26 NEW YORK STATE MUSEUM 



dack region Gushing says i 1 " It is thickest on the northeast, thin- 

 ning out to disappearance both to the south and west. As, further- 

 more, it appears to be the upper beds which persist, and the lower 

 ones which disappear in these directions, it seems certain that, so 

 far as the immediate region is concerned, the marine invasion came 

 on it from the northeast." This accounts for the fact that, in the 

 Broadalbin region, the Potsdam is so thin and represents only the 

 upper part of the formation. In Clinton county it is thickest, being 

 certainly over one thousand feet. The evidence is clear and con- 

 cise that, within the quadrangle, the Potsdam was deposited on a 

 fairly uneven surface [see page 51] which accounts for the rapid 

 local changes in the strata from place to place. Although the 

 formation is widespread under cover of later sediments, the present 

 outcrops are limited to a few comparatively small areas as shown 

 on the geologic map. 



The base of the Potsdam in the southeastern portion of the 

 quadrangle is characterized by a coarse conglomerate of unusual 

 interest. The best exhibitions of this basal member are in the 

 vicinity of Kimball's Corners one-half mile northeast of North Gal- 

 way. Here are fine exposures of Grenville quartzite and the con- 

 glomerate, resting upon the Grenville, shows in large almost con- 

 tinuous outcrops for nearly half a mile. The actual contact may 

 be seen at one or two points as shown in plate 5. The conglom- 

 erate varies in thickness from nothing to eight or ten feet and the 

 grayish-white fragments, ranging in size up to two or three feet, are 

 often angular so that the term breccia might well be applied. The 

 fragments are imbedded in a matrix of sand and are all of quart- 

 zite which have been directly derived from the immediately under- 

 lying Grenville by wave action. The surface on which the con- 

 glomerate was deposited was locally very irregular and the large 

 boulders seem to have got into the small depressions along the shore 

 as shown by the occurrence of heavy beds of conglomerate either 

 side of a little tongue or ridge of Grenville near Kimball's Corners. 

 Only at times is there evidence for very crude stratification and 

 this, together with the large size and angular character of the 

 fragments suggests a rapid deposition of the material. Above the 

 conglomerate there is very little sandstone, the succeeding passage 

 beds being within a few feet, so that the Potsdam is represented 

 nearly altogether by the conglomerate. 



Smaller outcrops of similar conglomerate practically in contact 

 with the Grenville occur one-third of a mile southeast of North 



1 N. Y. State Mus. Bui. 95, p. 354. 



