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Show 138 AMOUNT OF EARTH . CIIAP. III. chalk the insoluble matter, including a vast number of unrolled flints of all sizes, has circular space, several feet in diameter, sudoc~ly fell in, loavi~g on the field an open hole with perpendicular s1des, so~o f?et m depth. This occurred in one of my own field s, whtlst_ 1t waf; being rolled, and the hinder quarters of th~ f;haft ho.rso fellm; two ot· three cart-loads of rubbish were reqmrod to fill up the hole. The subsidence occurred where there was a broad depression, as if the surface had fallen in at several former periods. I heard of a hole which must have been suddenly form ed at the bottom of a small shallow pool, where sheep had been ,...-ashed during many years, and into which a man thus occupied fell to his great terror. 'l'he rain-water over this whole district sinks perpendicularly into the ground, but the chalk is more porous in cc rt~in places than in others. 'l'hus the drainage from tho overlymg clay is directed to certain points, where a greater amount of calcm ·eous matter is dissolved than elsewhere. Evon narrow open channels are sometimes formed in the solid chalk. As the chalk is slowly dissolved over the whole country, but more in some parts than in others, the undissolved residue-that is tho overlying mass of red clay with flints,-likewise sinks slowly dowJJ, and tends to fill up tho pipes or cavities. But the upper part of the red clay holds together, aided probably by the roots of plants, for a longer timo than the lower parts, and thus ~or ms a roof, which sooner or later falls in, as in tho above montwncd five cases. Tho downward movement of tho clay may be compared with that of a glacier, but is incomparably slower ; and th is movement accounts for a singular fact, namely, that the much elongated flints which are eml;>edded in the chalk in a nearly horizontal position, arc commonly found standing nearly or qui te upright in the red clay. This fact is so common that the workmen assured me that this was their natural position. I roughly measured one which stood vertically, and it was of the same ll;lngth and. of the same relative thickness as one of my anus. rl'hese elongated flints must get placed in their upright position, CHAP. III. BROUGIIT UP BY WOHMS. 130 been left on the surface and forms a bod of stiff red clay, full of flints, and generally from 6 to 14 feet in thickness. 0 ver the red clay, wherever the land has long remained as pasture, there is a layer a few jncbes in thickness, of dark-coloured vegetable mould. .A. quantity of broken chalk was spread, on December 20, 1842, over a part of a field near my house, which had exidted as pasture certainly for 30, probably for twice or thrice as many years. The chalk was laid on the land for the sake of observing at :some future period to what depth it would become buried. At the end of November, 1871, that is after an interval of 29 years, a trench was dug across this part of the field; and a line of white nodules could be traced on both sides of the trench, at a depth of 7 inches from the surface. ':rhe mould, therefore, (excluding the turf) had on the same principle that a truul· of a true left on a glaci er assumes a position parallel to the line of motiou. 'l'he flints in the clay which form almost half its bulk, arc very oftcll broken, though not rolled or abraded ; and this may be accounteu for by their mutual prcHsurc, whilf;t the whole mass is subsiding. l may add that the chalk here appears to have been originally covered in parts by a thin bed of fin e sand with some perfectly rounded flint pebbles, probably of Tertiary age; for such sand often partly .fills up the dec1Jer pits or cavities in the chalk |