Page:VCH Northamptonshire 1.djvu/31

 GEOLOGY IN the following pages it is proposed to give, as far as possible, a geological history of Northamptonshire as a succession of physical events in defined time periods, and to use the composition and natural sequence of the rocks, as well as the present configuration of the ground, or modern scenery, chiefly in evidence. Geological Maps On looking at the geological map which accompanies this descrip- tive matter, it will be observed that the county is coloured in what may appear to be a very erratic manner. Each colour is supposed to represent the area over which a particular rock formation outcrops, or is otherwise exposed at the surface. The colours are of course purely conventional. The expression ' supposed to represent ' is used advisedly, for over a considerable part of the county is spread a mass of gravel, sand, or clay of glacial origin, which partially or wholly obscures the particular formation represented on the map. The term ' Glacial Drift ' or simply ' Drift ' will be used in a comprehensive sense to include the deposits of all kinds and of relatively different ages directly or indirectly due to ice action. In the maps of the Geological Survey, on the scale of one inch to the mile, the chief areas covered with this confusing Drift are provisionally indicated by lettering, and where the covering is so thick as to leave the extent of the immediately underlying rock in doubt, dotted boundary lines are used for the latter. In some cases there is uncertainty as to whether the rock represented is even present. It may be noticed on the map that the colours tend to form bands lying approximately from north-east to south-west ; this direc- tion of outcrop is spoken of as the ' strike ' of the beds, and along it they preserve more uniformity of height above ordnance datum than in any other direction. The general ' dip ' of the beds is towards the south-east or at right angles to the strike, so that in traversing the county from north-west to south-east newer rocks successively occupy the surface without any concomitant rise of the ground level, but rather the reverse. In the direction of the county's greatest length however, which is nearly that of the strike, a single formation, the Great Oolite for instance, may be met with at the surface almost uninterruptedly from one extremity to the other. We now proceed to give in tabular form the various geological formations to be found in the county, together with their most distinctive characters and approximate maximum thicknesses. Further details with regard to each will appear in succeeding pages. 1