Another interesting exposure of these Pre-glacial gravels is seen near Newell Wood, between Pickworth and Holywell. (Fig. 17.) b. Light-blue clay with a few fragments of chalk and flint c. Reddish-brown sands with a few pebbles and waterworn 1 ft. 1 to 2 ft. 12 to 18 inches. d. Well stratified gravels, almost wholly made up of pebbles (all well waterworn) of the Lincolnshire Oolite Limestone with some of the harder beds of the Northampton Sand. Note. The upper surface of these gravels is very irregular, the sands and clays above being let down into the hollows of its surface. Frequently we find at a similar level upon opposite sides of a modern valley, or at two points on the skirts of the same outlier of Boulder Clay, small patches of this gravel, and in some cases evidence has been obtained in wells, of the existence of these gravels at intermediate points under the Boulder Clay. All the characters presented by these point to the conclusion that they occupy the beds of old rivers which drained the country before the deposition of the great marine glacial series. It is even possible, by comparing the positions and levels of these patches of gravel, to arrive at some approximate results as to the courses in which these old pre-glacial rivers flowed. The existence of similar old river channels under the Boulder Clay of Scotland has been described by Messrs. Croll and R. Dick. At Ring Haw Wood, one mile west of Yarwell, we have a section of these beds displaying very interesting features. Lying on the Great Oolite we find beds of white gravel 8 to 12 feet thick, made up of water-worn fragments of the Lincolnshire Oolite limestone. This white gravel gradually passes up into, and is covered by, beds of dark brown gravel, made up almost exclusively of the detritus of the Northampton Sand. Bearing in mind the relative position of these parent rocks, we are at once led to conclude that the river, which formed this gravel, must have flowed from the west, and have cut its valley in the higher part of its course, first through the Lincolnshire Oolite and then down into the Northampton Sand. In some places, as near Holywell Lodge, we find these gravels cemented by calcareous matter into great masses of solid rock. b. Pre-glacial brick-earths.-There is only one point within the limits of Sheet 64 at which I have found these beds exposed, namely in the brickyards at Melton Mowbray. Similar beds, however, occur at Billesdon, at Moulton, near Northampton, and at a number of points in the district of the Keuper. In all cases they appear to be formed of the detritus of a local rock, rearranged and finely stratified. At Melton this local rock is the clay of the Lower Lias, and the beds might at the first glance be easily mistaken for the undisturbed beds of the Lias, especially as they include numerous derived fossils from that formation. At this place these brick-earths are overlaid by beds of sand, and these by the ordinary Boulder Clay, which near here attains a thickness, as proved by well sections and borings, of not less than 200 feet. c. Pre-glacial Sands and Gravels.-These beds, which are much more widely distributed than the two former classes, present very variable characters. Sometimes they consist of beds of well stratified sand, with a few well rounded pebbles; but they pass by insensible gradations into gravels, in no respect different from those intercalated in the Boulder Clay, and from which they are only distinguished by their position below that formation. They are probably of marine origin, but as yet, unfortunately, neither bones nor shells have been detected in any of the Pre-glacial beds of this district; and we have no palæontological evidence to assist us in determining their age. * On two river channels buried under drift, belonging to a period when the land stood several hundred feet' higher than at present, by James Croll. Trans. Edinb. Geol. Soc., Vol. I. p. 330. On the discovery of a "Sand-dyke" or old River Channel running north and south from near Kirk of Shotts to Wishaw, Lanarkshire, by Robert Dick. Ibid, p. 345. d Pre-glacial? Lacustrine deposit.-In the Casewick cutting of the Great Northern Railway a deposit, of small extent, containing plants and shells of freshwater and terrestrial species, was Fig. 18. Section of the Gravel, Freshwater Bed, and the Oolites of the Casewick cutting. Length, 29 chains. Vertical scale, 120 feet to 1 inch. d. Sandy bed of the Oxford Clay. c. Freshwater deposit. g. Cornbrash, along the base of the cutting. f. Dark laminated Oxford Clay. e. Sandy rock (Kellaways). (Inserted by permission of Professor MORRIS and the Council of the Geological Society.) observed and described by Professor Professor MORRIS' description of the section "Casewick Cutting. Freshwater beds. The Bithinia tentaculata and opercula, plentiful. Planorbis marginatus, rare. imbricatus, only one. Limneus pereger, rare and immature. obtusale? mostly immature. * Quart. Journ. Geol. Soc., vol. ix. (1853), p. 321, Fig. 2. t "The above list has been corrected by Mr. PICKERING, who has kindly examined some portions of the clay from this deposit. To Mr. T. R. JONES I am obliged for determining the above mentioned Cyprides." The freshwater deposit on one side of the cutting appeared to be intercalated with the superincumbent gravel, but on the eastern side there appeared a welldefined line between it and the overlying gravel, as if the freshwater deposit had been eroded; the gravel forming a continuous and uniform covering over this bed