GEOLOGY OF THE NORTH CREEK QUADRANGLE 77 



After the intrusions the whole region was subjected to intense 

 compression and metamorphism when the gneissic or foliated 

 structure of all the rocks so far mentioned was developed. This 

 structure is now shown in the rocks at the earth's surface; but, 

 since such a structure can develop only in rock masses which are 

 deeply buried, we know that at the time of the compression the 

 present surface rocks were deeply covered. 



The rocks represented by the coarse grained pegmatite dikes of 

 the quadrangle were intruded after this period of intense com- 

 pression because they lack the gneissic structure, and before the 

 diabase because the latter rock cuts the pegmatite. 



The great elevation of the region, above referred to, inaugurated 

 a long period of erosion to be measured by at least some millions 

 of years, and extending into the early Paleozoic era. 



After the removal, by erosion, of some thousands of feet of 

 rock materials, the last igneous activity of the region occurred 

 when the molten diabase was forced through narrow fissures in 

 the earth to cool in the form of dikes. The utter lack of meta- 

 morphism and the fact that they cut all the other rocks show that 

 this diabase is the youngest of the intrusives. That it must have 

 cooled rather close to the earth's surface is evidenced by the fine 

 grained to even glassy texture. 



As a result of the vast erosion, the whole area was worn down 

 to near sea level and presented only a moderate relief. Then a 

 gradual sinking took place when the sea steadily encroached upon 

 the old land from the east, and the early Paleozoic sediments were 

 deposited upon the old land surface. As the nearby outliers show, 

 the first sediment to cover the area of the quadrangle was the upper 

 Cambric (Potsdam) sandstone, followed in turn by the Theresa 

 passage beds and the Little Falls dolomite which are also of Cam- 

 bric age. 



Recent studies have shown that, toward the close of the Cam- 

 bric, there was a gentle upward movement of the Adirondack region 

 to above sea level after which some erosion took place. Then, early 

 in the Ordovicic period, the Champlain and Mohawk valley regions 

 sank below the ocean surface when the thick limestones and shales 

 of that period were laid down. It is highly probable that the North 

 Creek area was also submerged under the Ordovicic sea, though 

 positive evidence is lacking. At any rate we have no reason to 

 think that this area, or in fact any of the Adirondack region outside 

 of the immediate Champlain and St Lawrence valleys, was ever 

 again below sea level after the Ordovicic period. 



