September i, 1910] 



NATURE 



279 



of England. The sand is inconstant in thickness, being 

 sometimes hardly represented, sometimes as much as 200 

 feet. The upper clay runs on its more eastern side up 

 to the chalky Boulder Clay, and extends on the south at 

 least into Worcestershire. On the western side it merges 

 with the upper member of the drifts radiating from the 

 mountains of North Wales, which often exhibit a similar 

 tripartite division, while tas we learn from the officers of 

 the Geological SurveyJ Boulder Clays and gravelly sands, 

 which it must suffice to mention, extend from the high- 

 lands of South Wales for a considerable distance to the 

 south-east and south. Boulder Clay has not been recog- 

 nised in Devon or Cornwall, though occasional erratics 

 are found which seem to demand some form of ice-trans- 

 port. A limited deposit, however, of that clay, containing 

 boulders now and then more than a yard in diameter, 

 occurs near Selsey Bill on the Sussex coast, which most 

 geologists consider to have been formed by floating rather 

 than by land ice. 



.Marine shells are not very infrequent in the lower clays 

 of East Anglia and Yorkshire, but are commonly broken. 

 The well-known Bridlington Crag is the most conspicuous 

 instance, but this is explained by many geologists as an 

 erratic— a piece of an ancient North Sea bed caught up 

 and transported, like the other molluscs, by an advancing 

 ice-sheet. They also claim a derivative origin for the 

 organic contents of the overlying sands and gravels, but 

 some authorities consider the majority to be contem- 

 poraneous. Near the western coast of England, shells in 

 much the same state of preservation as those on the pre- 

 sent shore are far from rare in the lower clay, where they 

 are associated with numerous striated stones, often closely 

 resembling those which have travelled beneath a glacier, 

 both from the Lake District and the less distant Trias. 

 Shells are also found in the overlying sands up the valleys 

 of the Dee and Severn, at occasional localities, even as 

 far inland as Bridgnorth, the heights of the deposits vary- 

 ing from about 120 feet to more than 500 feet above the 

 sea-level. If we also take account of the upper Boulder 

 Clay, where it can be distinguished, the list of marine 

 molluscs, ostracods, and foraminifers from these western 

 drifts is a rather long one.' 



Marine shells, however, on the western side of England 

 are not restricted to the lowlands. Three instances, all 

 occurring more than 1000 feet above sea-level, claim more 

 than a passing mention. At Macclesfield, almost thirty 

 miles in a straight line from the head of the estuary of 

 the Mersey, Boulder Clays associated with stratified 

 gravels and sands have been described by several 

 observers." The clay stops at about 1000 feet, but the 

 sands and gravels go on to nearly 1300 feet, while isolated 

 erratics are found up to about 100 feet higher. Sea shells, 

 some of which are in good condition, have been obtained 

 at various elevations, the highest being about 1200 feet 

 above sea-level. .About forty-eight species of molluscs 

 have been recognised, and the fauna, with a few excep- 

 tions, more Arctic in character and now found at a greater 

 depth, is one which at the present day lives in a temperate 

 climate at a depth of a few fathoms. 



The shell-bearing gravels at Gloppa, near Oswestry, 

 which are about thirty miles from the head of the Dee 

 estuary, were carefully described in 1892 by Mr. A. C. 

 Nicholson. He has enumerated fully sixty species, of 

 which, however, many are rare. As his collection ° shows, 

 the bivalves are generally broken, but a fair number of 

 the univalves are tolerably perfect. The deposit itself 

 consists of alternating seams of sand and gravel, the one 

 generally about an inch in thickness, the other varying 

 from a few inches to a foot. The difference in the 

 amount of rounding shown by the stones is a noteworthy 

 feature. They are ■ not seldom striated ; some have come 

 from Scotland, others from the Lake District, but the 

 majority from Wales, the last being the more angular. 

 Here and there a block, sometimes exceeding a foot in 

 diameter, and usually from the last-named country, has 



1 W. Shone, Quart. Journ. Geol. Soc . xjcxiv. (1878), p. 383. 



2 " Memoirs of the Geolozic.il Survey : Country around Macclesfield." 

 T. I. Pocock (1006). p. 85. For seme notes on Moel Tryfaen and the aUi- 

 tudes of other localities at which marine organisms have been found, see 

 J. Gwyn Jeffreys, Quart. Journ. Geol. Soc, xxxvi. (18S0), p. 351. For the 

 occarrence of such remains in the Vale of Clwyd see a paper by T. McK. 

 Hughes in Proc. Chester Soc, of Nat. His., 1884. 



^ Now deposited in the Oswestr>- Museum. 



