NATURE 



[SEPTtM!J^R I, I9IO 



signs of its erosive no less llian of its transporting power. 

 But what are tlie facts? In these lowlands we can see 

 where the ice has passed over the Molasse (a Miocene 

 sandstone) ; but here, instead of having crushed, torn, and 

 uprooted the comparatively soft rock, it has produced 

 hardly any effect. The huge glacier from the Linth 

 Valley crept for not a few miles over a floor of stratified 

 gravels, on which, some eight miles below Zurich, one of 

 its moraines, formed during the last retreat, can be seen 

 resting, without having produced more than a slight super- 

 ficial disturbance. We are asked to credit glaciers with 

 the erosion of deep valleys and the excavation of great 

 lakes, and yet, wherever we pass from hypotheses to facts, 

 we find them to have been singularly inefficient workmen ! 



I have dwelt at considerable, some may think undue, 

 length on the Alps, because we are sure that this region 

 from before the close of the Miocene period has been above 

 sea-level. It accordingly demonstrates what effects ice can 

 produce when working on land. 



In America also, to which I must now make only a 

 passing reference, great ice-sheets formerly e.xisted : one 

 occupying the district west of the Rocky Mountains, 

 another spreading from that on the north-west of Hudson's 

 Bay, and a third from the Laurentian hill-country. 

 These two became confluent, and their united ice-flow 

 covered the region of the Great Lakes, halting near the 

 ■eastern coast a little south of New York, but in Ohio, 

 Indiana, and Illinois occasionally leaving moraines only a 

 little north of the 39th parallel of latitude.' Of these 

 relics my first-hand knowledge is very small, but the 

 admirably illustrated reports and other writings of 

 American geologists " indicate that, if we make due allow- 

 ance for the differences in environment, the tills and 

 associated deposits on their continent are similar in 

 character to those of the Alps.' 



In our own country and in corresponding parts of 

 Northern Europe we must take into account the possible 

 cooperation of the sea. In these, however, geologists agree 

 that, for at least a portion of the Ice .'\ge, glaciers occupied 

 the mountain districts. Here ice-worn rocks, moraines and 

 perched blocks, tarns in corries, and perhaps lakelets in 

 valleys, demonstrate the former presence of a mantle of 

 snow and ice. Glaciei's radiated outwards from more 

 than one focus in Ireland, Scotland, the English Lake 

 District, and Wales, and trespassed, at the time of their 

 greatest development, upon the adjacent lowlands. They 

 are generally believed to have advanced and retreated more 

 than once, and their movements have been correlated by 

 Prof. J. Geikie with those already mentioned in the .Alps. 

 Into that very difficult question I must not enter : for my 

 present purpose it is enough to say that in early Pleisto- 

 cene times glaciers undoubtedly existed in the mountain 

 districts of Britain, and even formed piedmont ice-sheets 

 on the lowlands. On the west side of England, smoothed 

 and striated rocks have been observed near Liverpool, 

 which can hardly be due to the movements of shore-ice, 

 and at Little Crosby a considerable surface has been cleared 

 from the overlying Boulder Clay by the exertions of the 

 late Mr. T. M'. Reade and his son, Mr. A. Lyell Reade. 

 But, so far as I am aware, rocks thus affected have not 

 yet been discovered in the Wirral peninsula. On the 

 eastern side of England similar markings have been found 

 down to the coast of Durham, but a more southern exten- 

 sion of land ice cannot be taken for granted. In this 

 direction, however, so far as the tidal valley of the Thames, 

 and in corresponding parts of the central and western low- 

 lands, certain deposits occur which, though to a great extent 

 of glacial origin, are in many respects different from those 

 left bv land ice in the .Alpine regions and in Northern 

 America. 



They present us with problems the nature of which may 

 l)e inferred from a brief statement of the facts. On the 

 Norfolk coast we find the glacial drifts resting, sometimes 



1 Some of the glacial drifts on the eastern side of the continent, as we 

 shall find, m 'V have heen deposited in the sea. 



- S»*e the "Reports of the United States Geological Survey" (from vol. 

 lii, onwards), Joimtnl -/ Geology, American Journal of Scifnce, and local 

 publications too numerous to mention. Among these the studies in Green- 

 land by Prof. Chamberlin are especially valuable for the lieht they throw 

 on the movement of large glaciers and the transport of debris in the lower 

 part of the ice. 



cannot always he so sure of the absence of the sea. 



