Page:VCH Bedfordshire 1.djvu/40

 A HISTORY OF BEDFORDSHIRE The Cornbrash consists of tough pale-grey rubbly limestone, usually shelly and sometimes sandy, and occurring in irregular layers parted by bands of softer limestone and occasional seams of marly and sandy clay. In most of its exposures it is seen to be crowded with fossils, chiefly Lamellibranchiata, the prevailing genera being Avicula, Ostrea, Pecten, and Lima. It is by some geologists considered to be distinct from the Great Oolite, and by others to be a member of it, being everywhere its highest subdivision. It appears to have been laid down at a considerable distance from land in a rather shallow sea varying in depth from time to time. Rarely exceeding 10 feet in thickness, and being a persistent and well-marked formation, it is of value in the determination of the general dip of the strata. The river Ouse, in its devious windings through Bucks and Beds, has cut through it into the Great Oolite Limestone, the in- clination of the river-bed being a little less than the dip of the Corn- brash. Thus at Bradwell near Stony Stratford the river-bed is 200 feet above mean sea-level, and the Cornbrash just comes up to the 300 feet contour-line ; at Harrold the height of the river is 130 feet, and that of the Cornbrash 210 ; and at Bedford the river is 80 and the Cornbrash 120 feet above sea-level. There appears to be a local upheaval at Brad- well in Bucks, but in its range through Beds the dip of the Cornbrash seems to be very slight and fairly uniform, its direction being considerably to the east of south-east. It follows that this is the direction of the dip of the Jurassic strata above and below which are conformable with it, while the Cretaceous rocks dip in a more southerly direction. The Cornbrash is quarried for building, road-making, and lime- burning. Its presence near Bedford was known in the early days of geology (so far back as 1818), when it was called the ' Bedford Lime- stone.' To the rubble on its surface the name ' Cornbrash soil ' was given, this being an old agricultural term for certain stony or brashy soils which are well suited to the growth of corn. 1 The average width of its exposure in the county is about a quarter of a mile and does not vary greatly. UPPER JURASSIC r ,. ... f Kimerida;e Clay. . Zone of Ammonites tip/ex K.mendgian { Ampthil f Cla / c „ A. pluatUh ( = Corallian) (^ „ A. perarmatus Oxfordian -J Oxford Clay and J „ A. cordatus Kellaways Rock „ A. jason J o> The Kellaways Rock is usually considered to be a subordinate division of the Oxford Clay, coming in above the lower portion of the clay, the great mass of which is always above it. Although it is so mapped by the Geological Survey, it would perhaps be better either to call the lower bed of clay the Kellaways Clay, as suggested by Mr. H. B. Woodward, or to group the clay and ' rock ' together under the name of Kellaways Beds. While the clay indicates a rather deep sea, the rock 1 H. B. Woodward, Jurassic Rocks of Britain, iv. 380, 451. 6