1853.] TRIMMER ON THE GRAVELS OF KENT. 291 



sand formation. The gravel also contained some small waterworn 

 boulders of a dark ferruginous sandstone, some rolled eocene pebbles, 

 and fragments of a conglomerate, in which those pebbles are cemented 

 by a base of ferruginous sandstone. I could find no other detritus 

 foreign to the chalk. 



General conclusions. — In former communications* I have divided 

 the erratic tertiaries, commonly called northern drift and drift, into 

 Lower Erratics, consisting of Tillf or Boulder Clay, and Upper Erratics;]: 

 composed of rolled gravel with erratic blocks. I endeavoured also 

 to prove, by means of extracts from the narratives of the Polar Expe- 

 ditions §, that the boulder clay was the littoral deposit of an arctic 

 sea, which gradually crept, with the subsidence of the land, to certain 

 elevations in the interior, at which it ceased to be deposited, and was 

 replaced by the rolled gravel of the upper erratics, which covers it 

 at lower levels. I also endeavoured to prove, by the same means, 

 that these upper erratics were the deposits of a more open sea, like 

 that of Davis's Straits, though they might be in part the result of a 

 less rigorous climate during their formation. 



I also adduced proofs of the existence of a former terrestrial 

 surface in the Forest of Cromer, buried beneath the mass of the erratic 

 tertiaries || ; and I concluded that the gradual advance of a frozen sea 

 over subsiding land, and the gradual retreat of the arctic climate 

 northwards during re-elevation, — the subsidence and re-elevation com- 

 mencing in each case from the north, — would explain all the peculia- 

 rities, whether of composition or distribution, of the erratic tertiaries 

 north of the Thames. I believe that they will also explain the ano- 

 malies of the erratic deposits of Kent, which I have just described. 



The boulder clay does not extend across the valley of the Thames, 

 though it comes down to its northern edge (see PI. XIII.). It ap- 

 pears to have been cut off by a ridge of eocene tertiaries, of which 

 the Highgate range and Shooter's Hill may be considered the re- 

 mains. This ridge appears not to have been submerged until the 

 causes which produced the boulder clay had ceased ; but it was ul- 

 timately submerged, and the gravel of Shooter's Hill must be con- 

 sidered as an outlier of the upper erratics which overlapped the lower 

 erratics on a ridge of eocene tertiaries, since in great part removed, 

 which then occupied the site of the Thames Valley. See Diagram, 

 PL XIII. 



North of that river, the valleys in general appear to have been 

 formed before the submergence of the erratic period^, — to have been 

 filled with the erratic deposits, and re-excavated during the period of 

 emergence. The valley of the Thames, on the contrary, appears to 

 have been originally excavated in the London clay during the period 



* hoc. cit. 



t Journ. R. Ag. Soc. vol. vii. pp. 461-464. Quart. Journ. Geol. Soc. vol. vii. 

 p. 21, par. 3, 4, 5. % Ibid. par. 8. 



§ Unpublislied paper on evidence of Extreme Arctic Climate during the forma- 

 tion of the Erratic Tertiaries : Loc. cit. And Quart. Journ. Geol. Soc. vol. vii. 

 par. 5 and 7. 



II Vol. vii. p. 20, par. 1, 2, 3. 



\ Quart. Journ. Geol. Soc. vol. vii. p. 21, par. G. 

 VOL. IX. — PART I. X 



