ELIZABETHTOWN AND PORT HENRY QUADRANGLES 63 



The exposures in the neighborhood of Port Henry are by far 

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

 canibric rocks, the original surface of deposition of the latter 

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

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

 northern part that the irregular Precambric surface is exposed 

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

 exposures is in the southern part of Port Henry where West street 

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

 with the Precambric rocks with about 3 feet of conglomerate of 

 arkose character, consisting prevailingly of dark quartz pebbles, 

 smaller grains of fresh feldspar and a yellowish loamy-looking 

 matrix. The quartz pebbles are mostly small and never surpass 

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

 wdth irregular conglomeratic bands or streaks, measuring about 20 

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

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

 quartz grains. This is overlain by the typical white to yellowish 

 Potsdam sandstone. This basal portion is ver}^ indistinctly bedded 

 but exhibits clear evidence of current action such as cross striae 

 and plunge structure. Another contact of the Precambric and 

 Potsdam is shown at another inlier along the lower course of 

 ]\IcKenzie brook below the highway bridge. Here the bottom layer 

 consists of 3 feet of greenish gray arkose sandstone with scattered 

 pebbles of quartz of the size of cherries, over which beds of fine 

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

 McKenzie brook, a rather fine grained reddish sandstone, about 20 

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

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

 contact are the only indications of the nearness of the great un- 

 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 suflicient to completely plane the sea 

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

 time when the basal conglomerate was deposited, by the hummocks 

 of Precambric rocks protruding through the Potsdam sandstone as 

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

 with Potsdam sandstone and exposed in places along McKenzie 

 brook. 



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

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



