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geology

Moons Hill and Stoke Quarries
The rocks extracted from Moons Hill and Stoke Quarries are volcanic in origin and were formed 425 million years ago during the Silurian period of geological time. They form part of a renowned feature of British geological history and have been the subject of much debate and research over the years. The volcanoes which produced the lavas, ashes and other rock types extended over a wide area of what is now south west England. Rocks of the same type and age can be found in Devon and Gloucestershire.

Volcanic lavas, described as Andesites, were extruded over a wide area forming beds up to 100 to 150 metres deep. During this period of volcanic activity, accumulations of volcanic ash also built up, sometimes overlying the lavas, sometimes interbedded with them. The fine-grained ashes are known as Tuffs and often contain blocks or cobbles of Andesite lavas; these formations are known as Agglomerates. Because of the presence of shale and mudstone below the volcanic rocks and sometimes interbedded with them, we know that this volcanic activity was taking place in a marine environment. The whole sequence of lavas, ashes and agglomerates can be readily identified and measured in the Quarry because the beds are now standing vertically on end like books on a bookshelf.

After being deposited in a semi-molten state in horizontal layers, the volcanic rocks cooled and solidified and were eventually buried by thousands of metres of sedimentary rocks including terrestrial sandstones and limestones formed in oceans and lagoons. Many millions of years later these rock beds were subjected to an intense period of structural uplift resulting in the formation of the Mendip anticlinal folds, in the core of which the most ancient formations can be found. The idealised cross section through Moon's Hill quarry shows how the volcanic rocks now lie in relation to the other formations.

At Moons Hill and Stoke Quarries we concentrate on producing aggregate from the Andesite beds, these being generally stronger and more durable than the Tuffs. The latter are however quite suitable for most specifications. The Andesite aggregates are regarded as roadstone of the highest quality.

In order to develop faces of the highest quality material it is first necessary to excavate and strip between 20 to 30 metres of the weathered rock from the surface. Some of this material has to be placed in waste tips, but more importantly, utilised in the screening landscaping and reconciliation of the quarrying operation. Much of this weathered rock layer can, however, be crushed to produce a range of secondary grade aggregates and fill materials which are marketed by Wainwright. The main rock mass is hard Pyroxene-Andesite and described by its trade group as "Basalt". The basalt is surrounded by the much softer Old Red Sandstone. The Basalt after treatment in the plants is supplied both dry and coated to Highway Authorities and Contractors alike.

Chemicals and mineral analysis of typical Moons Hill Basalt Rock gives the following result:

Chemical percentage %
Silica 60.34
Alumina 15.57
Iron Oxide 6.07
Soda 3.62
Magnesia 0.12
Potash 1.88
Calcium Carbonate 5.20
Titanium 0.72
Manganese 0.19
Phosphorus 0.28
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A birds eye view of the Moons Hill operation.
geology image
The Asphalt plant photographed shortly after completion in 2006.
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Stone is carried on high conveyors as it is processed from the hewn rock to the final grades.
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