Dartmouth. Report from the Geology Section

Most of Dartmouth is built on the slates and siltstones of the Dartmouth Group which crops out in the South Hams between Adurn Point in the west and Scabbacombe Head in the east and is of Lower Devonian age (Lochkovian-Pragian). If one ignores the Start Complex (of uncertain age), these rocks form the base of the Palaeozoic succession in Devon south of the Dartmoor Granite.

The Group used to be called the Dartmouth Slates or just the Dartmouth Beds but British stratigraphic nomenclature has now been standardised by the British Geological Survey so that every major rock unit is now called a formation, even although that may only amount to adding the word ‘Formation’ to the old familiar name. So how come the Dartmouth Group is a group not a formation? The answer lies in east Cornwall where two subdivisions are recognised, the Whitsand Bay Formation and the Bin Down Formation which together make up the Dartmouth Group. As a consequence and in spite of the name, the type section for the Dartmouth Group is in West Cornwall.

Exposure of the Group around Dartmouth is excellent on the coast but most of the coastal exposures are inaccessible to the ordinary enthusiast not willing to risk life or limb. The rocks can be seen in Castle Cove and Sugary Cove south of Dartmouth Castle and are also well exposed in the cliffs at Strete Gate, a county geological site. The rocks are typically described as slates but strictly speaking, the composition of much of them is that of a siltstone or impure fine-grained sandstone, rich in clay minerals or micas. They are strongly cleaved rocks with the cleavage planes exhibiting a shiny finish of recrystallised clay minerals or very fine-grained micas. While in a few localities, it is possible to distinguish compositional banding (bedding) oblique to the cleavage, in most outcrops these are coincident. The main cleavage is penetrative, that is, it affects all of the rock without any earlier oriented fabric visible between the cleavage planes. Later folding of the cleavage is visible in some outcrops. Typically the style of the folds is rather open with approximately east-west axes and steeply dipping axial planes but in places a spaced cleavage defined by zones of micas re-orientated in the fold axial planes defines a second splitting direction or strain-slip cleavage. Small-scale folding of the cleavage surfaces defines a crenulation lineation parallel to these later fold axes.

The characteristic that perhaps distinguished these rocks most easily from the overlying slates of the Bovisand Formation is the presence of intervals of red, pink or purple slates which, so long as they can be differentiated from slate reddened by weathering, are usually referred to the Dartmouth Group rather than the Bovisand Formation. There is however a difficulty caused by the fact that large parts of the Dartmouth Group lack slate with this distinctive pigmentation. Moreover, it is often difficult to differentiate between original pink pigmentation and that caused by weathering. The boundary between these formations is admitted to be somewhat schematic in parts of the South Hams remote from the coast, reflecting the likely confusion. The present geological map shows unrooted patches of Dartmoor Group rocks within the Bovisand Formation which it is hard to explain by folding; it is likely a pragmatic solution to a mapping problem in an area of poor exposure of very similar rock groupings.

The northern part of Dartmouth is built of rocks of the Bovisand Formation which rests on top of the Dartmouth Group but is also Lower Devonian in age. Because it is further from the sea, there is very little current erosion or exposure of it in the banks of the estuary. It was extensively quarried in medieval times further west in a line of quarries around Frogmore Creek, part of the Kingsbridge Estuary, from where it was widely exported for use as roofing slates. The yellow-weathered pelitic slate from the Bovisand is the main building stone of Kingsbridge.

Those of us of a certain age will recall being taught about rias in one of our earliest geography lessons, about the same time as we were obliged to draw freehand maps of British coalfields or learn about the excellence of the administration of the Belgian Congo. A ria is a flooded river valley caused by rising sea level relative to the land. In the case of South West England, they originate after the last ice age when sea levels rose as the ice melted. They are preserved in the south west because this part of England never had the benefit of glaciation or an ice cap. Further north, flooding of valleys over-deepened by the erosive action of glaciers have fiord-like sea lochs which are completely different from rias and raised beaches which record, not the flooding of the land as the water rose relative to the land, but rising of the land relative to the sea as the earth’s crust reacted to unloading consequent on the disappearance of land ice.

The Dart and Kingsbridge estuaries are classic examples of rias. The coastline records the distribution of the main streams and their tributaries and the steep wooded slopes follow the edges of the former flood plains. There is however one very important difference. The Dart is a large enough river to maintain an adequate flow of water through the mouth of the estuary during the fall of the tide. Consequently, there is relatively little deposition of sediment as river meets the sea and the town remains an important port capable of hosting ocean-going liners. The Kingsbridge estuary was formed by streams of insignificant flow. The loss of velocity and contact of fresh with saline water at the mouth has led to the dumping of sediment there and the formation of a bar which restricts access to Salcombe and further up the estuary for even vessels of quite small draught except at high tide.

The stone used in the older buildings of Dartmouth reflects the local geology but modified to a significant extent by the advantage the town has of being a port, allowing access to sources of stone over a wider area brought to the town by river or sea. Dartmouth Group slate and fine-grained sandstone is the most plentiful building stone but many older buildings and early port and defensive structures are made of middle Devonian limestone either brought down the river from quarries at Galmpton, or brought by ship or barge from Torbay.

Slate and metasediment of volcanic origin is also quite widely used. While rocks of this kind are distributed throughout the buildings of the South Hams, the stone in Dartmouth is likely to have come from the Ashprington Volcanic Formation which crops out on both sides of the Dart estuary downstream from Totnes. Also fairly widely used though only in small amounts and mainly for quoins and dressings is red sandstone of Permian age, quarried around Waddeton and Stoke Gabriel. Added to these local building stones, the town has its fair share of high quality stone used in the town’s churches, mainly again for quoins and dressings, including Beer, Bath, Portland and Doulting Stone.

Britannia Royal Naval College
Image from Wikimedia Commons – click on it for details

Finally, a description would not be complete without mentioning the Royal Naval College Dartmouth, built at the turn of the nineteenth century, with decorative detail embellished with mainly Bath Stone internally and white Portland Stone externally. The plinth of the front of the imposing main building is of granite from the Bodmin granite intrusion while many of the external structures are of Middle Devonian limestone like much of the town. The chapel includes Bath Stone figures, and flooring of Ashburton, Carrara and a black marble. The walls are decorated with green Connemara marble and a black-and-white breccia, likely from Italy.

Mike Barr

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