Chilmark Stone

An example of walling restored with Chilmark stone

After briefly examining the gatehouses at the Tower of London, a variety of building stones - including Kentish rag and Reigate stone - can then be seen in an expanse of walling beyond the Bell Tower, where several phases of construction and restoration are also clearly evident.

The Triton Stone Library

When devising the Triton Stone Library back in 1997, I had investigated possible alternatives for these glauconitic stones and, at the time, the only source of geologically similar Cretaceous stone was from the Nickolls quarry near Folkestone - now a watersports centre and a haven for birds.

The Geology around Tisbury and Salisbury

Another stone that was included was Chilmark stone, a calcareous and glauconitic sandstone from around Tisbury in Wiltshire – from the Jurassic Portlandian beds. It has been used by the Historic Royal Palaces for very many years to restore Reigate stone at the properties in their care, including the Tower of London and Hampton Court.

Chilmark stone used for edging

When living in London, I had briefly explored the area around Salisbury and had collected some samples from the mine at RAF Chilmark and later had an opportunity to cut large quantities of Chilmark stone, when working for a few months at the Gregory Quarries in Mansfield – which mainly produced White Mansfield stone and Ancaster limestone. 

A detail of thinly bedded Chilmark stone with calcite veins

I didn't see any blocks of this stone but on a few occasions, when the crane was bringing a large slab down from the primary saw, it fell into bits and I had a few smaller slabs crumble on my saw table. It was generally pervaded with calcite veins and physically weak and friable, which I thought made it particularly unsuitable for the paving slabs into which it was being sawn.

Testing Chilmark stone for calcium carbonate with hydrochloric acid

Although the samples of Chilmark stone, including the pale coloured stone from the Chicksgrove quarry, were disposed of along with the rest of my building stone collection, the pieces that I use as edging in my garden display this friability and effervesce moderately when tested with hydrochloric acid - and are also easily scratched with a steel knife.

Effervescence indicates the presence of calcium carbonate