OCR Text |
Show 306 itEPRObUCTIVE EFFECTS OF TIDES AND CUliRENTS, portions of bold cliffs itltervening betwe.en the mouths o~ rivers consurtied by the sea, this has merely ans:n fro~ the accidental set of the currents and tides during a brief penod. The current which flows from the north-west and bears against our easterh coast, tra~sports, as we have seen, materi.als of various kinds. It undermmes and sweeps away the gramte, gneiss, trap rocks, and sandstone of Shetland, and removes the gravel and loam of the cliffs of Holderness, Norfolk, and SufFolk, which are between fifty and two hundred feet in height, and which waste at the rate of from one to six yards annually. It bears away the strata of London-clay ?n :he c?ast of Essex and Sheppey-consumes the chalk with Its flmts for ma?y miles continuously on the shores of Kent and Sussex-commits annual ravages on the fresh-water beds, capped by a thick covering of chalk flints, in Ham.pshire, and contim~ally sap~ the foundations of the Portland limestone. It receives, besides, during the rainy months, large supplies of pebb!es, sand ~nd mud which numerous streams from the Grampians, CheviOts and ~ther chains, send down to the sea. To what regions, then, is all this matter consigned? It is not retained in mechanical suspension by the waters o~ the s~a~ nor do.es it mix with them in a state of chemical solut1on,--1t Is deposited somewhere, yet certainly not in the immediate neighbourh~od of our shores; for in that case there would soon be a cessatiOn of the encroachme~ t of the sea' and larO'e tracts of low land, like Romney Marsh, , b . d h would everywhere encircle our island. As there IS no'; .a ept of water, exceeding thirty feet, in some spots where cities flourished but ·a few centuries ago, it is clear that the current not only carries far away the materials of the wasted cliff's, but tears up besides many of the regular strata at the bott~m o~ the sea .. The German Ocean is deepest on the Norwegian stde, wheie the soundings give one hundred and ninety fathoms; but the mean depth of the whole basin may b.e state~ at only about thirty-one fathoms''* . The bed of this ~ea IS encu~bered in an extraordinary degree with accumulatiOns of debns, especially in the middle or central parts. One ~f the great central banks trends from the Frith of Forth, m a north- · • Stevenson, on the Bed of the German Ocean, or No1·th Sea.-:Ecl. Phil.Journ., No. v., p.44 i 1820, FitLING TJP OF TRE GERMAN OCEAN. 307 easterly directioh, to a distance of one hundred and ten miles; others run .from Denmark and Jutland upwards of one hundred and five miles to the north-west; while the greatest of a11, the Dogge~ Bank, extends for upwards of three hundred and fiftyfour miles from ~orth to south. 'l'he whole superficies of these enormous shoals Is equal to about one-fifth of the whole area of the German Ocean, or to about one-third of the whole extent of England and Scotland*. The average height of the banks measures, according to Mr. Stevenson, about seventy-eight feet; and, assuming that the mass is uniformly composed to this depth of the same drift matter, the debris would cover the whole of Great Britain to the depth of twenty-eight feet, supposing the stu-face of the island to be a level plain. A great portion of these banks consists of fine and coarse siliceous sand, mixed with fragments of corals and shells ground down, the proportion of these calcareous matters being extremely greatt. As we know not to what distance our continents formerly extended, we cannot conjecture, from any data at present obtained, how much of the space occupied by these sands was formerly covered with strata, subsequently removed by the encroachments of the sea, or whether certain tracts were originally of great depth, and have since been converted into shoals by matter drifted by currents. But as the sea is moved to and fro with every tide, portions of these loose sands must, from time to time be carried . ' mto those deep parts of the North Sea where they are beyond the reach of waves or currents. . So great is the quantity of matter held in suspension by the tidal current on our shores, that the waters are in some places artificially introduced into certain lands below the level of the sea; and by rep.eating this operation, which is called ''warp- . " c mg, 10r two or three years, considerable tracts have been raised, in the estuary of the Humber, to the height of about six feet. Large quantities of coarse sand and pebbles are also drifted along at the bottom: and when such a current meets with any deep depression in the bed of the ocean, it must ~e~essarily fill it up; just as a river, when it meets with a lake lh Its course, fills it gradually with sediment. But in the one *Stevenson, on the Bed of the German Ocean, or North Sea.-Ed. Phil. Journ., No. V., p. 47 i 1820. t Ibid. · |