THE FARMER'S MAGAZINE. 



27 



Let us uow trace tlie distribution of the erratic 

 deposits over the British Islands. We shall flud 

 that a large portion of them has been covered with 

 transported matter, which must modify considerably 

 the characters which the soil would derive exclu- 

 sively from the rock immediately below it. We shall 

 ilnd, moreover, that those portions of the country to 

 which the erratic tertiaries have not extended are 

 covered by another and subsequent deposit, indica- 

 ting some amount of transport from a distance, and 

 which modifies considerably the characters which the 

 soil would derive from the rock on which it rests if 

 it were composed wholly of the detritus of that rock. 

 This deposit, to which the name of "warp-drift" has 

 been given, is not only spread over the denuded sur- 

 face of the erratic tertiaries, but, where they are 

 absent, is spread over the regular rock-formations of 

 all parts of the series. 



Kent is one of those districts which have been 

 exempt, in a great measure, from the operations of 

 the erratic tertiaries, but in which the warp-drift 

 has produced considerable modifications of soil. As 

 regards the distribution of the erratic tertiaries over 

 Great Britain, we find, commencing in the north, that 

 there are very deep and extensive accumulations of 

 them in Scotland ; but as M'c cannot speak of them 

 in that district from personal observation, we will 

 trace them southwards from the Scottish border. 

 From that point they extend on both sides of the 

 central chain popularly known as the Backbone of 

 England, or the Yorkshire and Derbyshire hills, and 

 called by geologists tlie Penine chain, from its having 

 been the Alpcs J'euina; of the Bomans. On the 

 western side of that chain which forms the water- 

 sh.ed of Eugland, wheuce the rivers flow to the sea in 

 opposite directions, there are considerable accumu- 

 latious of these deposits, and in Iloldernessor thecast- 

 crupart of Yorkshire. These deposits extend, with 

 some interruptions, from more recent alluvial de- 

 posits over Norfolk, SufTolk, Essex, and part of 

 Cambridgesliire, to the edge of the Valley of the 

 Thames. There the boulder clay terminates abruptly, at 

 a high level. The erratic deposits of this part contain nu- 

 merous blocks of stone and smaller fragments, which 

 have been transported from Scotland and Norway, 

 mixed with others of northern origin, but not derived 

 from such great distance. They form the tail of a stream 

 of detritus, which may be traced from Norway and 

 Sweden, through Denmark, Ilolsteiu, Pomtrania, and 

 Wesiphaliu ; while a similar line of transported blocks 

 derived fi-om Lapland extends over Russia and Poland. 



Then, again, on the western side of the Penine chain 

 there are considerable accumulations of transported 

 raattcv, which may be traced, in like manner, to their 

 parent rocks in the lake district of the North of Eng- 

 lanif. Those blocks are of peculiar mineral characters, 

 and being derived from snudl and well-defined areas, 

 are easily identified. The deposits containing these 

 blocks and shells of existing species extend as far south 

 as Bridgnorth. Smaller erratic detritus, derived from 

 the same quarter, may be traced still further, till it is 

 lost in the fine alluvial detritus of the lower part of the 

 valley of the Severn. This line of erratic blocks has 

 crossed the central ridge at one point, ^nd one point 

 only, and that the lowest pass, which opens directly to 

 the west, namely, the Pass of Stainmoor. Descend- 

 ing thence the valleys of the Tees and the H umber, 

 they become mixed with the transported matter which 

 has been before described on the eastern side of the 

 phainj so that block? of rocks deiived from Cum- 



berland are found on the coast at Redcar and in the 

 Vale of York. In the same way the Cambrian erra- 

 tics have travelled eastwards to the mouth of the Tyne 

 along the depression caused by the Tynedale fault, at 

 the northern termination of that cliain ; yet the streams 

 which flow in that direction are quite unconnected with 

 the mountains from which these blocks have been de- 

 rived, thus clearly cstablisliing the fact that they have 

 not been transported by the actipn of existing rivers, 

 under the present conditions of drainage ; while it is 

 equally certain that the general configuration of the 

 surface, as regards hill and valley, was established" 

 before the distribution of the erratic deposits. Not- 

 withstanding this general drifting from north to south, 

 the erratics have in one instance been borne northwards, 

 namely, along tlie valley of the Eden to Carlisle. There 

 they are mixed with blocks of granite, which have 

 been transported from Scotland across the Solway 

 Firth. This partial transport northward is an im- 

 portant fact in corroboration of the formation of these 

 deposits beneath an arctic climafe. In such seas, 

 loaded as they are with ice, the action of tides is un- 

 important; and, besides a general deep current having 

 a ciiustant direction southwards, there are minor capri- 

 cious movements of the ice in an opposite directioUj 

 caused by superficial currents set in motion by the 

 winds. In the narratives of the polar expedition, we 

 find frequent instances of the large ice-bergs, which 

 float with five-sixths of their bulk under water, drift- 

 ing steadily southward with the deep under current, 

 while the lighter floes were curried in an opposite 

 direction by superficial currents set in motion by the 

 wind. 



One peculiarity in the gravel on the northern and 

 southern flunks of the chalk escarpments demands 

 notice. That is the slight degree of abi-asion which the 

 flints composing it have undergone. They are, of the 

 largest size, and have undergone no degree of wear 

 when they are found imbedded in that clayey deposit 

 in the summits of the chalk hills, which in parts of 

 Kent bears the local name of " cledge." There is a slight 

 diminution of size with every stage of descent, but 

 scarcely any increase of abrasion till the alluvial de- 

 posits of the valleys are reached. These facts are dif- 

 ficult of explanation, if we suppose the flint gravel 

 once to have formed a continuous sheet at the higher 

 levels, and to have been slowly removed to the lower 

 levels by ordinary denuding action. It would rather 

 indicate rajjidity in the movements of elevation, which 

 a])pcar to have taken place by pitches. Perhaps, also, 

 this condition of the gravel may be the result of some 

 modified glacio-marine action by no means so intense as 

 that which formed the boulder clay north of the Thames. 



Lastly, there is the angular flint gravel associated 

 witii the loamy de])osit of the warp-drift, which fre- 

 quently passes into masses of such gravel, mixed with 

 fragmentary chalk, somewhat more water-worn, but 

 still only slightly abraded. By some geologists, who 

 have not made sufficient distinction between this angu- 

 lar flint gravel, and the sliglitly worn-gravel be- 

 forenientioned, this fracturing of the flints has been 

 attributed to violent movements, by which they suppose 

 the chalk to have been broken up, and the denudation 

 of the VVcalden area to have been eftected. This an- 

 gular flint gravel, however, is associateed with mam- 

 malian remains, and with shells, as well those of the 

 land as fresh-watsr shells, which are, as was previ- 

 ously stated, all of living species, and identical with 

 the group now inhabiting the neighbourhood, with the 

 exception of one or two species, which are not known, 

 or very rare in this county, but inhabit the opposite 

 parts of the continent, white one species is found no 

 nearer than the Nile. 



