Pawn of the Pumice Castle: On the Geology & Topography of Cornwall

Recently completed a spatial analysis of Cornwall. While scrutinizing the land, naturally my mind wanders towards investigating the county's geology. What I found was quite a unique hodgepodge of different mechanisms & features which make Cornwall quite distinct from the other 7 counties in the southwest of England that I have also done analyzes for.

Land's End headland on southwest tip of Cornwall.
Resistant sandstone has been shaped by erosion into pillars

Upon sweeping around the place, I noticed how similar land development was to, well, every other county I've looked at. Towns are few, with farms taking up about 90% of the land, with quarries scattered here and there (you'll find out why further down). Being on the southwest peninsula, Cornwall is exposed to the brunt of the Atlantic's influence, so winds are consistent and the temperature & precipitation regimes, thanks to the Gulf Stream, are mild and broken up only by occasional storms. Headland erosion is a process that has made a home in Cornwall, with the northern coastline getting pounded with more voracity than the south. The geomorphology of the northern coast interchanges between sandy beaches and steep cliffs (High Cliff ~240m face; sandstone cliffs near Bude). Resistant metasandstones at Land's End, morphosed due to contact metamorphism, highlight some of the rugged coastline typical of Cornwall. The southern coastline is more reserved, with more gradual gradients from the interior hills and more fluvial delta deposition. Energy from erosive forces have in past periods focused more on the south, so wave-cut platforms are more prevalent.

Looking to the interior of Cornwall, which is never greater than 50km across, one notices broad open landscapes broken up occasionally by tors and moors. These are the result of a granite underbelly that interjected pluton masses from a large batholith, formed during the coming together of our last supercontinent Pangaea. The tor/moor uplands contain quite acidic soils fueled by detritus of granite parent material from the plutons, and serve as the gathering point for headwaters of most of Cornwall's rivers. Bodmin Moor is a textbook example of broad radial drainage pattern from a high peak.

Cornwall surficial geologic units

To set the stage for Cornwall's geological underpinnings, I have to start back in the Devonian when marine sand was being lain down, preparing for its inevitable lithification and rapid uplift (is there any part of England without Devonian sandstone?). The mechanism to bring it above sea level was the Variscan orogeny of the Permian, northern Europe's part of the formation of Pangaea, with a mobile belt stretching in a northeast trending arc from Portugal - Poland. The orogeny uplifted the sedimentary basins, and also introduced folds & faults into various sedimentary formations throughout Cornwall. One particular fold is visible near the aforementioned town of Bude on the northeast coast, where a picture-perfect chevron fold shows the changing direction of thrusting during the length of the Variscan. The only Precambrian rocks exposed at the surface in Cornwall are scattered remnants of Man of War gneiss wherein Phanerozoic strata eroded to expose them; those are mostly found in the tors and moors of the Cornish uplands.

Chevron folding at Millook Haven, part of the Culm Measures formation

An ophiolite was also uplifted during the Variscan, but surprisingly its emplacement/obduction upon the southwest tip of Cornwall was not part of the closing & consumption of the Paleotethys ocean. Rather it was already there, dated to the Devonian and coinciding with the subduction of an arm of the Prototethys. This Lizard complex ophiolite, which is discussed further in this post, was named after the Lizard peninsula which serves as the southernmost extreme point of mainland England @ 50°N 5°12'W.

Cornwall truly differs from other county's in England based on what underlies the strata, and that is the Cornubian Batholith. The Cornubian Batholith (using the Roman name for Cornwall) is essentially a discordant granite batholith that has roughly 8 plutonic extensions that have intruded into the upper crust and been exposed via erosion (though some remain submarine). The batholith emplacement has been dated, using typical radiometric techniques, as Permian, thus matching the theorized Variscan orogeny. The mechanism(s) of emplacement/intrusion into the overlying sedimentary strata are still under debate, with different hypotheses for plutonic injection/growth. There is one concerning a typical diapir mechanism, one that favors feeder dykes supplying magma that was kept hot enough via frequent injections into laccolithic or lopolithic structures in which the shape of plutons were controlled by fault geometry, and finally a hypothesis that favors partial melting and branches of lamprophyre dykes as the source of pluton material via access to mantle magma through a network of extensional faults. These various methods are highlighted in the "Geology of southwest Cornwall" website link, which happens to be someones doctoral research into 'The Tectonics of Variscan Magmatism & Mineralisation in South West England'. It goes into extensive detail about the batholith's petrology, mineralogy, etc... Ultimately the complex of plutons intruded into the Devonian - Carboniferous sandstones above, and as one would expect there is plenty of contact metamorphism that produced schists & metasandstones surrounding the fringes of the plutons.

