N. II. DARTON — TWO OVERTHRUSTS IN NEW YORK. 



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dipping monocline, but in their eastward extension to the vicinity of the Hudson 

 river they are traversed by small but steep and characteristic flexures of the north- 

 ern prolongation of the Appalachians. Davis - lias described two typical areas in 

 this flexed belt, and in a memoir "On the Helderberg and associated formations 

 of eastern central New York," to accompany the report of the state geologist for 

 1892, I shall describe the entire area from Schoharie to Ellen ville. It is the pur- 

 pose of this paper to exhibit two particularly interesting details of the structure 

 of the region: one an overthrust fault near Rosendale, in the great cement region, 

 and the other a composite overthrust west of South Bethlehem. 



In Mather's report on the southeastern district of New York faults were proposed 

 to account for nearly every prominent topographic feature in his district, but I find 

 only a few faults in the Helderberg belt and these only of small amount and local 

 influence. Mather's report is very meagre of details regarding the structure of this 

 region, and his statements and sections are nearly all erroneous. 



The relations of the overthrust near Rosendale are shown in the following figure : 



Figure 1. — Cross-section of Ridge on south Side of Rondout Creek, at Rosendale, Ulster County, New 



York, looking North. 



The fault is on the eastern flank of the great corrugated anticlinal of the 

 Shawangunk mountains, which pitches northward in the vicinity of Rosendale 

 and involves the cement series and Helderberg formations. To the north the fault 

 extends across the creek, through the village and up a depression, dying out in 

 about two miles. To the south it soon runs out into the high, sand-covered terrace 

 lying east of the Shawangunk ridges. Its maximum displacement is about 200 

 feet, which it attains at the village of Rosendale. The details of the fault are finely 

 exposed in an abandoned cement quarry on. the slope just south of the creek, and 

 it is here that my section passes. The wedge of cement has been worked out for 

 a length of 200 feet and the fault plane is the hanging wall of the quarry. Many 

 minor features of slate wedges and crumpling arc not represented in the figure, bu1 

 I have shown at D a small wedge of grit which is faulted and cross faulted into the 

 main slate wedge at one point. The principal fault plane is along A-B, huf three 

 has been considerable movemenl along A-C, which has beautifully slickensided the 

 surface of the grit. Cross-faults and minor crumples are irregularly intermingled 



-The Little Mountains east of the Catskills : Appalachia, vol. iii, p. 20. The Folded Helderberg 

 Limestones east of the Catskills : Harvard College, Bull. Mus. Comp. Zo61 , vol. 7, p. 311. Norn ion- 

 formity at Rondout: Am. Jour. Sei., 3d ser., vol, xwi. p, 



