Fieldwork in the Atacama Desert, Chile

Wednesday, 23 August 2017

3 Days, 3 Rock Types and 100 Million Years: Climbing in Devon

This (extended) weekend just gone, Dan, one of our regular ex-Norwich climbing partners, took us on a climbing tour of his home crags in Devon. This was an excuse for me to look at some nice rocks and we managed to get on three different rock types, covering over 100 million years of the geological history of the UK, over the three days.

Simplified geological map of Devon showing the three locations (adapted from Kirkwood et al., 2016)

Day 1: Baggy Point - Sandstone

Saturday, starting off at Baggy Point near Croyde. The climbing here is on the 360-370 million year old, Upper Devonian Baggy Sandstone Formation. They overlie the Upcott Slates and are themselves overlain by the Pilton Mudstones, all together a making up a 450m thick succession of interbedded sands, silts, muds and thin limestones charting a changing river delta succession which at first built outwards as sea level fell and then retreated inland as sea level rose again.

Due to their age and the pressures and temperatures they have been subjected to these sandstones are slightly metamorphosed and so are much harder than more recent sandstone deposits (such as those known to climbers as the Southern Sandstones around Kent) and so are a lot more solid to climb on. Routes (such as the classics Lost Horizon and Shangri-Lai) follow angular fractured cracks up otherwise sheer faces while harder routes tackle the blank slabs themselves, relying on delicate footwork and careful movement with little means of protection.

Rob leading Lost Horizon following a steep crack system

The sheer faces plunging steeply into the sea that characterise the climbing here were originally horizontal as the sedimentary sequence was deposited on the sea bed. However, during the Late Devonian and Carboniferous Periods these rocks felt the distal effects of the mountain building event known as the Variscan Orogeny. The continents of Gondwana and Laurussia collided to form the supercontinent Pangea highly folding and faulting the rocks as they were compressed together. At Baggy Point this tilted the sequence very steeply to bring the ancient sea bed to a near vertical orientation and creating the sheer, almost featureless, delicate slabs which are a feature of the climbing here.

Dan leading while I belay on the second pitch of a route up one of the steeply dipping slabs at Baggy Point (credit: C. Wade)

Unfortunately due to the interbedded weaker muds and the highly erosive sea cliff environment a lot of the rock here is quite fragile and we pulled a few dangerous chunks off into the sea as we climbed.

Day 2: Daddyhole - Limestone

Sunday, now we’re climbing a little further back in time to the mid-Devonian at the cliffs of Daddyhole in Torquay. This is very close to the Devonian type section at Torbay (which I have written about before as part of the UEA Slapton fieldtrip). The plan was to climb on the lower part of the sequence at Daddyhole Main Cliff, however, due to the long commiting nature of the routes down there and the incoming rain we were forced to visit the uppermost part of the sequence instead at Daddyhole Upper.

The mid-Devonian Limestones here were deposited around 400 million years ago when the UK was located within the tropics and Devon was beneath a warm, shallow, tropical sea. The limestones here represent a sequence from a thriving offshore reef system, well away from any polluting terrestrial input, with corals, sponges, shellfish and other organisms being highly abundant in the fossil record. Overtime (up the sequence/cliff face) the limestone becomes ‘dirtier’ as more sand and mud reaches the area from the nearby landmass and the reef life is gradually choked out, a process helped by nearby volcanoes occasionally burying the reef in ash deposits.

Spot the Dan, he's pretty much at the boundary between the cleaner massively bedded limestones and the siltier, finer bedded sequence above

The impurity of the limestone and interbedded siltier layers mean that the climbing at Daddyhole Upper is somewhat ‘esoteric’ with plenty of loose, crumbly rock and so it is not the most popular venue. This does however mean it has not taken on the smooth mirror-polished quality of more popular limestone crags (such as much of the climbing in Portland or Cheddar) and the combination of weathered out juggy limestone cracks and grippy rock is a rare delight (as long as you don’t think about how sketchy all the gear placements are. Unfortunately rain quickly stopped play here and we only got a single route in before retreating for a seaside Devonshire cream tea.

Charlotte's favourite type of climbing

Day 3: Dartmoor - Granite

Monday and its back to the future on the granite of Dartmoor. This is part of the massive Cornubian Batholith (batholith = large body of magma) that welled up underneath the southwest of England around 300-275 million years ago, during the Late Carboniferous-Early Permian. This outcrops at various localities all the way from the Isles of Scilly to Dartmoor, but is known (from a low density gravity anomaly) to extend more than 100 km further southwest under the sea.

The formation of this huge body of molten rock is related to the same mountain building event as the folding of the Devonian sediments (that we climbed on the previous two days) it intruded into. Partial melting of the lower crust occurred at a late stage during the mountain building process (after the majority of the folding had already occurred) and extension of the crust allowed the batholith to rise irregularly as ‘blobs’ to higher levels.

Over time, the softer sedimentary rocks that were intruded into have preferentially eroded away, leaving behind these granite ‘blobs’ exposed as Tors on areas such as Dartmoor. As the overlying rocks were removed the granite was unloaded and expanded, fracturing both horizontally and vertically and peeling itself apart. This produced the horizontal breaks, vertical cracks and juggy flake systems that characterise climbing on the Tors of Dartmoor.

Dan wedging himself in a nice big crack, note the horizontal joints too (credit C. Wade)

Large, sharp phenocrysts (big crystals) of plagioclase stand out proud from the otherwise surprisingly smooth and slippery blank sections of granite face. These tell the geologist that the granite cooled in stages, the big crystals grew slowly at depth before the magma rose upwards to shallower, cooler levels and finished solidifying quicker so the rest of the crystals (the groundmass) are much smaller. For the climber, delicate, precise footwork or desperate crimping and hauling with the fingertips on these small protrusions is often the only way to make progress on the harder routes. The sharpness of these crystals tears into the skin restricting the number of attempts you can have at a hard move before bloody fingers stop play, but gives excellent friction allowing your climbing shoes to stick to the smallest nubbin.

Me climbing Vandal & Ann, run out and sketched out trying to figure out the best way of using a series of crappy little crystals to get to the safety of the next big break (credit C. Wade)

Clearly in three days it is only possible to scratch the surface of the number of different rock types and climbing venues available in the southwest and we will return soon to sample more of their esoteric delights. Although it’s back onto my favourite Peak Grit for the upcoming bank holiday weekend.

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