166 A. J. Julies-Broime — The Vale of Marslncood. 



and Greensand. They ran, of course, bip;!! above the present surface, 

 and their courses were prolon£2;ed far to the southward before reaching 

 the sea ; indeed, during the Miocene and again in the later Pliocene 

 time it is probable that most of the Englisb Channel was dry- 

 land, and that these Dorset streams were merely 

 tributaries of a large river which ran westward 

 down the valley of the Channel. 



Now the slope along which these streams made 

 their way was planed across the summit of the 

 low dome or pericline, which has been described ; 

 and as we have calculated the base of tbe Green- 

 sand on this summit to bave been about 100 feet 

 higher tban it is at Lewesdon, where the thickness 

 of Greensand at present is not more than 130 feet, 

 and as tbe surface sloped southwards from Lewes- 

 don, there cannot bave been much Greensand left 

 over tbe central area of the pericline when the 

 streams began to make their valleys. Hence, as 

 they deepened tbeir channels they would quickly 

 cut through tbe Greensand on tbe central area 

 and would soon enter the Jurassic beds on whicb 

 the sand rests ; these beds are the Midford Sand, 

 the Upper Lias clay (whicb is thin), and tbe 

 Marlstone Sands. 



As soon as any stream cut into tbe Upper Lias 

 tbe water on the overlying sands would issue in 

 the form of springs. Thereby the volume of tbe 

 streams would be increased and at the same time 

 landslips would take place, as is alwaj's the case 

 where springs issue from sand overlying a clay. 

 The valleys would be rapidly widened, and during 

 periods of upheaval they would be deepened also. 

 Much of this work was probably done during tlie 

 Glacial Period, and was finally completed during 

 the time when the raised beaches of the South 

 Coast were being elevated to tbeir present height. 

 Over the western part of tbe pericline the Midford 

 Sands and Upper Lias are absent ; that is to say, 

 they were planed off before the Greensand was 

 deposited, and tbe latter rests directly on the 

 Marlstone Sands. Here the process of valley 

 erosion would continue till tbe base of these 

 sands was reached, when strong springs would 

 be thrown out by the underlying margaritatns 

 clays, and these clays would be for a certain 

 distance exposed along tbe valley bottoms. 



We must remember that all this time tbe slope 

 of the valley-ways was less than the southerly 

 inclination of the beds on the southern curve of 

 the pericline ; hence the rivers, after cutting 



=i Ph 



