14 



GEOLOGICAL NOTES 



However, my object is not to c^ive a list even of the human 

 reHcs found in the excavations, hut only to allude to them so far 

 as may be necessary to illustrate the formation of these recent 

 river-deposits, and consequently that of those which are older 

 and more obscure. Where a river has cut its way through com- 

 paratively hard rocks, it may leave terraces clear enough to be 

 traceable for a considerable distance marking its former position. 

 But this is not the case where the river's course, as in South 

 Essex, has been through a formation like London Clay. Then, 

 as we know, it may be possible to note a broad flat composed of 

 river-deposits which rises very gently northward, but it is im- 

 possible to trace a distinct terrace except for a few yards here 

 and there. x\s in a river-deposit the MoUusca usually vary but 

 little, it has naturally happened that geologists examinmg 

 sections in the older and more obscure fluviatile beds have often 

 dwelt mainly on the human relics or mammalian remains as the 

 chief guide to their x-elative antiquity, and have disregarded the 

 stratigraphical evidence. For illustrations of the extraordinary 

 differences of opinion as to the older river-deposits of the Thames, 

 and the relative antiquity of various members of the series which 

 have thence resulted, see tlie chapter on the literature of the 

 River Drift in Mr. Whitaker"s memoir on The Geology of London 

 and of Fart of the Thames Valley, Vol. I., pp. 328-387. 



Now in the case of these reservoir sections there can be no 

 controversy ab;jut their stratigraphical position. No one can 

 assert that the beds at one spot iu either reservoir belong to an 

 older series than those elsewhere simply because a pre- Roman 

 dug-out canoe has been found here, and tobacco pipes only there, 

 at the same level. The excellent sections, long but varying in 

 direction, have illustrated, as no other sections could have done 

 so well, how incessantly a river in its windings tends to change 

 its course, and yet how often a spot a few yards from a recent 

 or existing channel may have remained unmodified for centuries. 

 And consequently it becomes e\'ident that in older river-deposits 

 there can be no legitimate excuse for assimiing stratigraphical 

 changes not only unsupported by, but contrary to, the available 

 stratigraphical evidence, to account for apparent discrepancies 

 in the nature of the fossil evidence analogous to those presented 

 by the human relics found in the marshes of the Lea. In the 

 absence of evidence like that furnished by these reservoir excava- 



