaciers, all of which fed into the main Ullswater valley glacier

Red Tarn lies in the eroded hollow (cove) below Helvellyn, with the arêtes of Striding Edge and Swirral Edge on either side

Ice-eroded volcanic bedrock (a roche moutonnée) with horizontal striations caused by the surface being scratched by rock fragments embedded in the glacier. Aira Force.

There are many other topographic features, large and small, that have resulted from structural or surface processes and would require more space than is available in this simplified summary. This brief account is intended to stimulate those who live in or visit this remarkable place to see more, to understand and appreciate the geological evolution of the wonderful Ullswater Valley.

by John Rodgers, President of Cumberland Geological Society, teacher and u3a lecturer

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