Page:VCH Bedfordshire 1.djvu/35

 GEOLOGY THE strata which form the groundwork of Bedfordshire, under- lying the surface-soil and other superficial accumulations, con- sist of rocks of Jurassic and Cretaceous age. With the ex- ception of the oldest bed that comes to the surface in the county, which is an estuarine deposit, they are entirely of marine origin. The freshwater or lacustrine beds which were deposited in the south of England at the commencement of the Cretaceous period are absent, and so also are the estuarine which were laid down in that area before the close of the Jurassic period. With this break in the succession of the strata there appears to be a slight unconformability, the Cretaceous rocks not lying quite evenly upon the Jurassic. A few small outliers of beds of Eocene age rest upon the Cretaceous rocks, indicating the former extension of the Eocenes of the London Basin far to the north of the main mass. The strata dip more or less towards the south-east, so that, proceeding in a north-westerly direction, they are seen to crop out successively in descending order. Bedfordshire is almost entirely within the catchment-basin of the Great Ouse. Its surface slopes on the whole from south-west to north- east, towards the great marshy tract of the Bedford Level through which the Ouse flows on its way to its embouchure in the Wash. It presents a series of shallow valleys and gently rising hills which follow in the main the same direction as the general inclination of the surface and coincide with the strike or trend of the strata. A rather prominent and very picturesque ridge crosses the county from Leighton Buzzard to Sandy, defining the limits of the Jurassic and Cretaceous rocks and form- ing the water-parting between the valley of the Ouse and that of its tributary the Ivel. Each of these valleys, when viewed from the hills on its southern margin, owing to its shallowness presents the appearance of a plain, that on the north consisting of Jurassic rocks with hills of but slight elevation, and that on the south of Cretaceous rocks rising along its south-eastern margin into hills which attain a considerable altitude. The most prominent of these are known as the Barton Hills and the Dunstable Downs. They form the north-eastern termination of the Chilterns, a range of hills situated entirely on the Chalk and gradu- ally approaching its escarpment in its passage through Oxfordshire, Buckinghamshire, and the west and north-west of Hertfordshire. In crossing the south-east of Bedfordshire this range forms the water- parting between the catchment-basin of the Ouse and that of the