84 
surface drainage. It remains surface drainage until the rapids of the 
larger streams have deepened their valleys 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 differeuce in elevation of the surface 
of the tributary, and of the water table between the tributaries, and the 
Fig. 2. Section of a funnel-shaped solution hole (enlarged joint) in limestone, 
illustrating the origin of solution sinks. 
main stream. As this is slowly drawn off 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 situated with respect to free circulation below 
and supply from above, soon begin to be enlarged by solution. This solu- 
tion is most active where the water first Comes into contact with the 
limestone and the upper part of the opening will be dissolved most rapidly, 
resulting in 2 funnel-shaped hole. The larger this funnel becomes the 
