Ketton Stone capital from St Pancras Station
 


York Stone paving
from Lisson Grove

Sand grain detail of York Stone
showing faint bands of mica.

 

York Stone

The traditional London paving has been York Stone and this continues even today where the boroughs can still afford it. Even up to the present day, new slabs are being laid on City of London streets. Good examples of this stone can be seen outside the older shops in Harrow Road, Lisson Green and Hampstead. Many pavings become polished and slippery with wear, but this does not happen with York Stone for good geological reasons.

York Stone is a sandstone which was formed as a sediment on the bed of a great river system, 300 million years ago in the Carboniferous period. Sand grains were deposited layer by layer by the flowing water. The sand layers were separated by thin spreads of the silvery mica flakes which settled in layers like leaves when the waters were still. Wear and tear of York Stone cuts down the stone mica layer by mica layer, so that the non-slip surfaces are constantly renewed. Sometimes, we can pick out ripple-marks as bands running across the surface - they were formed when the sand grains were ruffled by the currents, as we can notice on a beach or sandy stream edge today.

Metamorphic Rocks

Granites rind other igneous rocks worked themselves through the older rocks by melting their way upwards from the deeper crust of the Earth. They found weak spots in the crust, especially in the roots of mountain chains, melting and heating the surrounding rocks. Older, sedimentary rocks which had lain undisturbed for millions of years were suddenly heated and put under great pressure by the burning magma. Sometimes the melt reached the surface and poured out as white hot lava, like a steel furnace being tapped. Sometimes it burst out as clouds of ash and flying debris, as it did when Vesuvius buried Pompeii, filling every crevice with powder and covering fleeing people in shrouds of stone.

When melts fail to reach the upper crust of the earth, they slowly cool and solidify, the mineral content crystallizing into well-formed shapes. Slow cooling allows large crystals to form and we then have a typical granite forming perhaps five miles below the surface of the earth. Millions of years later the softer covering rocks may have been eroded away by water or wind to leave the granite exposed at the surface.  These are the sudden granite towers seen in cowboy films, with goodies and baddies shooting at each other.

Where the surrounding rocks have not been eroded away, we find that the immediately surrounding rocks have been changed by the heat and pressure of the. melt into very different rock types. These are the Metamorphic Rocks.


Metamorphic rocks altered by heat and pressure.

The metamorphic aureole

 

Carrara Marble

Marble is a metamorphic rock (Greek: meta - change, morphe - form).  At one time it was limestone, a sedimentary rock similar to Portland Stone. Then the limestone was subjected to intense heat and pressure by some geological event such as mountain building stresses. When the rock cooled it had been changed (metamorphosed) into marble. Any fossils in the limestone would have been destroyed and turned into marble with the rest. Sometimes the heat was not sufficient to completely destroy the fossils and they can still be traced in the marble. Snails (gastropods) can still be seen in the Pietra Persichina di Pram between the Marks & Spencer's windows in Chapel Street, Edgware Road. In other cases the fossils are completely destroyed and the marble is re-crystallized, as in the Blue Crystal Marble in Edgware Road, next to Marks & Spencer's.

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