LITMIR.BIZ

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flat-topped cliff or one of the hog’s-back type. The former type, under natural conditions, may be spectacular, the latter rather uninteresting, e.g. falls at Glenthorne and Woody Bay. Litter Water has a vertical fall of 75 feet, and illustrates the rapid incutting of the sea. But some examples of this type of fall have been altered as a result of landslips; Hobby Water and Cleave Fall have, in fact, been obliterated in this way. On the other hand, the dip, strike, and hardness of the rocks play an important part. Milford Water illustrates this extremely well. It is one of the bigger streams, and there are really five individual falls. The uppermost (see Pl. III) is a dip fall, at the bottom of which there is a right-angled turn and the stream flows along a gutter following the strike. There is then another right-angled turn to form a short steep fall running contrary to the dip (the one at the bottom right of Plate III) and beyond it are two smaller falls.

      In one or two cases the streams used to run in their lower courses roughly parallel with the coast. Erosion cutting into the more vulnerable parts of the cliffs more quickly than in other places has succeeded in interrupting their courses, so that a stream may now cascade into the sea rather higher up its valley than was originally the case. The lower valley is now dry. The valley of Speke’s Mill has been truncated by the sea near St. Catherine’s Tor.

There are numerous other examples. In the Old Red Sandstone coasts of eastern Scotland falls are not uncommon; that in the geo¹ at Crawton is a perpendicular cascade. But perhaps the finest coastal waterfalls in Scotland are in the lava areas of the Western Isles. The northern part of Skye is almost wholly formed of lava flows and sills, except that on the east coast the lavas overlie more or less horizontal Mesozoic rocks. The streams draining the interior often tumble over the abrupt cliffs in beautiful cascades. The fall of the stream draining Loch Mealt is well known, but others of like nature occur in Loch Bracadale, Talisker Bay, and elsewhere. At Invertote and Bearreraig Bay, both on the Sound of Raasay, are two handsome falls. In the first the stream has cut through the lava, and cascades over the Mesozoic rocks; in the second the lava is still largely intact. Waterfalls of this type usually imply small streams; a major river might have cut down to sea level. Moreover, their volumes fluctuate a great deal with the rainfall. Since the streams are short and drain but small areas, they are best after heavy rains.

      Another feature that has a great effect on the form of cliffs are the coastal plateaus (see here). The lower ones, and presumably the newer, are the best preserved, and often form extremely level surfaces cut across rocks of different types. The plateau at 180 feet in the Tenby peninsula is very sharply cut (see Pl. XXIII).

      Whatever their origin, the cliffs cut in the lower platforms necessarily show a flat and even crest line, a feature clearly visible in parts of South Wales and Cornwall. Sometimes traces of older platforms can be found inside the one now forming the cliff top, and appear as flat-topped cliffs inside and above the modern ones. It is relevant to note here that rivers cut down through these platforms and have their mouths at sea level. Thus the great powers of marine and fluviatile erosion in comparatively recent geological times can be contemplated.

      It would be possible to expand this account of cliff form almost indefinitely. Each line of cliff has its own special features, and all are worthy of study. Nevertheless the reader will easily imagine variation of form and will undoubtedly know of particular cases. The boulder clay on the chalk at Flamborough Head, the soft sandy cliffs in the Crags and Westleton Beds of the Suffolk coast, the easily eroded Tertiary rocks of Bournemouth Bay, are all steep, all soft, but all quite different. The possibilities are endless, but each in its natural condition is beautiful and interesting.

      There is another way in which cliffs can be considerably modified. If circumstances are favourable landslides may take place. There are several well-known examples, but that at Dowlands, east of Axmouth, is perhaps the most impressive. In order to appreciate the causes which produced the slipping, the following table of rock succession is relevant:

      The dotted lines represent unconformities. Briefly, the cause of the slips depends on the dip, the unconformity beneath the Gault, and the relation of both to sea level. If the junction plane between the Foxmould and underlying clay occurs above sea level, and if it also slopes seawards, erosion of the cliff face removes the outward support of the beds, and so the upper layers slide forward over the lower after periods of heavy rain. This is what has happened at Hooken, and between Axmouth and Lyme Regis. At Beer Head and Whitecliff the unconformity is nearly all below water, and since the cliffs above are wholly of Cretaceous rock, falls of chalk drop directly on to the shore. Slipping of this sort produces undercliffs, the form of which depends among other things on the angle of dip, and also on the nature of the rocks which have slipped. If the rocks are coherent, large unbroken masses may slide down, as at Dowlands; in clays and softer rocks a species of mud glacier may be produced.

Near Dowlands the inland cliffs are of chalk, and the undercliffs are made of much disturbed Cretaceous material. The great chasm was formed in 1839. Since the previous June there had been much rain, and several gales. Fissures and cracks began to appear on the cliff top before Christmas, 1839, and on December 23 one of the cottages began to subside. By 5 a.m. on the 25th it was settling rapidly, and other cottages soon followed suit. The great slip itself occurred on Christmas night. “During December 26 the land that had been cut off by the fissures in the cliff-top gradually subsided seawards, and by the evening had reached a position of equilibrium in the undercliff. A new inland cliff, 210 feet high in its central portion and sinking to east and west, had thus been exposed, backing a chasm into which some twenty acres of land had subsided. The length of the chasm was about half a mile, while its breadth increased from 200 feet on the west to 400 feet on the east.”¹ In all, some eight million tons of earth foundered. The movement also caused a ridge of Upper Greensand (Foxmould and Cowstones) to rise in the sea near the beach; it was about three-quarters of a mile long, and reached 40 feet above sea level at high water. The beds were much broken, and the mid-part of the ridge was connected to the mainland by shingle. The reef very soon disappeared, but the main chasm remains much as it was, except that most of the large pinnacles have gone. Nearly the whole of the area is now covered with vegetation, and the relative movement of the strata has produced a very uneven terrain and varied plant habitats. In Dowland’s Chasm, ash has sown itself abundantly and in several places natural ashwoods, including quite large trees, have developed. They are fine examples of self-sown virgin woodland, a rare phenomenon in present-day Britain. There is no part of the British coast on which there exists such a great extent of varied wild vegetation—a rich field for future research.

There have been several other slips in this neighbourhood, all of which were similarly caused. Another example of major slipping is found at Folkestone Warren. Various ideas have from time to time been put forward to explain these slips, but the most recent views of W. H. Ward² are interesting. His sections show that the slips are primarily caused by the erosion of the Gault toe by the sea, and that the slips are rotational (i.e. turning about a horizontal axis) in character. The plane on which slipping takes place penetrates the full thickness of the Gault, and there is no suggestion that the Chalk is sliding down the surfaces of the Gault. Ward is also of the opinion that the Axmouth slip was rotational.

      There are many other examples of local slipping, including the Tertiary beds of the Isle of Wight facing the Solent, and the undercliff near Niton in the south of the Island. Landslips include a wide variety of phenomena from the great slip at Axmouth to the almost constant loss of sand and fine pebbles that takes place on the high cliffs at Beeston and Skelton Hills, near Sheringham. This constant loss is not a landslip in the normal sense of the word, but any slide forward or downward, whether big or little, ought to be included.

      This chapter began with some reference to the development of the profile of cliffs. It would have been perhaps more logical

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