GEOLOGY As the southern counties of England were not glaciated when Northamptonshire was, so at an earlier period Northamptonshire was not when more northern counties were. Certain accumulations of sand in old depressions (valleys) of Pre-glacial age, sometimes described as Pre- glacial deposits,' may have been formed contemporaneously with the Pliocene beds of the east coast, but they may have been formed later, and since they are certainly connected with glacial conditions not far away, they will receive attention in the next section. The Glacial Period Clear evidence exists of two distinct periods of refrigeration in Northamptonshire, and of three periods of diluvial action, corresponding with the advent, temporary recession, and final decline of glacial con- ditions. A common-sense view of what would be likely to happen during the advance and retirement of an ice sheet appears to afford an adequate explanation of the extra-ordinary phenomena of the Drift deposits. Considering the main mass of a glacier to have passed from snow, through neve, to compact ice, a great thickness of ice presupposes a plentiful supply of moisture, but not necessarily great cold, indeed a mean annual temperature not far below the freezing-point of water would suffice for glacier formation. Northamptonshire was never far from the southerly limit of complete glaciation, and so probably fulfilled the above conditions. Suppose an ice sheet advancing from the north. This would necessarily imply, either as a cause or effect, a lowering of the mean annual temperature, but is quite compatible with warm, if short, summers during which the melting of local snow and ice would give rise to great floods, and these would distribute sand and gravel derived from local rocks along their course of flow, the old river valleys, whilst disen- gaged blocks of ground ice and masses of frozen ground would disturb the soft wet clays on which they impinged or grounded. Most of the evidences of this preliminary glacial (or pre-glacial) action were necessarily obliterated by subsequent events, but some remain. There are certain sand beds, from 20 to 40 feet in thickness, with a width varying up to half a mile, occupying an elongated depres- sion, which have been traced for about eight miles on the south side oi the Nene valley, through the parishes of Heyford, Bugbrook, Rothers- thorpe, Milton, Collingtree, Courteenhall, and beyond towards Pid- dington. These sands, finely laminated and apparently quite free from erratics, are certainly to great extent derived from the Northampton Sand. Recently these beds were cut right through at Courteenhall for draining purposes, and it was then seen that at and towards the base of the cutting gravel rather than sand predominated, that this gravel
- John W. Judd, ' The Geology of Rutland,' etc., Memoln of the Geological Sur-vey,
p. 240. 23
