Holiday Geology: Dartmoor

For my practice DofE expedition last April, we headed down to Dartmoor… and it’s safe to say that coming from the relatively flat central south England, the hills were a shock! But why is Dartmoor so hilly, compared to the rest of the South?

The dichotomy is often depicted on maps by the imaginary Tees-Exe line: everything above/to the left is more resistant, generally older rock, so tends to be hillier!

The easy answer is that Dartmoor sits on granite, an igneous rock very resistant to weathering and erosion, whereas the bedrock where I live (and much of the South East) is made of softer rock which is more easily eroded down into flat plains by rivers, rain and the wind. To understand why this is, we need to go back in time… about 300 million years back in time!

Time Travel Time!

We find ourselves in the almost alien world of the late Carboniferous/early Permian period, a time of great change. The giant swamps and rainforests of the Carboniferous that eventually would turn into the fossil fuels we used to power early society were slowly being replaced by conifers. The amphibians that dominated the Carboniferous were giving rise to reptiles (well more specifically the ancestors of reptiles, the sauropsids, a group which today is survived by crocodiles and the like). Also presiding over the land were the synapsids, the ancestors of mammals.

Amongst this landscape, we find Great Britain sat roughly equatorially, near the edge of the supercontinent Pangaea, which had only recently formed.

Location of the UK in the Permian (via BGS)

The supercontinent formed when plate tectonics brought 2 smaller supercontinents (Laurasia- which included the land that became the UK- and Gondwana) together, with the subduction of an ancient ocean and eventual collision of the 2 landmasses. And it’s this collision, known as the Variscan Orogeny (a mountain building event), which is the origin of the granite that yields the classic Dartmoor landscape!

As well as Dartmoor, the Variscan Orogeny also left mountains across Europe, such as the Pyrennees, Sardinia and the Harz Mountains, which would have all made up part of the central Pangean mountains during formation.

The Magma Reservoir

Granite is an intrusive igneous rock, meaning it forms when molten rock cools slowly beneath the ground. This means that it tends to have large crystals- more on that later! The origins of the granite in Dartmoor can be traced to this mountain building era of the Variscan Orogeny. It’s suggested that much of the batholith (the name for this sort of large-scale granite intrusion) was formed due to crustal extension- stretching of the earth’s crust. This means there’s less pressure on the rock below, leading to the partial melting of the lower crust, forming a magma chamber! By looking at the exact composition of the granite, we can actually trace the story back even further, and say that the rock that made up the lower crust, before it was melted, was a metamorphic rock from the Proterozoic age- at least 500 million years old but perhaps up to 2.5 billion years old! The unusually high presence of Boron, contained within tourmaline crystals, suggests that some of the magma can be traced to the mantle. This eventually cooled, and now sits below the majority of the South West, stretching from Devon to the Isles of Scilly- the Cornubian Batholith!

But this is only part of the story- the Variscan Orogeny also altered the pre-existing sedimentary rock above the batholith… The compression that caused the layers (strata) to buckle and fold into the dramatically hilly landscape we see today, also resulted in faults and cracks as the pressure built up and up. It is along these lines of weakness that igneous rock from the batholith was able to make its way up to the surface, resulting in the outcrops visible today, including Dartmoor. It also altered the actual lithology (rock type), as the sedimentary rock at the edges of the intrusion was subject to sufficiently high temperatures due to the presence of the magma that the minerals making up the rocks were fundamentally changed, resulting in entirely new rocks, in a process known as thermal contact metamorphism!

Fast Forward

The area is much more tectonically stable now, and the standard processes of deposition (forming new sedimentary rock) and weathering (particularly during the wet Cretaceous period and more recently, periglacial- ie. not quite glacial- activity) began to expose the granite, as the newer layers of sedimentary rock which had previously hidden the granite were eroded away.

The surface processes have also resulted in other interesting formations: On the way up to Cox tor, and around some other tors, we spotted some interesting features. I think these might be thufurs, cryogenic earth hummocks, formed due to interactions between the soil and ice. Or they could be sleeping rock trolls from frozen…

The exposed towers of granite are found at the tops of hills, and are formed when granite with widely spaced joints is eroded, leaving isolated stacks of granite- a tor! Although this initial process of tor formation may have began some millions of years ago, the tors exposed today have only been so for a few hundred thousand years.

As mentioned above, a key feature of the Dartmoor granite is all the large crystals found within it, particularly on the tors (eg. Haytor), which tend to mark the edges of the batholith (hence why they’re more likely to be exposed), and may be areas that cooled very slowly, so the crystals had time to grow.

As the hot granite solidified and began to cool, mineral rich hot water escaped through cracks, eventually cooling itself into crystals of tourmaline or mineral seams of tin, for example. Water is also involved in the formation of another economically important mineral mined in the region- kaolin (china clay), is formed due to water coming not from the intrusion, but infiltrating from the surface. The water becomes acidic (often due to the boron or fluorine present in the intrusion), and reacts with the minerals in the granite, changing feldspar into kaolin.

A unique geology shaping cultural history

The mineral seams of tin have been an important part of the region’s economy since at least the medieval times, but has likely been used for much longer, as acquisition of tin may have been a motive for Romans invading Britain, and perhaps even earlier, given that many of the famous stone monuments come from the Bronze Age (a time when societies began combining copper with minerals like tin to make it stronger). These prehistoric monuments remain visible today, millenia later, due to the resistant nature of the granite used to build them- and amongst the impressive physical geography, are what makes the area so popular with tourists today… including DofE-ers!

I’d originally written this article back in summer, with hopes of creating a series documenting the geology of places during my post A-Level travels… however it turned out I was too busy on said travels, followed by an incredible (and incredibly busy) first term at uni! But better late than never 🙂

Up next in the series (at some point!): the Peak District & the Swiss Alps

Sources/further reading

Time Travel Time!

The Magma Reservoir

Fast Forward

A unique geology shaping cultural history

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