Geology

The Quarter's underlying rock is Keuper Marl, a stiff red clay 800 feet thick. This is impervious to water, its fine particles being waterbound; it will mixes readily with surface water to form a soft sticky mud. Upon the clay are variable deposits of glacial drift, consisting chiefly of sand and gravel, with some boulder clay, which was strewn thickly over the area at the end of the last Ice Age. This material has been washed off the valley sides by melt-water torrents, but patches of it remain at places in the Cole bed where they form fording points. Drift survives also as a capping on the inter-fluvial ridges, where it forms the highest ground; being relatively porous it soon dries out on top, but it holds water well and springs flow from its edges across the clay below. Overlying both drift and clay is a hundred centuries' accumulation of topsoil, which is wetter and richer in humus on the latter.

The larger stream beds are flowed with alluvium, fine-grained silt, brought down and deposited by flood-waters at the end of the last Ice Age; in natural conditions this is waterlogged. The Cole valley and the lower courses of the Chinn, Shirley, and Spark Brooks are narrow strips of silt bordered by clay slopes (see maps 2 & 3). There are V-shaped exposures of clay extending from the Cole up the valleys of Billesley, Primrose, Robin Hood, and Showell Green Brooks.

An illustration of the accumulation of silt is provided by Titteford Mill Pool. At the upper end, where the headrace dumps its suspended load when checked by the static water of the lake, a delta of black sediment forms. In twenty recent years the bird-sanctuary islands became accessible from shore in dry seasons, and only a narrow channel twisted past them to deeper water. By 1972 the pool became too shallow for boats, and a costly mechanical operation was required to dredge it out. The silt was used to raise the river bank upstream.

The water-worn riverbed gravel which provided a ford and a district name - Greet - can be seen on the upstream side of the Stratford Road bridge over the Cole. It creates an island there unless periodically removed, like that which used to exist at the Warwick Road crossing; to some extent the bridge piers prevent the stone from rolling on downstream. Our gardens contain hundreds of these smooth pebbles, fragments of rock brought by glaciers from Welsh mountains; cultivation mixes topsoil and drift together. Street excavations reach the clay, which is like plasticine when first exposed to air and becomes concrete-hard when dry. It is difficult to work, but will crumble and weather down eventually to a good tilth. Because it was more fertile than drift, it was customary from Georgian times to dig out marl and spread it on the surface, Every farm had its marlpits, the clay being used both for its potential fertility and for making excellent bricks and tiles. The ponds filled with rainwater and were then used to water the livestock.