I58 NEW YORK STATE MUSEUM 



erosion, but in eastern New York between the Hudson and the Massachu- 

 setts line and in the direction of the Devonic rocks of Bernardston lies an 

 extensive sheet of coarse clastic material known as the Rensselaer grit 

 which at this point requires brief attention. 



Rensselaer grit. Rensselaer and Columbia counties, New York, lyino- 

 east of the Hudson river and in the general direction of continuity between 

 the Helderberg-Catskill escarpment and the Bernardston Devonic outcrops 

 of the Connecticut valley, are extensively mantled by heavy arenaceous 

 deposits lying unconformably on the unfolded Cambric and Lower Siluric 

 strata beneath. The character and distribution of this rock was clearly 

 outlined by Lieutenant Mather in his report on the First geological district 

 (1843) and it was regarded by him as equivalent in age with the Shawan- 

 gunk grit of Ulster and Orange counties on the west of the river. 



The early geologists held the Shawangunk grit to be an eastern repre- 

 sentation of the Oneida grit of central New York and this conception has 

 been quite generally promulgated. Mr T. Nelson Dale has been one of 

 the latest investigators of this region and has acquired an intimate knowl- 

 edge of the stratigraphic relations of this terrane to the unconformable 

 rocks beneath and we owe to him the conclusion that the upfolding of the 

 lower and upper terranes pertains to different dates, the former to the 

 Taconic and the latter to the Postdevonic or Carbonic movement which 

 also produced the more southerly synclines now represented by Becraft 

 mountain, Columbia county. Mr Dale has correlated the Rensselaer grit 

 with the entire Oneida-Medina sedimentation of eastern New York. In 

 recent investigations carried on by C. A. Hartnagel [see Mus. Bui. 107. 

 1907. p. 51*] it is shown with approximate conclusiveness that in the typical 

 sections of central New York the Oneida conglomerate is not a formational 

 unit but actually lies within the Medina sandstones ; that further, the Sha- 

 wangunk grit, on stratigraphic evidence alone, is of an age much later than 

 the Medina formation and being overlain by rocks of Postsalina age is pre- 

 sumably the eastern representation of Salina deposition. The confirmation 

 of this conclusion as to the value of the Shawangunk grit, was afforded by 



