A HISTORY OF HERTFORDSHIRE and arrangement, and contain many sub-angular flints and some un- broken and almost unworn ones, with drifted Oolitic and Liassic fossils, chiefly gryphasas and belemnites, and echinoderms and other fossils de- rived from the Chalk. Much false-bedded sand also often occurs, and sometimes a bed of loam or clay. ' These deposits are exposed and may be examined in nearly all the valleys south of the Chalk escarp- ment. They may be seen in the railway-cutting north of Hatfield, and in a pit on the hill-side east of Horn's Mill. They can be traced all along the hill-side from that place to Hatfield, near Cole Green station, and south of the Mimram near Tewin. In the road-cutting south of Broad Oak End Farm, and along the west side of the Beane between that place and Hertford, some boulder-clay, with glaciated stones, occurs at the base of the gravels. In the gravel-pits near Ware, some finely-laminated brick-earth, belonging to the Mid-glacial series, is seen to be folded and crumpled up and then covered by horizontal beds in the way usually ascribed to ice-action. At Camp's Hill there is also a brick-earth in the Mid-glacial beds, beneath which bones of reindeer, mammoth, and rhinoceros have been found. Mr. S. V. Wood found at Stevenage, in the brick-earths intercalated in the Middle Glacial series, several specimens of Ostrea edulis, a non-arctic shell . . . the only instance of [contemporaneous] fossils being found in the Mid- glacial of the county.' ' The Middle Glacial beds are thus seen to be widely spread over the county, and to be very variable in their origin as well as in their nature. Possibly the prevailing impression with regard to this period does not quite accord with the facts, the term Interglacial which has been applied to it being to some extent misleading. Although the only con- temporaneous fossils known indicate a temperate climate, there are in- dications that the seas of the period were not free from icebergs. The (so-called) ' foreign rocks ' found in our Mid-glacial gravels, which must have been carried a great distance from the north, being fragments of much older rocks than occur in Hertfordshire, and the fossils derived from distant formations, indicate some other transporting agent than rivers or ocean-currents, while the presence of local patches of boulder- clay with glaciated stones, confirms the view that ice-action was not entirely absent. A temperate climate is not incompatible with the occasional presence of icebergs drifting from the north ; but the more likely explanation of the anomaly is that this period was one of long duration, generally cold but with mild intervals when a temperate molluscan fauna migrated to the seas of the British archipelago from the warmer southern waters. Such milder intervals would be most likely to occur when the depression of the land was greatest, and the 1 Elsden, 'The Post-Tertiary Deposits of Hertfordshire,' Trans. Herts Nat. Hist. Sac., vol. i. p. 105 (1881). Prestwich has recorded the finding of pieces of the tooth and tusk of an elephant in gravel, which he believed to pass under the boulder-clay, at Bricket Wood near Watford, but there is some doubt as to the position of this gravel. Geologist, vol. i. p. 241 (1858). 22
