. The Quarterly journal of the Geological Society of London. orges. Where the plain was undercut by the sea it was bounded bycliffs, and the rivers formed waterfalls when they reached the edgeof the cliffs. These falls immediately commenced sawing down-wards and backwards into the land, so rapidly in some cases thatthey led to the formation of gorges. 1 Q. J. G. S. vol. lxiv (1908) pp. 384-400. 2 See The Geology of Dartmoor Mem. Geol. Surv. 1912, p. J. G. S. No. 285. f 66 MB. H. DEWEY ON THE ORI&IN OF SOME [vol. lxxii, The most conspicuous features in these gorges are the numerouspothole
. The Quarterly journal of the Geological Society of London. orges. Where the plain was undercut by the sea it was bounded bycliffs, and the rivers formed waterfalls when they reached the edgeof the cliffs. These falls immediately commenced sawing down-wards and backwards into the land, so rapidly in some cases thatthey led to the formation of gorges. 1 Q. J. G. S. vol. lxiv (1908) pp. 384-400. 2 See The Geology of Dartmoor Mem. Geol. Surv. 1912, p. J. G. S. No. 285. f 66 MB. H. DEWEY ON THE ORI&IN OF SOME [vol. lxxii, The most conspicuous features in these gorges are the numerouspotholes or marmites, and it is not too much to say that they wereinstrumental in drilling out the chasms where now the rivers flowas cascades and torrents. Of these gorges the most characteristicin Cornwall are near Tintagel and Boscastle, and, inland, thoseat Lydford, Luxulyan, and the Bovey Valley. The following descriptions are arranged geographically, com-mencing in North Cornwall and following the coast southwards,and thence to inland localities. Fig. L. The base of the degraded line of cliffs runs nearly parallel with, and between,the 400- and the 500-foot contours. The cliff is marked by the crowding-together of the contours above 400 feet. The plateau (see map, fig. 1) near Boscastle covers an area ofsome 10 square miles, with a coast-line 6 miles long and muchindented with bays, havens, and coves. There are five valleysdissecting this plateau, and, although they are several hundred feet part 1] RIVER-GORGES IN CORNWALL AND DEVON. 67 deep, their existence would not be suspected if the plain were viewedfrom a distance ; for then it appears simply as a wide expanse oflevel fields and waste land. The area is divided by these streamsinto flat-topped blocks of land, where no rock is exposed at thesurface, separated one from the other by rocky chasms. These five valleys exemplify the several stages in the developmentof gorges, and, when seen from the sea, the degree to which theyha
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