SUB-SURFACE WATER 



317 



SINK-HOLE IN TENNESSEE LIMESTONE. 



folded porous rock layer, below which is a rock through 

 which it cannot penetrate. The water will then accumu- 

 late throughout the porous rock. If this rock layer in 

 another part of its extent is overlaid by an impermeable 



GREAT NATURAL BRIDGE, UTAH. 