Cornubian batholith granite outcrops. Diagram shows negative
gravity anomaly caused by granite plutons having a lower density
than surrounding metasandstones & schists + uplift of the terrain

First-hand accounts of geologists examining the tor rocks of the plutons denote that what is visually obvious at first glance is numerous megacrysts and sandstone xenoliths (remember back to early days of first-year earth science for the principle of inclusions and aphanitic - porphyritic - phaneritic - pegmatitic crystal formation). Computer modelling, gravity anomalies (above figure) and density contrasts reveal a total volume for the batholith of 68,000 km³. The overall appearance of the batholith is a tabular body, which is 50-60 km wide at its base, with steep sides, a sloping base (possibly tilted 2-3° further south during post-emplacement movements) and an irregular upper surface that is continually being shaped by denudational forces. Think of it as the area of Cornwall + Devon, but with a depth of sea level - Mt. McKinley.

The batholith has resulted in mineral richness for Cornwall, with mining operations throughout the last century acquiring mineralized Silver, Copper, Lead, Tin, and Zinc, along with quarrying the ammonium-rich granite and kaolinite, that nice soft moldable clay mineral that provides us pottery and loo's. A lot of the mineralization comes from metasomatism processes that allowed elements to coalesce from mass fluid movements.

The icing on the Cornish geology cake has to be the Lizard complex ophiolite, located on the southwest tip of the county. It was obducted onto the protocontinent of Laurasia in the Devonian, and sheared by thrust & extensional faults, especially during the Variscan orogeny. It's true that the vast majority of ophiolites are rarely an intact specimen from META/IGNY basement - pillow basalts on top, but amazingly the Lizard does not stretch its oceanic crustal segments over a large area, certainly not compared to the ophiolites I've studied in the southern Appalachians as part of the Alleghenian orogeny.

Lizard peninsula in Cornwall. Blue line
represents Helford river, a ria that prior
to the last ice age was a true river flowing
along a normal fault

The current composition of the ophiolite fragments are serpentinite, amphibolite, and schist facies, respectively representing the harzburgite basement, the gabbro magma chamber, and the sheeted dykes. There's even some high grade gneiss found in the Goonhilly Downs, which is derived from the Man of War gneiss mentioned previously. Pillow basalts are still pillow basalts for the most part, though they were poorly preserved in this instance. Topographically the peninsula is mostly a raised platform between 50 - 100m, with the Goonhilly Downs satellite dish network (a future space science center) at the highest part. The serpentinite material, along with poor drainage, has given rise to a heathland where dwarf shrubs dominate due to the basic soils (ex. Cornish heath).

Whilst doing some work involving the European Soils Database, I distinctly noted the properties of Cornwall's soils. Looking at the parent material, slate and metasandstones are prevalent for the majority non-moor expanses, whereas acidic soils from the granite plutons make up most of the rest; patchy spots of boulder clay are the only unfeasible spots for agricultural activity. Very thin layers of loess are widespread on the Lizard peninsula. When compared to the rest of the country, Cornwall has quite a shallow depth to rock + a medium-strong erodibility class. Since uplifted sedimentary basins make up the majority of Cornwall's surficial geology, the eroded sandy overburden makes for decent aquifer material, allowing for a high topsoil & subsoil water capacity that is easily available for crops and other plants to soak up.

To conclude this post, I would have to say that observing Cornwall has been a treat. It wasn't immediately unique compared to the rest of southwest England, but after several minutes in I started noticing the big differences, and along with Devon it makes up a distinct geological history from Devonian to the present. A granite backbone, tough sandstone cliffs, and an ophiolite to boot. Makes it a treat to write a lengthy post about the county ... as a way to have a deeper understanding of the place I'm analyzing and to just plain learn more geographical geology. I invite anyone also interested in European history to look into Cornwall's Celtic heritage, and how the Arthurian legends are tied to the place. Those cognizant will certainly know of Tintagel Castle.

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Pawn of the Pumice Castle

January 22, 2011

On the Geology & Topography of Cornwall

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