GEOLOGY OF THE NORTHERN ADIRONDACK REGION 281 



gestfi the possibility that the larger part of the formation on the 

 north was deposited in a closed or nearly closed basin. 



Beekmantown formation. By the close of Potsdam time the sea 

 had encroached for a considerable distance on the present north- 

 ern and eastern portion of the Adirondacks, but there was 

 yet a large land area remaining in the heart of the region which 

 on the west and southwest extended somewhat beyond the present 

 surface limit of the Precambric rocks in these directions. Dur- 

 ing Beekmantown time submergence was in progress on all sides 

 of the Adirondacks, but it was most rapid, and to greatest 

 amount on the northeast and diminished to the south and west, 

 the rocks having treble the thickness in the lower Champlain 

 valley that they have along the Mohawk. On the extreme west 

 the amount of subsidence was but slight and little deposit took 

 place. 



The Beekmantown rocks are in large part peculiar, and except 

 for the fact that they are clearly water-deposited rocks, the 

 precise conditions under which they were deposited are difficult 

 to understand. In the upper portion of the formation are many 

 pure limestone beds, often containing numerous fossils, and so 

 far as these are concerned the formation seems clearly a marine 

 limestone. These beds seem to be limited to the east and north 

 sides of the region and to be wholly lacking on the south and 

 west. The bulk of the formation everywhere is made up of beds 

 of sandy dolomite. The sand is mostly rather coarse and is 

 embedded in a fine mosaic of crystalline dolomite. There is little 

 mud in the formation and fossils are either wholly lacking or 

 else exceedingly rare. The sands imply vigorous water action, 

 sufficiently so to transport them to their present resting place 

 and to wash away all fine mud. Yet the sand forms less than 

 25^ of the whole formation, the bulk being dolomite, along with 

 some calcite. The nature of the deposit would suggest a chemi- 

 cal, rather than an organic origin for this material, since the 

 waters must have been shallow, and this would imply estuarine 

 or closed basin conditions of deposit, stream waters holding lime 



