Castle-an-Dinas Quarry

The pit exposes one part of the Land’s End Granite and is comprised of Permian-aged fine-grained two mica and muscovite granite. Towards the top of the pit, the granite is a pinkish-white, changing to grey at the bottom of the pit. In many publications, the granite is mapped as a biotite granite (i.e. more biotite mica than muscovite mica), but recent research indicates that muscovite is commonly more prevalent; therefore the granite is a muscovite granite.

Other minerals within the granite include quartz, plagioclase feldspar (albite), tourmaline (schorl) and alkali feldspar. Biotite micas are either siderophyllite or lithiu, siderophyllite in composition. There are rare alkali feldspar phenocrysts, typically over 25 mm. Accessory minerals in the granite include rutile, zircon and apatite; these are typically best observed using a microscope. A number of barren quartz-tourmaline veins crosscut the granite, as is common throughout the Cornubian granites.

Previously, the granite has been termed an elvan, a sandstone and a freestone due to its fine-grained nature; it differes substantially in texture from a “typical” Land’s End Granite, such as the one quarried at Lamorna Cove.

On the left is the plane polarised image, with the cross polarised image on the right. Mineral abbreviations: Fld – Feldspar; Msc – Muscovite mica; Qtz – Quartz; Tur – Tourmaline. For more information about looking at rocks under the microscope click here.

The Land’s End Granite is part of the large mass of granite that lies underneath southwest England, termed the Cornubian Batholith. At Castle-an-Dinas, the granite formed in the Early Permian and is approximately 276 million years old. All of the granites in southwest England formed at the end of the Variscan Orogeny, a mountain building event. During mountain building events, heat and pressure results in melting of the crust. As the earth’s crust was stretching apart at the end of the Variscan Orogeny, these melts were emplaced through fault systems over millions of years. The Land’s End Granite is comprised of several distinct pulses of granitic melt, of which the Castle-an-Dinas quarry is one.

The prehistoric hill fort is comprised of three rings, although the third wall is not visible at the site. Very little of the site, including the ditch, is now visible, and is largely overgrown. There is an Ordnance Survey Trig Point in the middle of the fort. The little castle on the south-eastern edge of the site is a little folly named Roger’s Tower. This was built in the late 1700s by an influential local family (the Rogers) and used during the Second World War by the Home Guard as a lookout. The folly was restored in the early 2000s BY English Heritage after being neglected for much of the 20^th Century.