SIXTH REPORT OF THE DIRECTOR I909 I7I 



Gravity also still aids in holding the cutting material against the 

 surfaces, retains it there when motion ceases, and adds new mate- 

 rial thereto from the water and the air above. On account of the 

 fineness of this material the vortex holds most of it in suspension 

 as an air vortex does dust, and while it cuts down at its apex, 

 gravity is powerless to keep the greater part of the load there and 

 a very marked widening of the older portion of the cut is allowed 

 to take place.^ Sections atong their axes therefoire give us so^me- 

 what V-shaped outlines but the sides are never straight lines and 

 the figure is more nearly that of a parabola or hyperbola. 



In dentpi't formation we are dealing with still smaller vortexes and 

 usually with a greater number per unit area. If the vortex is acting 

 against steeply inclined or overhung surfaces at some distance from 

 the bottom, or source of load, it will be able to use only such 

 abrading material as is already in mechanical suspension in the 

 water or which may be supplied to the vortex through the action of 

 vertical currents or through detachment from the rock surface. 

 The deeper the water the finer will the particles be that the vortex 

 is allowed to use. Instead of pressing the abrading material 

 against the surface, as in pothole and cuphole formation, gravity is 

 always working to keep such material away from overhung sur- 

 faces and is constantly trying to withdraw from the grasp of the 

 vortex all particles whose specific gravity is greater than that of the 

 water. The material used must therefore be somewhat uniformly 

 distributed throughout the vortex rather than loaded in or confined 

 to a smaller portion, that would thus be itself drawn down by 

 gravity, and the cutting in consequence will be less localized and 

 confined to no one direction. In other words, one portion of a 

 vortex can not cut more than another because of greater load, 

 unless the vortex segregates its abrasive material through centrif- 

 ugal action, and if so segregated, the result would be a cut show- 

 ing that widening is carried on greatly in excess of deepening, and 

 this is the form of cut that we actually find. The fact that the 

 effect of gravity on the sediment is here made a negative rather 

 than a positive quantity is in itself a distinction which places dent- 

 pits in a distinct class and we must see that they really differ more 

 markedly from cupholes than do the latter from potholes. We 

 may also note that when this cutting is on overhung or nearly ver- 

 tical surfaces gravity does not allow abrasive material to remain on 



^ It is in this manner that cupholes, such as those shown in State Museum 

 Bulletin 133, plate 4, are cut in the sides of steeply sloping surfaces. 



