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THE GEOLOGY
of Hopton-Wood Stone

Millions of years ago, long before the appearance of mankind, most of the British Isles lay under water, and the grand mountain country which we know as Derbyshire was merely a bed of limey mud. Conditions in this part of the sea were favourable to the existence of certain marine organisms and particularly to certain types having hard parts composed of calcium carbonate. They included corals, crinoids (sea-lilies), brachiopods (twin shelled molluscs), and foraminifera (minute organisms with a chambered shell); and their skeletons, accumulating on the sea floor, were buried, not only by successive deposits of skeletal remains, but also by quantities of calcium carbonate precipitated from the sea water. Terrific pressure over millions of years from higher and higher sediments and from earth movements that sank our valleys and flung up mountain peaks, compacted them in their partly recrystallised calcite matrix to make layer on layer of limestone.
It was the era known to geologists as the Carboniferous period for, towards its close, vast primeval forests were buried and crushed, and became coal; so the 'Mountain Limestone' of this period is more scientifically called Carboniferous and a very beautiful variety of Lower Carboniferous Limestone is the unique Hopton-Wood Stone.
Unique, because there is only one Hopton-Wood Stone, which is quarried by The Hopton-Wood Stone Firms Limited, at 

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Middleton-by-Wirksworth, near Matlock Bath, in Derbyshire. In a district famous for remarkably pure limestones, Hopton-Wood stands out as exceptional. The calcium-carbonate content of other stones is very high, frequently exceeding 94% of the rock mass, yet the average calcium-carbonate content of Hopton-Wood is more than 99%, while its iron content stands at the extraordinarily low figure of 0.02% This purity, this absence of foreign sediments of grit and sand and mud, shows that no streams were draining into that part of the sea-bed during the long ages when the Hopton-Wood Measures were being deposited. Tranquil and relatively constant settling led to very slight variation in chemical purity from bed to bed, which are of an unusual thickness. A bedding plane usually indicates a pause or changed conditions in the deposition; the thick beds of the Hopton-Wood Measures prove that interruptions were less frequent here than in the formation of most other mountain limestones.
For some reason yet to be explained, Hopton-Wood is less erratically jointed than adjacent limestones. joints are caused by shrinkage of the mass in drying-out and in compacting, or by folding and faulting of the strata: in Hopton-Wood the effect of these actions seems to have been reduced to the minimum, and large blocks of the stone can readily be quarried.
Visitors to Middleton Quarry observe across the face a band of fine-grained impervious clay, which is the decomposed remains of lava that flowed from some submarine volcanic eruption, scaling off the sediment below. This feature, known locally as the 'Great Clay', was the last of Hopton-Wood's primeval blessings, for it has formed a sill, preventing the percolation into the lower measures of water which would not only have enlarged the joints but filled them with quantities of clayey material whose iron constituents must have ruined the delicate even colouring which is one of the great charms of Hopton-Wood Stone.
The lava overflow ceased ; upon it the depositing of limestone was resumed, but not as uniformly as before. Above the 'Great

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Clay' lies the ordinary mountain limestone of Derbyshire. This makes excellent road-surfacing and furnace-flux, but has not the solid, uniform texture which renders the Hopton-Wood Measures so attractive to architect and sculptor.
Hopton-Wood Stone is Dark or Light, according to whether it comes from the top or the bottom of the quarry face. Perhaps the beds which lie immediately below the band of igneous clay were impregnated with volcanic ash blown into the sea before the onset of erupting lava. That is a geological point which has not been settled, but the two complementary yet contrasting tones of Hopton-Wood have an attractiveness that needs no arguing.
The qualities which distinguish Hopton-Wood Stone were early recognised and its reputation as an English stone of great beauty has grown steadily during the hundred and fifty years since its first uncovering. Architects and builders know that for embellishment and enrichment no stone can surpass it, for it may be supplied in all lengths from ten feet downwards and in thicknesses varying from three feet to three quarters of an inch; while its fine and compact texture combined with extreme hardness enable the most intricate designs to be worked sharply and reliably, with a polish as brilliant as that of any marble obtainable.

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Sir Herbert Baker, RA, and T.Scott FRIBA

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Copyright © 2001 Jeremy Hewitt
Last modified: May 14, 2002

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