1 64 THE ANTIQUITY OF MAN 
gravel, in which implements of the later Acheulean type 
occur — St Acheul II. The stratum of gravel (" upper " 
gravel), marking the commencement of the third series, 
lies under the surface soil (fig. 57). The sands and brick 
earths which were probably present when the terrace was 
finished have been washed away long ago. To find the 
continuation of the third and last series of the 100-foot 
terrace, we must descend from the level of that terrace at 
Swanscombe to the level of the 50-foot terrace. That 
terrace has been already mentioned — at Crayford and at 
Baker's Hole (p. 103). Lying at the base of the brick 
earths, in which the culture of the Mousterian period is 
preserved, occur Palaeolithic floors of the late Acheulean 
period. Acheulean man lived on the floor of the valley 
before the brick earths of the 50-foot terrace were 
deposited. In 1880, Mr F. C. J. SpurrelP found such 
a floor under the brick earths of the 50-foot terrace at 
Crayford. From the floor he gathered not only a finished 
implement, but also the chips which the workman had 
struck off^ in fashioning it, and part of the lower jaw of a 
rhinoceros — all of which he presented to the Natural 
History Museum at South Kensington. It is clear, then, 
the period we have now entered — the Acheulean — must 
be one of great duration. At its close the valley of the 
Thames was excavated almost to its present depth, for at 
such a level, buried beneath the deposits of the middle 
terrace, we find the work-floors of Acheulean man. 
The commencement of the Acheulean period is re- 
corded, as we have just seen, in the upper series of the 
100-foot terrace. It is plain, then, that the Thames 
carried out an enormous task during the Acheulean 
period. At the beginning of the period the floor of the 
Thames valley lay flush with the 100-foot terrace. A 
process of land submergence was then in operation, 
attended by a filling up of the valley, and by the 
formation of the upper series of Acheulean deposits on 
the 100-foot terrace. Afterwards an opposite movement 
set in, one of elevation of the land, leading to an excava- 
i Quart. Journ. Geol. Soc, 1880, vol. xxxvi. p. 544. 
