A HISTORY OF BEDFORDSHIRE the existing outlines of the hills overlooking the valley of the Lea in South Bedfordshire. The most instructive exposure is that on the Mid- land Railway between the 28^ and 28| mile marks. At the deepest part of the cutting the following beds are seen in descending order : — The base of the Upper Chalk with numerous flints, about 20 feet ; upper seam of Chalk Rock, 2 feet; white chalk with few flints (the Micraster-bed) , 10 feet; lower seam of Chalk Rock, 2 feet; Middle Chalk with few flints, 30 feet. The beds are approximately parallel and are interrupted with occasional faults, the dislocations being very apparent in the seams of Chalk Rock. The series of beds thus shown indicates several variations in the depth of the sea which were accompanied by changes in the fauna. That for the Middle Chalk below the Chalk Rock suggests a deep sea of 1,000 fathoms or more ; that for the lower bed of Chalk Rock much shallower water ; again a deepening for the Micraster-bed, followed by another period of shallower water during the formation of the upper bed of Chalk Rock ; and then a great depression of the sea-floor during the deposition of the Upper Chalk. Much original work on the palaeontology of the Chalk Rock has been accomplished in recent years, especially by Dr. Morison 1 and Mr. Henry Woods, 2 the latter of whom has figured and described several species found near Luton. The most prevalent forms which occur in this district are the sponges Ventriculites, Cepbalites, and Plocoscypbia ; the echinoderms Micraster, Ho/aster, and Ecbinocorys ; the gastropods Trochus, 'Turbo, Avellana, Aporrhais, and Pleurotomaria ; and the cephalopods Nautilus, Ammonites, Scaphites, Turrilites, Baculites, Ptycboceras, and Hetero- ceras. Taken in the aggregate these genera present a group which is incomparable with any other in the Chalk series. The absence of flints in the Chalk Rock, and the finding of a pebble of quartzite in it near Luton, are noteworthy. The deepening of the sea at the commencement of the Senonian or Upper Chalk period was accompanied in this area by a great palason- tological break which indicates a cessation of deposition of strata for a prolonged interval after the formation of the Chalk Rock. The presence of dislocations in the lines of bedding in the Middle Chalk and their absence in the Upper Chalk are confirmatory of the assumption that there were great oscillations in the level of the sea-floor at the close of the Middle Chalk period. The surface of the Chalk Rock also some- times appears to show indications of erosion. The Upper Chalk, of which only the lower portion is present in Bedfordshire, covers the summits of the hills in the south of the county, and is often concealed by accumulations of gravel, loam, and clay. That it has been subjected to an enormous amount of denudation is attested by the numerous flints which are present in the superficial deposits of the district. Those in the drift gravels are usually much 1 ' Notes on the Chalk Rock,' Trans. Herts Nat. Hist. Soc. v. 199-202 (1889). 3 'The Mollusca of the Chalk Rock,' Quart. Journ. Geo!. Soc. lii. 69-98, liii. 377-404 (1896-7). 22
