84 



surface draiuage. It remains snrface drainage until the rapids of tbe 

 larger streams have deepened their A-alleys well across the plain, leaving 

 their tributaries out of adjustment with them. At this stage underground 

 drainage first takes place to a considerable degree. The rocks are satu- 

 rated with ground-water and at the level of the larger streams is under an 

 unbalanced static head equal to the difference in elevation of the surface 

 of the tributary, and of the water table between the tributaries, and the 



Fig. 2. Section of a funnel-sliaped soliilion bole (enlarged joint) in limestone, 

 illustrating the origin of solution sinks. 



main stream. As tbis is sloAAiy drawn ofl' more is supplied from above 

 and a subterranean circulation is begun. The development of sinks goes 

 along with the development of the subterranean drainage channels. The 

 cross-joints most favorably siiuared with respect to free circulation below 

 and supply from above, soon beijin to bo onl.-'rged by solution. This solu- 

 tion is most active where the water first cduics into contact with the 

 limestone and the upper part of the opening will be dissolved most rapidly, 

 resulting in a funnel-shaped hole. The largej.- this funnel becomes the 



