Sedimentary Rocks

- the limestones and sandstones.

Portland Stone is a high quality limestone which cam take two forms. The first is a compact and solid limestone with only a few clearly visible fossil shells standing out from the surface. Look closely at the surface and you will see that the stone is made from thousands of close-packed rounded grains. It looks like cod-roe, and this fish-egg texture is why geologists call it OOLITE  (Ooion = Greek for egg). The white gateposts of Stoke Newington Town Hall, in Church Street, and St Paul's Cathedral both are made of Portland Stone. You will recognise it all over London

Lime is always likely to be attacked by even mildly acid waters, but as everyone knows these days, the rain which falls on cities has an acid character.  The pillars outside the Town Hall were very smooth in the 1930s, when the Town Hall was built. Very slowly the softer Limestone has begun to weather away under the effect of the rain. The fossils which are slightly harder, have slowly begun to emerge as rough patches. Over the years more and more will show themselves. Rub your hands over them every few years and feel how the process develops..

The second type of Portland Stone is one which is crowded with fossil shells, most of which have had their lime shells dissolved away to leave the stone full of cavities. This stone is known to the quarry men of Portland as 'ROACH'. Walls like this it gives us our best chance of seeing fossil shells. There does not seem to be any Roach in Stoke Newington, but there is plenty to be found in the City.

During the million years which we judge to cover the Ice Age, the ice melted arid froze again. When the ice melted, there were enlarged stream flows over the limestone outcrops of the north Pennines. The water dissolved the stone, producing the fantastic shapes which we now see in what are called 'limestone pavements'. These are huge white deserts, split and divided into thousands of irregular blocks, with ferns and tiny trees growing in the cool cracks.

A water-worn block of this material is illustrated, pitted and furrowed by solution. This limestone is Carboniferous Limestone, probably from Yorkshire, and at least 320 million years old has to be more than twice the age of the Portland Stone (130 million years old).

Imagine how much water must have flowed along the first channel to cut it away like this. Then, thousands of years later, the water was diverted. Perhaps a dropped into the channel and blocked the flow, but there is no stopping water.  Another channel was started and the water cut a fresh  route. Geological time seems endless.

 


Oolitic Limestone

A greatly magnified section showing the small spheres, about 1-2mrn in diameter, like fish eggs, which constitute Ketton Stone.


Shelly Limestone
from the Moberly School building,
Edgware Road, London N 1. (demolished).

 

 

This limestone contains the shells of animals which lived and died over immense periods of time. In the cornea of the playground are stones which came from the Moberly School building, now demolished. It used to be in the Harrow Road and was one of the schools which indirectly combined to form North Westminster Community School.

The School name was carved in a piece of Oolitic Limestone, which was suitable for letter carving, but this ornamental piece was made  of a lower quality Shelly Limestone.

Shelly Limestone from the Moberly School building. The decoration is an amphemion, or honeysuckle. This was a favourite stone decoration for the Greeks and was brought to England in the 18th Century when architects made the Great tour of Italy and Greece.

Working with Limestone

Oolitic limestones contain few fossils or shell fragments, so they can be cut easily in any direction. This is why sculptors prefer them and call them 'freestones' [meaning 'freely workable]. Stones which split easily into sheets, like York Stone, or flaggy limestone, or slate, are not really suitable for sculpture and are not freestones. In the curved wall is one famous type of politic limestone, Ketton Stone, and one Shelly limestone, Reigate Stone. The Reigate contains shells but it can be used for masonry work as they are not large enough to give the stone a flaggy texture. It would probably not be used for sculpture if a better stone was available.

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