172 NEW YORK STATE MUSEUM 



the surface when motion ceases, and brings no new material to this 

 surface during intervals of rest. From the aspect of the question 

 in regard to the influence of gravity we might be led to assert that 

 while potholes and cupholes can be cut only downward, dentpits 

 can be cut in any direction but downward ; and under all ordinary 

 conditions the statement would be true. We have seen however in 

 plates 12 and 13, that where waters are comparatively pure, typical 

 dentpits may cut very nearly downward. Where waters hold no 

 particles heavy enough tO' allow the influence of gravity to become 

 positive and thus interfere with the tendency of the vortex to 

 whirl them freely about, there is nO' apparent reason why dentpits 

 might not be cut directly downward. 



That the recognition of dentpits may become a matter of no 

 little importance, will be seen if we consider them for a moment in 

 their relation as geological timepieces. Niagara has long since be- 

 come a classic in this respect, and while we realize that the time 

 problem is not so simple as it was first thought to be its value is 

 greater today than ever before and its accuracy as a timepiece 

 will be more clearly understood in the near future than it now is. 

 Giant kettles must be cut with a speed which is in some respects 

 comparable tO' that with which cauldronlike cavities are excavated 

 at the foot of falls where vortex motion is both great and rapid. 

 The cutting of potholes is a much slower process. When we come 

 to cupholes we are dealing with phenomena which require a very 

 much longer period in which to produce any marked result. The 

 intermittent character of the vortex, its small size, the fineness of 

 the material used, the slight pressure allowed the abrading material 

 at the cutting point, these and other things make the above con- 

 clusion an inevitable one. These cupholes show conclusively that 

 at no time since the recession of postglacial Lake Vermont has 

 the water surface remained at any one level so long as it has re- 

 mained at the level on which these cupholes are' cut. Now in dent- 

 pit formation we must remember that we are dealing with smaller 

 vortexes and with abrading material so fine that gravity can give 

 it no pressure whatever on the surface undergoing the cutting. 

 This abrasive material is not only exceedingly fine but there is in 

 amount not to^ of that which is allowed or used in the cutting of 

 cupholes. 



Keeping in mind the statements just made, let us examine 

 the shores of Valcour island. Where cupholes have not become 

 confluent they are cut on distinctly glaciated surfaces and their 



