398 E. M. BUR AV ASH CHALK AXD FLINTS OF XORTHERX FRAXCE 



valley to which the stream was tributarv. the disappearance of the stream 

 from the surface would commence in the lowest part of its course. Once 

 established here, the underground channel would be extended headward 

 by the rapid flow of water into its head so long as the stream continued 

 to be absorbed by the fissures of the chalk. This would in turn lower 

 the ground-water level farther upstream and establish the conditions 

 already described. 



Eelatioxs of the Strea:ms to the Watee-table 



The relations of streams which flow over the chalk to the water-table 

 is indeed somewhat exceptional, since they are said seldom to flood, 

 owing to the ready absorption of the surplus surface run-ofl into the 

 water-table on either side of the stream through the interstices of the 

 chalk. 



When the stream had got down to grade throughout its course, the 

 process of deepening the dry valley above it by gradual sinking in of the 

 roof of the channel and its removal by solution would continue until 

 possibly the whole of it would be removed and the stream appear on the 

 surface again. Xo case of this sort was observed by the writer, however, 

 but parallel instances will easily suggest themselves from other lime- 

 stone regions. 



The tendency to meander probably depends generally on the same 

 causes in an underground as in a surface stream, but the possibility of 

 diversion by heavv' falls of the roof must also be taken into consideration. 

 In either case the meandering would result in the removal of a horizontal 

 layer, producing a flat-bottomed valley, and in either case this maturitv 

 would begin in the lower part of the stream's course and extend upstream. 

 Meandering is probably more pronounced in chalk than in harder lime- 

 stones, and for this reason complete unroofing may be rarer in the chalk. 



The rapid formation and recession of sea-clifls through wave-action 

 may have caused the adoption of a subterranean course by some streams 

 by rapid lowering of the water-tiible near the coast. Instances were ob- 

 served of wells near the coast which in recent years have gone dry from 

 tins cause. The larger streams would deepen their valleys fast enough 

 to keep pace with the uplift of the land, thus lowering the ground-water 

 level in adjacent uplands and compelling their tributaries of less erosional 

 power to disappear beneath the surface, following the sinking water-table. 