NO. 2 131, VOL. 84] 



on the chalk, sometimes on strata of very late Pliocene 

 or early Pleistocene age- The latter show that in their 

 time the strand-line must have oscillated slightly on either 

 side of its present level. The earliest of the glacial 

 deposits, called the Cromer Till and Contorted Drift, pre- 

 sents its most remarkable development in the cliffs on 

 either side of that town. Here it consists of Boulder Clays 

 and alternating beds of sand and clay ; the first-named, 

 two or three in number, somewhat limited in extent, and 

 rather lenticular in form, are slightly sandy clays, full 

 of pieces of chalk, flint, and other kinds of rock, some of 

 the last having travelled from long distances. Yet more 

 remarkable are the huge erratics of chalk, in the neigh- 

 bourhood of which the sands and clays exhibit extra- 

 ordinary contortions. Like the beds of till, they have not 

 been found very far inland, for there the group appears 

 as a whole to be represented by a stony loam, resembling 

 a mixture of the sandy and clayey material, and this is 

 restricted to a zone some twenty miles wide, bordering 

 the coast of Norfolk and Suffolk, not extending south of 

 the latter county, but being probably represented to the 

 north of the Humber. Above these is a group of false- 

 bedded sands and gravels, variable in thickness and 

 character — the Mid-glacial Sands of Searles V. Wood and 

 F. \\'. Harmer. They extend over a wider area, and 

 may be traced, according to some geologists, nearly to 

 the western side of England, rising in that direction to a 

 gi eater height above sea-level. But as it is impossible to 

 prove that all isolated patches of these materials are 

 identical in age, we can only be certain that some of them 

 are older than the next deposit, a Boulder Clay, which 

 extends over a large part of the lowlands in the Easteri} 

 Counties. This has a general resemblance to the Cromer 

 Till, but its matri.x is rather more clayey and is variable 

 in colour. In and north of Yorkshire, as well as on the 

 seaward side of the Lincolnshire wolds, it is generally 

 brownish or purplish, but on their western side, and as 

 far as the clay goes to the south, it is some shade of grey. 

 Near to these wolds, in mid-Norfolk, and on the northern 

 margin of .Suffolk, it has a whitish tint, owing to the 

 abundance of comminuted chalk. To the south and west 

 of this area it is dark, from the similar presence of 

 Kimeridge Clay. Yet further west it assumes an inter- 

 mediate colour by having drawn upon the Oxford Clav. 

 This Boulder Clay, whether the chalky or the purple, in 

 which partings of sand sometimes occur, must once have 

 covered, according to Mr. F. W. Harmer, an area about 

 ten thousand square miles in extent. It spreads like a 

 coverlet over the pre-glacial irregularities of the surface. 

 It caps the hills, attaining sometimes an elevation of fully 

 500 feet above sea-level : ' it fills up valleys," sometimes 

 partly, sometiines wholly, the original floors of which 

 occasionally lie more than 100 feet below the same level. 

 This Boulder Clay, often with an underlying sand or 

 gravel, extends to the south as far as the neighbourhood 

 of Muswell Hill and Finchley ; hence its margin runs west- 

 ward through Buckinghamshire, and then, bending north- 

 wards, passes to the west of Coventry. On this side of 

 the Pennine Chain the matrix of the Boulder Clay is 

 again reddish, being mainly derived from the sands and 

 marls of the Trias ; pieces of chalk and flint are rare fno 

 doubt coming from .Antrim), though other rocks are often 

 plentiful enough. Some authorities are of opinion that the 

 drift in most parts of Lancashire and Cheshire is separ- 

 able, as on the eastern coasts, into a lower and an uoper 

 Boulder Clay, with intervening gravelly sands, but others 

 think that the association of the first and third is lenti- 

 cular rather than successive. Here also the lower clay 

 cannot be traced very far inland, eastward or southward ; 

 the others have a wider extension, but they reach a 

 greater elevation above sea-level than on the eastern side 



1 Not far from Rovston it is found at a height of 52s feet .above O.D. 

 See F. W. H.armer, " Pleistoc-ne Period in the Kastern Counties," r. 115. 



2 At Old North Road Station, on a tributary of the Cam. the Boulder 

 Clay was pierced to a depth of 180 feet, and at Impington it goes t', 60 feet 

 below sea level Near Hitchin, a hidden valley, traced for seven or 

 eight miles, was proved to a depth of 68 feet below O.D.. afd one near 

 Newport in Essex, to 140 feet. Depths were also found of 120 feet at West 

 Horseheatb in Suffolk, of 120 feet on low ground two miles S.W. of Sandy in 

 Bedfordshire, of from 100 to 160 feet below the sea at Fossdyke, Lone Sut- 

 ton, and. Boston, and at Glemsford in the valley of the Stour. 477 feet of 

 drift was passed through before reaching the chalk. See F. W. Harmer, 

 Quart. Journ. Geol. Soc, l.xiii. (1907), p. 494. 



