Geology highlights: Penzance to St Ives

One of the stand-outs for me about this coastline is the spectacular geology - it’s incredibly varied, with some of the oldest formations I will see until I get up to Scotland. Here is a little summary of the geology that has stood out to me so far.

Penzance and Mount’s Bay

Mainly Devonian slates and sandstones with more recent alluvial deposits and beach sands in the bay,, and the beautiful St Michael’s Mount a granite outcrop connected to the mainland by a tidal causeway.

Mount’s Bay to Porthcurno

Metamorphosed mudstones and slates apparently called killas appear along side granite intrusions from the larger Land’s End Granite mass which I crossed on the second day. We started to notice striking black “dykes” - exposures of contact metamorphism, where the granite intruded into the sedimentary rocks. I hadn’t been aware of these before but they are called hornfels.

There are also quartz veins in the cliffs , part of the mineralization for which the are is famed for the historical mining of tin and copper which I was to see increasingly on subsequent days.

Zennor to St Ives section

I will always remember this stretch for the classic granite cliffs, part of the Land’s End granite backbone, and a particular love of mine at Bosigran Castle. The coastline here is particularly rugged underfoot with a boulder field to navigate. There has remarkable weather g and erosion of this tough granite giving rise to these fabulous granite tors and offshore outcrops.

Inland is extensive granitic moorland, with erratic boulders - and a lot of surface water pouring off the slopes leading to pretty extensive boogy areas!

At Gurnard’s Head and Zennor Head, the metamorphosed killas reappear alongside the glorious granite outcrops and toes, leading to some imposing scenery and contrasting rock colour.

All along this coast there are some gorgeous coves with golden sand made up of eroded mica and feldspar - and the fabulous turquoise waters which lift my spirits so much.

There are also peri glacial head deposits - basically frost-shattered and consequently friable rock layers on the cliff slopes. What I hadn’t taken in - I’m ashamed to admit - were the raised beaches and wave cut platforms resulting from changing sea levels during the last ice age - obvious now I think about it, and will look out for going forward…… my “excuse” was I was too busy looking at the rocks!

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