STBATIGBAPHY OF THE RIVER DEPOSITS. 53 



More or less complete skeletons of mammoth have been 

 obtained at Baling, Endsleigh Street (London), Ilford and West 

 Thurrock. 



Bones of goose (Anser) have been obtained at Ilford and 

 Crayford. Remains of frogs and toads and of the grass snake 

 (Tropidonotus natrix) have been found near Grays. 



Remains of the freshwater fishes dace, pike, roach, rudd, 

 and ruff have been recorded from Grays ; the pike also from 

 Ilford. 



STRATIGRAPHY. 



It has been suggested that the different types of flint imple- 

 ments might supplement the evidence of fossils for the estab- 

 lishment of definite horizons in the river gravels; a fairly 

 definite sequence of these types has been established on the 

 continent, and much work has been done towards establishing 

 this sequence in England; but before a zonal classification of 

 geological deposits is accepted, it must be shown that it agrees 

 with the stratigraphical sequence where normally developed. 

 Unfortunately, the stratigraphy of the river deposits is, from 

 their nature, difficult to decipher with certainty. For our 

 present purpose it will be best to describe the stratigraphy first, 

 then to give a brief resume of the time-sequence of the flint 

 implements, and finally to discuss the extent of the agreement 

 between the two. 



Nomenclature. The Thames gravels and associated brick- 

 earths along the main valley were originally divided by Prest- 

 wich into high-level and low -level deposits. At a later date 

 Mr. Whitaker recognised three terraces which can be separately 

 mapped over the greater part of our area, though they are much 

 less distinctly separated in some parts than in others. These 

 terraces are shown by different colours and symbols on the new 

 maps in course of publication, but are not distinguished on the 

 four Special London Sheets ; they were described by Mr. T. I. 

 Pocock as Upper, Middle and Lower Terraces. 1 L^nfortunately, 

 a fourth higher terrace was at one time thought to exist, and 

 consequently a numerical system was used ; but some writers 

 numbered the terraces from above, others from below; accord- 

 ingly the upper terrace has been called the first and the third 

 and, by those who recognised four in all, the second. Another 

 method of naming the terraces was by the height above Ordnance 

 datum at which they occur. Mr. Pocock (op. cit.) shows that 

 the upper and middle terraces of the Thames maintain an almost 

 uniform level from Staines to Grays, that is to say, throughout 

 our area, corresponding approximately to the 100-ft. and the 50-ft. 

 contours respectively. The terraces have often been spoken of 

 as the 100-ft. and the 50-ft. But the only true basis for such a 



1 ' Summary of Progress for 1902 ' (Mem. Geol. Surv.), 1903, Appendix viii, 

 pp. 199-209. 



