SIXTH REPORT OF Tllli IJIKECTOR I.909 161 



JOINT CAVES OF VALCOUR ISLAND — THEIR AGE 

 AND THEIR ORIGIN 



BY GEORGE H. HUDSON 



Wherever the shore of Valoour island consists of the purer dolo- 

 mite limestones of the middle Chazy beds, it has been penetrated, 

 sometimes to co^nsiderable depths, by a number O'f small caves 

 which owe their origin in great part to the solvent power of water. 

 The majority of these caves seem to have had their beginnings in 

 exposed joint or fissure planes, and as a rule they have closely fol- 

 lowed them in their subsequent development. Such deviation as exists 

 is expressive of a well marked tendency to develop vertically. The 

 form is usually somewhat wedge-shaped with the apex of the wedge 

 uppenno'St, and in the smallest caves this edge may sometimes be 

 seen to be working upward across a dipping bed without any ap- 

 parent fissure or joint plane as a guide. If the small cave has 

 penetrated to considerable depths the inrushing volume of water 

 from a wave condenses the cave air and this in turn shoots out a 

 smaller volume of water at the cave top with great velocity. These 

 spouting caves lose the sharp upper edge of the wedge and develop 

 arched tops. The roofs of those caves which rise still higher and 

 reach or pass the present high water mark of the lake, lose also 

 their wedge form and develop wide arches or even flat roofed tops. 

 In the latter instances it is apparent that cave-widening agencies 

 have become more effective than those bringing about upward ex- 

 tension. A few caves have been developed upwards far enough to 

 reach the cliff top. Among these are to be found a still smaller 

 number that have had a portion of the roof fall in some distance 

 back from the shore line. These therefore present us with small 

 sink holes and natural bridges. Where these caves occur they de- 

 velop in rather close proximity to each other, so one has to go but 

 a few^ rods to pass many small mouths. 



The general external appearance of these caves where well de- 

 veloped, may be seen in plate I — a view of a portion of the north 

 shore of Paradise bay on the east side of the island. The wedgelike 

 form and close proximity of the caves are both well shown. The 

 cave near the middle of this view has a sink hole som^e distance back 

 and a natural bridge between it and the sea wall. A view of this 

 small sink hole is shown in plate 2. The sea wall here faces south- 

 east by south and is only subjected to strong wave action during 



