GLACIAL POTHOLES. 325 



at Brunswick. Here and there the water can be seen flowing over an 

 angular depression in the rock, where a portion of tlie granite has broken 

 away under the action of frost, ice gorges, the force of the water, etc. In 

 process of time the surface is sand carved and hollowed out into bowl 

 shape. The water falls into the cavity, rebounds in a curve, and swiftly 

 shoots up the other side. Up to this time the sand grains and stones of 

 various sizes used by the stream in this process are driven almost imme- 

 diately out of the cavity, along with the upward rebound of the water. 

 By degrees the cavity deepens, until some day a stone falls into the bowl 

 of such size that the water can 'not roll it up the steepened slopes. The 

 stream now sets this stone to rolling, at first with considerable vertical 

 motion, but more and more, as the hole deepens, the horizontal whirling 

 prevails. The grinding now proceeds with multiplied rapidity. 



The conditions for the formation of a pothole are the following: (1) A 

 rapid stream. (2) A rock firm enough to withstand the direct impact of 

 the water. Thus potholes are more frequently found in granites, sand- 

 stones, and indurated slates than in schists and shales easily weathered or 

 split and broken under the action of the watei'. (3) The formation of such 

 a cavity as to permit a vortical motion of the water. (4) A moderate 

 quantity of stones for the stream to Avhirl around in the hole. If there is 

 a larg-e quantity of sediment swept along- by the stream, the cavity will 

 soon be filled or partly filled with stones and the process of excavation 

 will be stopped. Sooner or later most potholes are filled in this way. 



It is important to note that the direct impact of the running water bears 

 a very subordinate part in pothole erosion. The principal agency is the 

 friction of the rolled stones and bowlders. It makes little difference whether 

 the water falls into the cavity from above or is shot horizontally} or nearly 

 so, across the mouth of the opening, provided the water is kept whirling. 



The best-laiown glacial potholes in Maine are situated near Riggs 

 Landing, on the island of Georgetown. They were examined and measured 

 by me in 1879. The region has since been explored by Mr. P. C. Manning, 

 of Portland, whose observations were presented in a paper read before 

 the Portland Society of Natural History. He found similar potholes in 

 several other of the islands situated east and southeast of Bath. Some 

 of these were called to his attention by Mr. Alexander Johnston, of Wis- 

 casset. Several times archeologists have asserted that these potholes were 



