September 27, 1907] 



SCIENCE 



417 



DISCUSSION AND CORRESPONDENCE 

 ORIGIN OF SINK-HOLES 



The writer has read with much interest 

 Professor A. H. Purdue's paper in the issue 

 of Science of July 26, 1907, on "Origin of 

 Limestone Sinlv-holes." In this connection it 

 may be of further interest to call attention 

 to a type of sink-hole very frequently met 

 with in Florida formed under conditions ap- 

 parently not included in Professor Purdue's 

 discussion. 



The surface deposits throughout much of 

 the interior of Florida consist of sands and 

 clays with occasional limestone layers. These 

 deposits are variable, being in some places 

 almost entirely absent, while in others they 

 are of considerable thickness. Beneath these 

 surface accumulations occurs a limestone of 

 undetermined thickness. This foundation 

 limestone, which is for the most part porous, 

 holds inexhaustible supplies of water, and is 

 traversed by solution cavities. 



When first formed, the typical sinli through- 

 out this area is an opening leading from 

 the surface through the superficial deposits to 

 or into the limestone below. Many of these 

 sinks are perfectly . cylindrical, not funnel- 

 shaped. This is especially true of the smaller 

 sinks. As a result of subsequent caving of 

 the banks, the bottom usually becomes clogged 

 and the sides sloping. The formation of 

 these sinks is practically instantaneous and 

 results from a sudden caving of the earth. 

 In size they vary from a few feet to many 

 rods in diameter. So frequent is their forma- 

 tion in certain sections, notably the phosphate 

 mining area of Alachua and Columbia coun- 

 ties, that one must be on the lookout in driv- 

 ing through the country for newly formed 

 sinks. Indurated layers exposed along the 

 sides of the sink are rough-edged and bear 

 evidence of fracture due to the sudden giving 

 away and breaking under the weight of the 

 load above. The depth of the sinks is prob- 

 ably quite variable. As a rule, they reach 

 through and connect, with the permanent 

 underground water horizon. Some reach 

 much below the water line. 



The type here described is not merely a 



modification of the type described by Professor 

 Purdue. This is evident from the fact 

 that the static head of the water in many, 

 though not all, of these sinks is such 

 as to bring it above the top surface of 

 the limestone. There is abundant evidence- 

 of solution in the limestone at all depths, both 

 above and below the static head of the mider- 

 ground water. It is apparent, however, that 

 the conditions existing in the limestone below 

 the water level are not such as to bring about 

 a funnel-shaped cavity. This point would 

 scarcely seem to call for emphasis were it not 

 that Professor Purdue considers the cave-in 

 sink the rare exception. 



A sink of this type was examined by the 

 writer within a few hours after its formation 

 about one mile south of Juliette in Marion 

 County in 1905. This was a small sink, not 

 more than eight feet in diameter, and of the- 

 usual cylindrical form. The sides down to 

 the water level were, so far as could be de- 

 termined, entirely of clay. The sink which had 

 formed directly under the railroad track was 

 caused possibly by the jar of a passing train,, 

 the engine of which had passed safely over. 

 The water rose immediately in the sink to- 

 the static head of the water of that locality. 



The writer recalls having often seen similar 

 tubular openings reaching from the surface 

 to the runway of abandoned coal mines, the 

 " cave-in " occurring in these cases through a 

 thickness of forty or fifty feet of clays and 

 shales. From analogy it seems probable that 

 the formation of the sinks in question results 

 from a gradual caving of the clay from the 

 bottom, assisted, perhaps, by the removal me- 

 chanically of a part of the material by under- 

 ground water. Finally a point is reached at 

 which the entire remaining mass suddenly 

 gives way. Wliile some of these sinks are in 

 clay formations entirely, others break through 

 considerable thicknesses of limestone. 



E. H. Sellards 



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