Page:VCH Leicestershire 1.djvu/46

A HISTORY OF LEICESTERSHIRE have a thickness of 20 ft., and which are not unlike the tea-green marls below. The Rhaetic Beds, which thus altogether have a thickness of about 40 ft., form a narrow band at the foot of the Lias. South of Wigston the outcrop has not as yet been traced, but there is no doubt that it occurs some distance to the west of Lutterworth, although the ground here is deeply covered by Drift gravels, which entirely conceal the rocks beneath. In the neighbourhood of Leicester the outcrop is fairly clear, but becomes very much obscured again where it crosses the valley of the Wreak. North of Barrow-on-Soar it becomes clearer, and these shales, together with the lowest beds of the Lias, form a conspicuous group of hills near West Leake in the next county. Throughout this extended outcrop the only clear exposure of the Rhaetic Beds is that in the brickyard at Glen Parva near Leicester. In the physical history of the earth the Rhaetic Beds show a transition from the continental and lacustrine conditions of the Trias to the open sea in which the Lias was laid down. They are of special interest as they mark the time when the great Triassic lake was invaded by the sea. The stunted character of the Mollusca shows that the conditions of this sea were not suitable for vigorous growth ; while the character of the Bone-bed, with its fragments of bones, scales, and teeth, testifies to the sudden irruption of the water which exterminated the saurians and fish previously existing. LIAS The Lias occupies almost the whole of the eastern half of the county, but it is so much covered over by Glacial beds that it is not exposed over a large part of this area. It forms heavy clay land which is mostly in permanent pasture, affording one of the most renowned hunting grounds in England. The formation is separable, from its petrological character, and the nature of its organic remains, into three distinct horizons, Lower, Middle, and Upper, each of which is further divisible into zones characterized by particular assemblages of fossils. LOWER LIAS This division consists of a series of thin argillaceous limestone bands and shales in the lower part, and a thick series of clays or shale in the upper. It occupies a large stretch of country extending from Lutterworth across the low ground east of Leicester to Melton Mowbray, having a breadth of six or seven miles. In the northern part of the county the Lower Lias covers a large area extending into Nottinghamshire. Over a large part of this district the beds are completely hidden by Boulder-clay ; it is therefore chiefly along the numerous streams, and on the steeper slopes at the foot of the Middle Lias escarpment, that exposures of these beds are met with. These sections show that the Lower Lias may be separated into the following subdivisions or zones characterized by species of Ammonites -.Ammonites (Psiloceras] planorbts, Ammonites (Schlotheimia] angulatus, Ammonites (Arietites} Bucklandi, Ammonites (Arietites) semicosfatus, Ammonites (Oxynoticeras) oxynotus, Ammonites (Aegoceras) Jamesom, and Ammonites (Aegoceras) capricornus. The two highest 12