The Brantwood Musical Stones

 

There are four rock types that are used for the keys of the main Ruskin Rocks Instrument. These are limestone, hornfels, shap "blue granite", and green slate. On this page we will explore these rocks in a little more detail.

Hornfels

The hornfels used in these instruments is the rock that was used in Victorian times to make the original Lake District lithophones. Recent research has shown that it came from Sinen Gill, north of Keswick, where our material was also collected.

Photo

The rocks of the northern lakes are mainly Skiddaw Slates, deposited originally as mud on the sea floor and buried, heated and compacted to make slates during the Caledonian Orogeny. Subsequently, the slates were intruded by the Skiddaw Granite magma, and close to the intrusion the high temperatures made the minerals react and recrystallise (a process known as metamorphism) to make a coarse-grained, tough, metamorphic rock known as hornfels.

Close up of the rock

The crystals form a strong, interlocking texture and the rock no longer has a cleavage, but it does weather into slabs reflecting the different beds in the original sediment.

Thin section - SEM images

Shap "Blue Granites"

Shap Blue Granite is a trade term for this rock, which is neither a granite, nor, to be honest, very blue! Strictly, it is another form of hornfels.

Image of Shap Blue quarry

It was a volcanic rock originally, formed during the extensive period of volcanic activity in the Lake District during Ordovician times, and was originally laid down as an ash deposit. Because of its composition, it is called an andesite, and is very similar to many young volcanic rocks forming around the Pacific today.

Thin section - SEM images

Unlike most of the volcanic rocks of the Lake District, this rock has undergone subsequent metamorphism due to heating by granite, in the same way as the Sinen Gill Hornfels. In this case the intrusion responsible for the heating was the Shap Pink Granite, a true granite.

Green Slate

The green slates used in the musical instrument are from High Fell Quarry located to the north of Coniston. The green slates are part of the Borrowdale Volcanic Group and were deposited originally as volcanic ash in shallow waters.

Inside the quarry?

They often contain fine layering produced as the ash settled in the water but they can also include larger fragments of volcanic rock. They were compressed to produce the typical Green Lake District Slates during the Caledonian Orogeny, around 400 million years ago. When the rocks were squeezed, the platy mineral grains in the rock re-grew at right angles to the direction of the pressure. Because they are now all lined up in the same direction, this means the rock splits into slabs parallel to the aligned grains.

Thin section - SEM images

This structure is termed cleavage and means that it is easy to extract thin slabs of rock, such as might be used for shelves, cills, tiles or work tops.

Carboniferous Limestones

At least two layers of ringing rocks are present in the Carboniferous Limestone worked at Stainton Quarry, which is located near the village of Stainton with Adgardley, south Cumbria.

Picture of inside quarry

Limestones are sedimentary rocks, and these were deposited in a warm, tropical sea that covered northern England about 350 million years ago. They are made of the mineral calcite, which is calcium carbonate and were originally deposited as shells, broken shell fragments and algal secretions in shallow seas.

Close up of fossils

After the sediment was buried, the calcium carbonate recrystallised and infilled the original pore spaces between the particles, making a hard, compact rock with surprising acoustical properties.

Thin- section - SEM images  


 

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Created by Ruskin Rocks Team, August 2010
Last updated: Rebecca Hildyard, 16 August 2010