ELIZABETHTOWN AND PORT HENRY QUADRANGLES 



63 



The exposures in the neighborhood of Port Henry are by far 

 the most interesting of all, suice here the contact with the Pre- 

 :ambric rocks, the original surface of deposition of the latter 

 ind the basal beds of the Potsdam are shown. The main part of 



le block is strongly tilted to the east and so deeply eroded in its 

 lorthern part that the irregular Precambric surface is exposed 

 ielow the Potsdam sandstone in several places. The best of these 

 exposures is m the southern part of Port Plenry where West street 

 crosses a brook. Here the Potsdam sandstone begins at its contact 



^ith the Precambric rocks with about 3 feet of conglomerate of 

 irkose character, consisting prevailingly of dark quartz pebbles, 

 smaller grains of fresh feldspar and a yellowish loamy-looking 



latrix. The quartz pebbles are mostly small and never surpass 

 lalf an inch in diameter. This is followed by reddish sandstone 



'ith irregular conglomeratic bands or streaks, measuring abo'Ut 20 

 Feet and above this follow about 10 feet of grayish white sandstone 



ath fine grained silicious matrix and many floating, large, rounded 



[uartz grains. This is overlain by the typical white to yellowish 



[Potsdam sandstone. This basal portion is very indistinctly bedded 



)ut exhibits clear evidence of current action such as cross striae 



ind plunge structure. Another contact of the Precambric and 



[Potsdam is shown at another inlier along the lower course of 



fMcKenzie brook below the highway bridge. Here the bottom layer 



;onsists of 3 feet of greenish gray arkose sandstone with scattered 



)ebbles of quartz of the size of cherries, over which beds of fine 



grained sandstone directly follow. In a third place, also along 



'cKenzie brook, a rather fine grained reddish sandstone, about 20 

 ifeet thick, is found to rest in an apparently original depression of 



le ancient sea floor. A few thin conglomerate streaks near the 

 ^contact are the only indications of the nearness of the great uri- 

 ^conformity. It is thus evident that the coarse basal conglomerate 

 ^seen in other places had here been worked up by wave action until 

 only a small amount of larger quartz pebbles was left. Neverthe- 

 ^less the wave action was not sufficient to completely plane the sea 

 [floor, for the latter is proven to have been very irregular at the 

 lime when the basal conglomerate was deposited, by the hummocks 

 ;of Precambric rocks protruding through the Potsdam sandstone as 

 jWell as by the presence of original channels in the floor now filled 

 [with Potsdam sandstone and exposed in places along McKenzie 

 Ibrook. 



The base of the Potsdam formation in this area differs not only 

 from that of others in the slight development of the basal con- 



