85 
more water it collects and the more rapidly it widens at the top and the 
larger the sink becomes. In this way the sinks develop at the same time 
that the subterranean channels do and in a region of mature sink topog- 
raphy where the channels are well below the surface, as in the Indiana 
Fig. 3. A more advanced stage of subterranean drainage than No. 1. Sinks 
have developed and all the water of the stream passes beneath the surface and 
enters the larger stream as a great spring. It may be considered a sort of vertical 
self capture, a common occurrence. The underground channels have become enlarged 
and subterranean drainage has worked headward along the stream. At this stage 
the sinks will be developed over considerable of the surrounding land surface. It 
may be regarded as approaching maturity. 
region, probably ninety-five per cent. of the sinks are formed this way.’ It 
is certainly true of the Bloomington region and aj! the Indiana region as 
far south as Wyandotte that has come under the writer’s notice. In some 
cases a sink may cover many acres and be as much as a hundred feet 
deep. he first surface indication of the sink is frequently the collapse 
of the soil into the funnel which has been dissolved in the surface of the 
underlying rock. This has probably given rise to the popular notion that 
sinks are usuaily formed by the collapse of the roofs of caverns.’ Incipient 
Fig. 4. An ideal section, similar to Nos. 1 and 2, in old age. Natural bridges 
are developed, much cf the roof of the subterranean channel has collapsed, revealing 
the underground stream, and the mouth of the cavern has retreated by collapse and 
erosion. Dotted line indicates the old land surface. 
>See Blatchley, 21 Ann. Rep. Ind. Dept. Geol. Nat. Res., p. 1383, 1896. 
®For a discussion of the solution of the Indiana limestones, see Cumings, Proc. 
this Acad., 1905, pp. 85-102, 1906. Caves, F. C. Greene, idem, for 1908, pp. 175-183, 
1909. 
*'This does not seem to apply to the caves of Florida and some regions of Cuba 
where the channels are very near the surface and the roof soon becomes so weakened 
that it gives way, and the extreme porosity of the rocks does not concentrate the 
solution to the joints to produce solution sinks. 
