i 3 2 PREHISTORIC EUROPE. 



must not expect always, or even often, to meet in nature with 

 the regular succession of beds that is indicated upon the 

 diagram. Although it is common to speak of high-level and 

 low-level terraces, the one series really passes down into the 

 other. Neither do these terraces occur continuously on the 

 valley- slopes, forming a series of broad steps or platforms 

 ascending from lower to higher levels. On the contrary, it is 

 often hard or even impossible to distinguish anything like a 

 terrace either in the gravels themselves or in the Cretaceous or 

 other strata upon which they chance to lie. It is doubtful, 

 indeed, whether the gravels would often be spread out so equally 

 as to form fiat-topped terraces. It seems much more probable, 

 judging from what we know of rivers that are subject to period- 

 ical floods, that they would be distributed very irregularly over 

 the valley -slopes and bottom, forming shoals here and banks 

 there. We must remember, moreover, that while the lower 

 terraces were being formed, the upper ones would tend to be- 

 come partly obscured by the scouring action of flood -waters, 

 and partly also by the deposition upon them of loam or brick- 

 earth. Again, we should not forget that so long a time must 

 have elapsed between the formation of the upper or oldest and 

 lower or youngest valley-gravels, that the former, after they had 

 ceased to be inundated by floods, would be subjected to the 

 slow but continuous and therefore effective action of the atmo- 

 spheric agents of waste. Thus, in course of time, it might well 

 be that all trace of a distinctly-terraced feature would disappear, 

 and the gravels would then be reduced to mere patches or 

 interrupted sheets cloaking the slopes of the valleys. Notwith- 

 standing all these changes, however, platforms excavated in the 

 older strata and covered by Pleistocene gravels and loam may 

 now and again be detected. 



The size of the stones and the quantity of the material 

 constituting what are called high- and low-level gravels suf- 

 ficiently indicate, as we have seen, the great transporting 

 power of the Pleistocene rivers, while the brick-earths, with 

 their delicate land-shells, covering all the gravels, and running 