and the adjacent sandy and argillaceous strata, in a depression of which the freshwater bed had been previously accumulated. The gravel deposit consists, chiefly of rounded and angular flints, rolled quartz pebbles, and a few other rocks, as oolite, &c., and some small sandstone boulders, irregularly stratified with occasional layers of small pebbles, seams of clay and loam, and others much mixed with a chalky paste, the larger pebbles occurring at the base. The gravel overlies 3 feet of greyish brown sandy clay, containing fragments of Belemnites and Gryphæa, with veins of gravel at the upper part, which is irregular and wavy." Three miles to the westward, in the valley of the Gwash, another freshwater deposit, about 6 feet thick, intercalated with gravel, has been met with; it contains land shells, &c., and bones, and may be of slightly later date than the one above described. B. GLACIAL DEPOSITS. This division includes a great mass of deposits, which, although they have suffered very extensive denudation, yet are often of great thickness in this district in places probably not less than 300 feet. They were evidently deposited during a period of intense cold, in which the land had undergone very extensive submergence; that portion of it which remained above the sea appears to have been enveloped with great glaciers, like those which are now only found in the arctic and antarctic regions, while all over the bed of the ocean transported fragments of rock were dropped by floating icebergs. a. Glacial or Boulder Clay.-No formation occupies so large an area in this sheet as the great mass of clay usually crowded with fragments of all sizes of rocks, for the most part foreign to the district, and which is known as the Glacial or Boulder Clay. This clay where unweathered is usually of a blue colour, and though it occasionally appears to be rudely stratified, yet it is generally characterised by the absence of any regular arrangement in its materials, the confused heaping together of which is a most striking feature. The rock fragments included in it, which often exhibit the polishing, striation, and grooving characteristic of glacier- or iceberg-borne masses, consist of very various materials; chalk and flint being the most abundant, especially in the eastern part of the area. In places the chalk is so abundant that the bed is little more than a reconstructed mass of that rock, and even produces the vegetation which characterises the chalk soils. Thus the Rev. M. J. Berkeley informs me that he, many years ago, found growing on a patch of very chalky Boulder Clay at Benefield specimens of the Orchis ustulata, Linn. ; a species which is usually confined to chalk downs and never appears on noncalcareous soils. It was probably a patch of this kind at Ridlington in Rutland which led to a notice in the Philosophical Transaction for 1821 on the discovery of chalk in that county, which has been referred to both by Mr. Lonsdale and Dr. Fitton.* - Next in abundance to the fragments of chalk and flint, are those of the Jurassic rocks, which become more numerous in the western part of the area; then follow blocks of coal-measure sandstone, millstone-grit, and carboniferous limestone, while the older Palæozoic and granitic rocks are comparatively rarely represented. The Boulder Clay is found in many places capping the hills composed of Jurassic rocks; but in other cases it may be seen extending to the bottom of some of the deepest valleys, and it even underlies a portion of the Fens. It appears to have been spread like a great mantle over the surface of the denuded and submerged older rocks. When a junction is seen, these latter often present the appearance of having been eroded or reconstructed to the depth of several feet before the deposition of the Boulder Clay. The far transported boulders in this district do not generally attain to any great size, though blocks of coal-measure sandstone and millstone-grit up to six feet in diameter are occasionally met with, as at Upton and Hallaton, which have been left on the surface by the denudation of the enclosing Boulder Clay. But the transported masses of local rocks are sometimes of enormous size, especially in the northern portion of this area, and in that to the north (Sheet 70). The attention of geologists was first directed to these great transported masses by Professor Morris, who found that at the south end of the Stoke tunnel on the Great Northern Railway, an enormous mass of the Lincolnshire Oolite limestone lay on undoubted Boulder Clay. During the mapping by Messrs. Holloway, Skertchly, and myself, of the districts which I have indicated, we have found a number of such transported masses, some of them far exceeding in size that described by Professor Morris, and composed both of the Inferior Oolite and the Marlstone Rock-bed. The position of these transported masses is indicated upon the drift map. They always appear to occur in the lower part of the Boulder Clay; and by the denudation of the softer surrounding material often make a distinct boss, rising above the general surface. Stone pits are often opened in them, and they sometimes give off springs at their base. The largest of these transported masses, that capping Beacon Hill in Sheet 70, is more than 200 yards across and is composed of the Marlstone Rock-bed. It is noteworthy that these masses always belong to *Phil. Trans., vol. lxxxi. pt. 2, p. 281, referred to by Dr. FITTON in Trans. Geol. Soc., 2nd ser., vol. iv., p. 308, and notes, p. 383*. |