QUATERNARY DEPOSITS OF BRITISH ISLES BROOKS. 359 



SOMME VALLEY. 



V. Commont (130) gives the following general succession: 

 At Amiens there are four principal zones of river gi'avels, sepa- 

 rated by slopes of chalk. 



1. Terrace of 10 meters (altitude 23 to 29 meters; bed of Somme + 13 



meters; low terrace. 



2. Terrace of 30 meters ( altitude 40 to 45 meters, St. Aclieul). 



3. Terrace of 40 meters, divisible into two beds. 



4. Terrace of 55 meters, which occupies the summit of the plateau. 



All these terraces are inclined downstream, their altitude above the 

 present bed of the Somme diminishing as they approach the sea, 

 and at Abbeville the second terrace reaches the present level of the 

 Somme. Gravels of the third terrace have furnished a flora of Plio- 

 cene affinities at Abbeville, and remains of a coarse " pre-Chellean " 

 industry at Amiens. The true Chellean characterizes the 30-meter 

 terrace, and " evolved Chellean " the 10-meter terrace, with Elephas 

 antiquus and HippopotaTrms. At the end of Chellean time, owing 

 to a relative subsidence of the land, the sea invaded the Somme Val- 

 ley up to Menchecourt, depositing calcareous sands with Corbicula 

 fuTninalis^ etc., overlying the gravel at a height of about 6 meters 

 above present sea level. At the same time on the flanks of the valley 

 were formmg the pebble beds and Umon moyen (old loess) with 

 Acheulian implements. 



A fresh lowering of base level enabled the river to cut its bed 

 deeper not only in the Chellean gravels of the low terrace but also 

 in the underlying chalk. The gravels deposited at this stage con- 

 tain Elcplias prlmigenms^ Rhinoceros tichorhinus^ and Cervus taran- 

 du.s, with IMousterian implements. From this period date also the 

 deposits of tufa and older peat of Longpre and Montieres, while on 

 the slopes was forming the " ergeron " (3'ounger loess) with Mouster- 

 ian and Aurignacian industries. A drj^, cold period followed this 

 moist period — the epoch of the formation of the Solutrean and Mag- 

 clalenian brick earth by alteration of the Ergeron. This period was 

 followed by renewed subsidence, permitting the filling up of the 

 river valley by loam, tufa, and peat. This peat commenced with a 

 formation of alder, hazel, oak, and walnut, the remains of which are 

 now below sea level, and extend to the coast, where they occur on the 

 shore below sea level, and evidently correspond to the submerged 

 forests of the opposite coast of England. 



The average thickness of the peat formation at Abbeville is nearly 

 10 meters and M. Boucher de Perthes found in it at a depth of 40 

 centimeters several large, flat dishes of Roman pottery, which were 

 at least 14 centuries old. If this rate of 3 centimeters per century 

 had not been exceeded — and the earlier peat is naturally more com- 