NO. 2 131, VOL. 84] 



been dropped among the smaller material, most of which 

 ranges in diameter from half an inch to an inch and a 

 half. The beds in one or two places show contortions ;. 

 but, as a rule, though slightly wavy and with a gentle 

 dip rather to the west of south, they are uniformly 

 deposited. In this respect, and iii the unequal wearing of 

 ihe materials, the Gloppa deposit diflers from most gravels 

 that 1 have seen. Its situation also is peculiar. It is on 

 tlie flattened top of a rocky spur from higher hills, which 

 falls rather steeply to the Shropshire lowland on the 

 eastern side, and on the more western is defined by a 

 small valley, which enlarges gradually as it descends 

 towards the Severn. If the country were gradually de- 

 pressed for nearly 1200 feet, this upland would become, 

 first a promontory, then an island, and finally a shoal. 



The third instance, on Moel Tryfaen in Carnarvonshire, 

 was carefully investigated and described by a Committee 

 of this Association' about ten years ago. The shells occur 

 in an irregularly stratified sand and gravel, resting on 

 slate, and overlain by a Boulder Clay, no great distance 

 from, and a few dozen feet below, the rocky summit of 

 the hill, being about 1300 feet above the level of the sea 

 and at least five miles from its margin. .About fifty-five 

 species of molluscs and twenty-three of foraminifers have 

 been identified. According to the late Dr. J. Gwyn 

 Jeffreys,- the majority of the molluscs are littoral in habit, 

 the rest such as live in from ten to twenty fathoms of 

 water. Most of the erratics have been derived from the 

 W'elsh mountains, but some rocks from Anglesey have also 

 been obtained, and a few pebbles of Lake District and 

 Scotch rocks. If the sea. were about 1300 feet above its 

 present level, Moel Tryfaen would become a small rocky 

 island, open to the storms from the west and north, and 

 nearly a mile and a half away from the nearest land. 



I must pass more rapidly over Ireland. The signs of 

 vanished glaciers — ice-worn rocks and characteristic 

 Boulder Clays — are numerous, and may be traced in places 

 down to the sea-Ievel, but the principal outflow of the 

 ice, according to some competent observers, was from a 

 comparatively low district, extending diagonally across the 

 island from the south of Lough Neagh to north of Galway 

 Bay. Glaciers, however, must have first begun to form 

 in the mountains on the northern and southern side of 

 this zone, and we should have expected that, whatever 

 might happen on the lowlands, they would continue tO' 

 assert themselves. In no other part of the British Islands 

 are eskers, which some geologists think were formed when 

 a glacier reached the sea, so strikingly developed. Here 

 also an upper and a lower Boulder Clay, the former being 

 the more sparsely distributed, are often divided by a wide- 

 spread group of sands and gravels, which locally, as in 

 Great Britain, contains, sometimes abundantly, shells and 

 other marine organisms : more than twenty species of 

 molluscs, with foraminifers, a barnacle, and perforations 

 of annelids, having been described. These are found in 

 Counties Dublin and Wicklow, at various altitudes," from 

 a little above sea-level to a height of 1300 feet. 



Not the least perplexing of the glacial phenomena in 

 the British Isles is the distribution of erratics, which has 

 been already mentioned in passing. On the Norfolk coast, 

 masses of chalk, often thousands of cubic feet in volume, 

 occur in the lowest member of the glacial series, with 

 occasional great blocks of sand and gravel, which must 

 have once been frozen. But these, or at any rate the 

 larger of them, have no doubt been derived from the 

 immediate neighbourhood. Huge erratics also occasionally 

 occur in the upper Boulder Clay — sometimes of chalk, as 

 at Roslyn Hill near Ely, and at Ridlington in Rutland, of 

 Jurassic limestone, near Great Ponton, to the south of 

 Grantham, and of Lower Kimeridge Clay near Biggles- 

 wade.'' These also probably have not travelled more than 

 a few miles. But others of smaller size have often made 

 much longer journeys. The Boulder Clays of eastern 

 England are full of pieces of rock, commonly ranging 

 from about half an inch to a foot in diameter. Among 

 these are samples of the Carboniferous. Jurassic, and 

 Cretaceous rocks of Yorkshire and the adjacent counties, 

 * Brit. Assoc. Report, 1899 (igco), pp. 414-423. 



2 Quart. lourn. Geol. Soc, xxxvi (i88o), p. 355. 



3 See T. iVI. Reade, Proc. Liverpool Geol. Soc. 1653-4, p. 1B3, for some 

 weighty arguments in favonr of a marine origin for these deposits. 



■1 H. Home, Quart. Jour. Geol. Soc, Hx. (1903), p. 375. 



