My thanks again to Richard Savage and the Crickley Hill Trust for permission to reproduce material from Richard's 1988 booklet "Crickley Hill: Village, Fortress, Shrine". If you click on the images they will enlarge, but for ease of reading I reproduce the text below.
"Between 210 and 135 million years ago, much of the area which is now Britain was covered by ancient seas. In their warm waters were deposited the clays, sands and limestones that make up the Cotswolds we see today. The plants and animals living in them are preserved as fossils in the rocks of the period, known as the Jurassic, which stretch obliquely across England from Dorset to Yorkshire. The hard limestones end in a scarp on their western side which drops abruptly for several hundred metres to the valleys. These rocks were lifted from the seabed by earth movements which also tilted them so that in the Cotswolds they sloped gently downwards towards the southeast.
Crickley Hill is a promontory of the Cotswold scarp, 8 km from Gloucester and 5 km from Cheltenham. The nature of the limestone uplands has had a profound influence on the use of the hill in ancient times and our approach to studying it. The Stone ("oolite") is permeable to water, providing a light well-drained soil with clear springs issuing at a lower level - 60 m below the summit of Crickley - where the limestone meets impermeable clay. Much of the stone, especially that near the surface, splits easily into pieces of the size and shape suitable for building. Post holes and stake holes can be made in it with simple tools, though the rock is hard enough to discourage casual digging. It is easily and permanently discoloured by fire, and at high temperature (900 - 1,000°C) turns to quicklime, which, when slaked, forms a powdery mortar-like aggregation. The Crickley stone is not of the best quality, so there has been only local demand for it as building stone, though it has been burned for line and used as road stone. Accordingly the damage to the promontory by quarrying has been limited; it has removed several metres from the south side, and made both sides steeper. They were always steep enough to make ascent troublesome and to prolong a climber's exposure to observation from above. (Illustration: Exposure of oolitic limestone in the Crickley Hill quarry)
Within 25 km of Crickley more than a dozen hill-forts take advantage of similar conditions.
Flint, on which everyday technology depended in Britain until after 2000 BC, does not occur in limestone, though small pieces of poor quality may be found in gravel deposits. In this, the inhabitants of Crickley and their neighbours could never be self-sufficient, and they had to obtain supplies from areas of chalk to the south and east, none nearer than 50 kms away. All flint found on the hill was brought by man.
A natural hollow, 100m long and 25 m wide, runs beside the southern edge of the hill at its western end. It represents a fissure in the rock which it will eventually break the mass to the south of it away into the valley, but it has been stable for at least 6000 years, since the neolithic period. The limestone along its bottom and has degraded to a matrix of small particles which is easy to dig into. The traces of neolithic houses the best preserved here, where substantial postholes could most easily be made with the tools of time, and here, too, we can follow the small grooves and stake holes intended for wattle fencing. (Illustration: Typical fossils of the Crickley Oolite)
The soil on the hill is shallow, seldom exceeding 15 cms thick. Because of this the stratigraphy - the sequence of deposited layers which can be distinguished from one another and analysed to establish a succession of events - is severely limited. There are substantial accumulations only in such large features as ditches and quarry pits, and not always there. On the other hand, all but the most trivial ground-work has always had to affect the bedrock, in which its traces are usually permanent and unmistakable.
Since the stone resists erosion fairly well, and even the normal methods of building "drystone" (mortarless) walls make surprisingly durable structures, we can usually recognize even slight remains of walling, the residues of ancient demolition. It is a feature of the site that the prehistoric walls have not been much robbed of their stone in recent centuries. Unofrtunately the Crickley soil preserves seeds, pollen, and other organic remains only when they have been carbonised by heat, and not always then. On the other hand, it preserves snail shells very well indeed. These can often be used as evidence of the nature of the environment during the snails' lifetime at the places where they are found; species can be identified which prefer grassland, for example, or shady woodland. It also preserves bone, when this has been buried, and thus protected, in pits and ditches.
Crickley Hill may have had a key position in the prehistoric geography of the region, offering a near approach of the Cotswold upland to the lowest part of the Severn at which it could be crossed into Wales without much difficulty, perhaps on the northern side of Gloucester. The hill has easy access eastwards to the Thames Valley and south-east Britain while any trade with running from south-west to north-east along the high ground of the limestone scarp must have passed close to it. It is tempting to speculate that Crickley may have stood near, and at some periods on, a cross roads, where apart from the river crossing in the Vale reached the uplands and divided into north, south and eastward routes, and this was recurrent motif in its long story. (Illustration: Prehistoric use of oolite. The wall of the second hillfort"
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