CONTEXT 182 : DECEMBER 2024 35 This Horsham stone slate roof was re-laid in 2023. It is a stone that is no longer quarried, putting in doubt the longterm survival of these roofs and the skills necessary to maintain them. This house at Higher Ashton, near Mortonhampstead, Devon, burned down over 20 years ago and has been extensively rebuilt. Buildings have to be maintained, but this can lead to changes in their appearance. Clifton-Taylor offered this jettied former shop in Colchester as an example of cement imitating stone. It has been overpainted so many times that the shallow imitation joints scored into the cement have been filled, and the effect has been largely lost. Understanding materials There have been tens of thousands of quarries and brickworks. Acquiring a knowledge of all these would be impossible. But sharing knowledge of the underlying processes by which materials and chemical compounds are formed and colours arise might help greatly. Stone is created by biological, chemical and physical processes. Igneous rock acquires different forms depending on how quickly it cools: rapid cooling leads to a uniform stone, whereas slow cooling gives time for large crystals to form. Clays are formed by weathering or chemical erosion of some of the minerals in igneous rock into sub-microscopic particles. Sandstone comes from the quartz component of igneous rock. The sorting effect of the flow of water, wave action or wind determine the stone’s grain size and texture. Chalk required the evolution of microplankton which formed calcite skeletons. With regard to colour, most materials are influenced by the absence or presence of iron compounds in their many different forms. The stones of the British Isles go back as many as three billion years in the case of the oldest stones in Scotland. They are a record of the story of the Earth and provide insights into its future. Over that period there have been immense changes. The atmosphere has gone from having very low levels of oxygen and high levels of carbon dioxide, to what we have now. Levels of oxygen have been relatively stable for the past 500 million years. Temperatures and humidity levels have fluctuated widely, as have land and sea levels. The position of the land mass that formed England has changed too, owing to continental drift. A billion years ago, the English land mass was south of the equator but heading north. About 500–400 million years ago, a collision with the supercontinent of which Scotland was a part led to the raising of mountains perhaps as high as today’s Himalayas, that subsequently eroded into sand and clays. Continental drift has taken England’s land mass through different climate zones that are driven by the Earth’s atmospheric circulation. There are three zones – the Polar circulation, the mid-latitude circulation and the ‘Hadley circulation’ at either side of the equator. It is here that the sun’s warming effect is greatest, heating the air, reducing its density and causing it to rise. Replacement air is forced towards the equator, bringing with it rainfall but drying the land on either side of the equator. The result is that the Earth tends to have deserts to the immediate north and south (though this can be strongly affected by the local land mass). This atmospheric circulation pattern is thought to have existed for several billion years. So as England’s land mass journeyed from south
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