536 C. SOHUCHEET — SILURIAN FORMATIONS 



The Shawangimk begins at Binnewater, New York, and thence con- 

 tinues unbroken for at least 200 miles to Bockville or Fort Hunter, on 

 the Susquehanna Eiver to the north of Harrisburg, Pennsylvania (see 

 summary table, page 539). The thickness in the northeast is 170 feet or 

 less (clown to 10 feet) ; at Minnewaska it is 500, at Otisville 800, at 

 Delaware Gap 1,565, at Lehigh Gap 1,250, at Schuylkill Gap 1,130, and 

 on the Susquehanna it is at least 170, and more probably nearer 800. 

 The greatest amount of conglomerate and the largest pebbles are in the 

 region of greatest thickness — that is, between the Delaware and Lehigh 

 gaps. In about 85 miles the Shawangimk has increased from 10 to 1,565 

 feet, or at a rate Of more than 18 feet per mile; then for more than 50 

 miles farther along the strike it does not thin under 1,250 feet. 



It should be added here that some of these upper beds are actually of 

 Clinton time, but how much was not determined. Probably all of the 

 pink and red zones in the Upper Shawangimk are of Clinton time. Far- 

 ther southwest the formation appears to thin rapidly, but the amount 

 can not be definitely stated. Northwestward across the strike from the 

 Shawangimk or Kittatinny Mountains to Jacks Mountain the Medimui 

 deposits lose their conglomerates in 10 miles, the sands are liner, there 

 is more shale interbedded, and Artlirophycus is common. At Jacks 

 Mountain it is 820 feet thick, or about 600 feet thinner than at Kittatinny 

 Mountain to the east. Across the strike westward for 35 miles more 

 (Tyrone Gap) the Medina has thinned to about 500 feet. On the other 

 hand, from Shawangunk Mountain east to Green Pond Mountain, about 

 23 miles across the strike, a thickness of about 1,200 feet appears to be 

 maintained, but the strata have considerably more conglomerate, the 

 pebbles average larger, and the sand is coarser. 



The annelid burrow Artlirophycus alleghaniense has been collected at 

 such various levels that it may be said to be present throughout the 

 Shawangunk (for detail see Part II). However, the burrow is not often 

 present in the lower part of any section, but is more common in the upper 

 half of the formation. It is the guide fossil for Medina time, not only 

 in the eastern exposures here described, but for the sandstones of western 

 ISTew York as well. 15 Here the Medina can be traced into the equivalent 

 Brassfield formation of Ontario, where it underlies the western phase of 

 the Clinton ; at Jacks Mountain, in eastern Pennsylvania, the Tuscarora, 

 the equivalent of the more easterly Shawangimk, is overlain by the typical 

 or eastern phase of the Clinton. These facts make it plain that the 

 Shawangunk is of the time of the Medina, and that both formations are 

 the deposits of the invading Silurian sea, and are characterized organic- 

 ally by Arthrophycus alleghaniense ("Harlan). ■ 



16 See Schuchert : Bull. Gaol. Soc. Am., vol. 25, 1914, p. 288. 



