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THE SCOTTISH

GEOGRAPHICAL

MAGAZINE.

ROCK-BORING ORGANISMS AS AGENTS IN COAST EROSION.

By Professor T. J. JEHU.

(With Illustrations.)

THE burrowing of marine organisms into wood, sand, mud, shells, and stone has been familiar for a long time to the zoologist, but it has received hardly the attention it deserves from the geographer and the geologist. To the latter, the main interest of these borers has lain in the evidence which their perforations sometimes afford of changes in the relative levels of land and sea. In the well-known instance of the ruins of the Temple of Serapis at Pozzuoli, near Naples, marble pillars were found pierced at a height of twelve feet above ground with holes made by the marine bivalve Lithodomus, the date-shell-a form now living only below the level of high-water in the neighbouring seas. Such occurrences afford clear evidence of a recent depression and subsequent elevation of the sea-floor. In geological literature the references made to the part played by boring organisms in the destruction of rocks are only cursory. The destructive action of certain mollusks, annelids, and echinoderms on coral reefs has been recognised. The coral rock is often found so riddled with cavities as to fall an easy prey to the pounding of the breakers along shore. Again, the deepening of the lagoons of atolls has been ascribed in part to the destruction of dead coral by boring animals.

But the importance of the activity of these organisms in the gradual disintegration of rocks along our coasts, and the aid which they afford to the erosive action of the sea have been almost overlooked. Their operations cannot be observed as they are carried on below sea-level, and consequently the true significance of their work has escaped notice.

VOL. XXXIV.

A

Interesting observations were made by the late Mr. Clement Reid 1 on the lowering of the foreshore and sea-bed off the Norfolk coast. At the present day extensive submarine areas occur for some miles off the coast, near Cromer, covered with large unworn flints, and forming celebrated crab and lobster grounds. The flints lie on bare chalk, which in this neighbourhood is softer than usual. These areas are known as the "Norfolk stone-bed." This stone-bed extends from between tide-marks down to a depth of about ten fathoms, and consists of an interlocking mass of angular flints with some erratics. Though a permanent feature, and a source of protection to the underlying chalk against the mechanical scour of the tides during gales, it has been shown that the level of the stone-bed does not remain constant, but is gradually lowered. Mr. Reid noted the growing inaccessibility of certain wreckage and large boulders in the sea near Cromer. "These I could formerly reach dry-shod at extreme low water; now I must wade knee-deep." Formerly the village of Shipden 2 lay between Cromer and the sea, and in the survey recorded in Domesday Book Cromer formed part of the lordship and parish of Shipden. This once flourishing village, with its famous church and spire dedicated to St. Peter, disappeared owing to the ravages of the sea about the time of Henry IV. For long the ancient site of Shipden could be reached at very low tide, but now it lies several fathoms below sea-level. The gradual lowering of the sea-bed to its present level is ascribed not to any sinking of the sea-floor, nor merely to ordinary mechanical scour, but to the long-continued eating away of the soft chalk by various rock-boring organisms, amongst which certain mollusks, annelids, and a sponge play the chief part. The chalk in this area, though soft, is exceptionally flinty. The chalk rock of the seafloor becomes riddled with holes, often large enough to admit the finger, and sometimes larger. The massive rock having been thus reduced to a net-work or honeycomb-like condition easily breaks down, and the included flints are left standing on pillars of the chalk, whence they are readily detached and shifted by mechanical action. The result of this continual cutting away of the soft chalk is that gradually a thick layer of loose angular flints accumulates. It has been estimated that near Cromer the destruction of a thickness of ten feet of chalk will yield a layer at least one foot thick of large flints. In time the sheet of interlocking flints becomes thick and more or less continuous, and the interstices get choked with sand, thus leaving no chalk bare. Organic erosion of the underlying chalk will then cease for a time, but heavy storms often result in the shifting of the flints and the laying bare of another surface of the chalk, when the boring action will begin again.

It would be interesting to obtain some definite evidence as to the rate and extent of this organic erosion. The available data, however, are very scanty. Mr. Clement Reid's remarks on our lack of knowledge regarding these points, and the results of his own observations, may be

1 Report Royal Commission on Coast Erosion, vol. i. Appendix xii. (B), pp. 166-167.