Loading...

Saturday, November 8, 2008

Geology

By kind permission of Professor Dixon: the geology section of "Crickley Hill and Gloucestershire Prehistory":

"The Cotswold Hills are part of a ridge of Middle Jurassic rocks, mainly limestones, that runs diagonally across England from Dorset to N.Yorkshire. The south-easterly tilt of the limestone strata produces a gentle slope towards Oxford and a steep scarp towards the Vale of Gloucester, from which project a number of spurs or headlands. Crickley Hill, pointing diagonally from the scarp edge, towards the west, is one of these. Its sides have been steepened, and its promontory-like shape accentuated, by the quarrying, over centuries, of the oolitic limestone of which it is formed. This rock, so named because it is composed of spherical grains of calcium carbonate resembling fish roe (Greek oion: an egg; lithos: a stone), can produced excellent building stone, formerly transported to areas outside the Cotswolds for use in prestigious buildings. Crickley's stone, however, is of poorer quality, and its use has been local: for the surviving prehistoric structures on the site, and recently for houses, field walls, and, after burning it, to improve the clays of the lowlands; the hill's most recently worked quarry closed only in 1963. Archaeologically, the effect of the quarrying has been most drastic on the southside, now cliff-like in profile, where part of the hillfort rampart and interior has been removed; the original extent can thus no longer be determined.

Although massive below, as may be seen in the recent quarry, the oolite near the surface on the hilltop has been horizontally laminated by exposure. In undisturbed areas, once the topsoil has been removed, the rock looks like great expanses of crazy paving, in which postholes and other man-made features are clearly visible. Many places, however, have been disturbed, not only by rabbit holes and tree roots, but also by geological phenomena. Along the axis of the promontory run large fissures of great depth. Originally vertical joints in the limestone, these had been enlarged to their present surface width of up to 20 feet by seeping water, which dissolves calcium carbonate, and by frost, and they are packed with the tiny fragments of the once solid rock thus destroyed. Known to quarrymen as "gulls", these fissures can be archaeologically confusing: not only do they sometimes superficially resemble back-filled ditches, but real features occurring in them can be less well-defined than the flat-bedded limestone; in them also, because of their softer matrix, are found most of the rabbit holes. In a least one instance the first Neolithic builders to took advantage of their softness and aligned a stretch of their causewayed ditch along the gull. On excavation the ditch was found to have no bottom, but instead a deep fissure, formed along the existing line of weakness only after the ditch was at least partially refilled.

In addition to the main axial fissure other cracks run along and parallel to the side of the hill. These demonstrate clearly the forces of erosion at work, by infinitesimal stages detaching blocks of rock from the slope, so that, over the millennia, the entire scarp recedes, leaving occasional residual masses of harder rock, such as Robins Wood Hill and Churchdown Hill, both of which can be seen from Crickley."

© Crickley Hill Trust, 1977.

No comments: