Navigation



by Mick de Pomerai


Introduction:

Part of a geologist’s training is to learn how to “read” the landscape and interpret it in the context of the rocks beneath - the processes that produced them, altered them, and wore them down. Since I moved to the village, I’ve always been intrigued by the hollow in which it sits and, a request to give a lecture for the W.I. in September spurred me into a closer study that results in this article.
Unfortunately (or perhaps fortunately?) it is impossible to include the whole of the story here, so I’ll just concentrate on the important bits!

Satellite view of South Hams

In order to understand our landscape and how its shape reflects the rocks beneath, it is paradoxically helpful to look from above. The satellite picture shows that our area has a definite “grain” running WSW-ENE, picked out by valleys such as Bow Creek and the clouds over the ridge north of Dartmouth. This is cut by a secondary grain running NW-SE, corresponding to a fault (or break in the rocks), picked out by the course of the river Dart. The same “grain” appears on geological maps of the area, showing how the rocks determine the lie of the land.

Geological Map of South Hams

I. The rocks beneath:

Most of the rocks in the South Hams date from the Devonian Period, around 400 to 450 million years ago (an unimaginably long time if you consider that the birth of Christ occurred only about ¾ million days ago), long before anything we would now recognise as the British Isles existed, and a time when our region lay in the southern hemisphere! The red, sandy and pebbly rocks around Paignton are the exception – these were formed much later in the hot deserts of Permian times, around 250 million years ago – note how they cover up the aforementioned “grain” of the Devonian rocks on the satellite photo.

Devon 420 million years agoDuring the Devonian, our area was covered by warm seas, and lay to the south of a continent edged by rapidly-eroding mountains, with a coastline positioned in what is now the Exmoor region (although at that time the distance in between would have been much greater). Ours was an active continental margin undergoing frequent earthquakes and volcanic eruptions, not unlike parts of present day SE Asia, or the southern Aegean. Our particular bit of the South Hams lay in relatively shallow water over an up-faulted block (or horst) which, at times, allowed the establishment of coral reefs that were particularly extensive around Torbay From time to time, these faults also provided passage to the surface for rising magma that erupted as lavas and volcanic ash, forming a volcanic island whose remains still form the backbone of Totnes Down. Ashprington, it seems, was once a rather exiting and dangerous place!
Devonian Fossils from Black Ness

The ocean basin in which the Devonian rocks were deposited continued to exist through most of the ensuing Carboniferous period. Sea-floor deposits of this age form the rocks of central Devon, whilst on the land further north, tropical forests produced the coal deposits on which Britain’s industrial revolution depended. However, the ocean in the south was gradually closing up, as what is now NW Europe approached southern Britain. As continents collided, around 300 million years ago, the crust beneath our feet was squashed and deformed into many folds and faults – giving us the WSW-ENE grain referred to earlier – and Devon became Folding & faulting at Hope’s Nose, Torquaypart of a new mountain chain into which thegranite mass that now makes up Dartmoor and the Cornish uplands was intruded.

Folding & faulting at Hope’s Nose, Torquay

Our own area does not record much of the geological past since the time of the late Carboniferous mountain-building, but rocks of progressively younger age form the cliffs of east Devon and Dorset, telling a story of Permian and Triassic Deserts and of Jurassic and Cretaceous Seas.Flints from the Cretaceous chalk seas top the Haldon hills and form many of the pebbles on our local beaches. Our own story resumes with the lowering of global sea levels and the shaping of our landscape from about 65 million years ago, during the Tertiary and Quarternary periods.

II. Shaping the landscape:

Wave-cut plateau forming Berry Head, BrixhamMuch of the Devon landscape consists of a series of dissected plateau that formed over the last 50 million years or so as global sea levels - which reached an all-time high around 70 million years ago - began to fall and the Earth’s climate began to cool. As the sea receded and more of Devon became land, it was the “grain” of the geology that began to control the formation of the valleys which drained that land. The dominant, roughly E-W trend of folds and faults dating back to 300 million years ago meant that rivers exploited weaker rocks along this trend – so much so, that clay weathered from Dartmoor is found as far east as Wareham in Dorset.

Aerial view of the Washbourne-Cornworthy valley

The ancient course of the Washbourne river through Cornworthy.

The global cooling that began in Tertiary times culminated, around 2 million years ago, in the establishment of ice sheets in the polar regions and the first of many ice advances that we call glaciations – in other words, it was the start of the Quarternary “Ice Age” in which we now live, albeit in a warmer interlude.

continued ...