NEW ENGLAND APPALACHIAN SYSTEMS 



157 



on the older structures of the Taconic orogeny. The section along the 

 Catskill aqueduct, Fig. 11.5, gives a good idea of the composition and 

 structure of the Catskills and adjacent Hudson Valley. 



The regional stratigraphy including the Catskills has been presented in 

 Chapter 8 on the southern and central Appalachians. See Figs. 8.10 to 

 8.12. 



Regarding the structural history, Chadwick and Kay (1933) say the 

 following: 



There is evidence in the region of at least two periods of deformation. In 

 several exposures, Ordovician beds lie in close contact with angular uncon- 

 formity beneath the basal Silurian sediments. Formations as young as Middle 

 Devonian have been folded and affected by faults of low angle showing relative 

 overthrust from the east. 



The first of these deformations is definitely assigned to the Taconian dis- 

 turbance, for which this is the classical area of study. The later deformation may 

 have been produced either in the Acadian disturbance at the end of the 

 Devonian or in the Appalachian revolution, or in both. Inasmuch as late 

 Paleozoic rocks are not present in the disturbed areas, it is not possible to date 

 the movements precisely. The tectonic movements that produced the coarse 

 clastic Upper Devonian sediments to the west may have been accompanied by 

 this folding and faulting; if so, the structures are Acadian. On the other hand, 

 the structures are similar to those formed farther to the southwest and north- 

 east in the Appalachian revolution, and it is probable that some of the effects 

 were produced at that time. 



Erosion has been dominant in the region since the end of Paleozoic time. 

 Remnants of a peneplain may be preserved in the accordant summits of the 

 higher peaks in the western part of the region, of which Plateau Mountain is 

 typical. The high areas that bear these remnants seem to stand above an erosion 

 level represented by the open upper valleys of the Catskills and by the beveled 

 surface of the Helderberg Plateau, to the north, seen from Windham Notch. 

 This lower level lies 2,500 feet (750 meters) below the supposed summit 

 peneplain and has been correlated by some geologists with the Schooley pene- 

 plain of Pennsylvania, by others with the Harrisburg peneplain, of later Tertiary 

 age. Further elevation and subsequent erosion produced a peneplain that bevels 

 the weaker folded rocks in the Hudson Paver Valley west of the river. This later 

 Tertiary surface is 1,500 feet (450 meters) below the last and has been called 

 the Albany peneplain. More recent movements have elevated this surface a few 

 hundred feet (100 meters or more) above present base-level, permitting the 

 excavation of valleys in the weakest rock belts. Thus erosion has brought about 

 the removal of a great mass of later Paleozoic sediments through several cycles 

 of erosion with intervening uplifts, exposing early Paleozoic rocks in the eastern 

 part of the region. 



Adirondack Mountains 



The Adirondack Mountains constitute a nearly circular uplift about 150 

 miles across, which extends from Lake Ontario on the west to Lake Cham- 

 plain on the east, and from the Mohawk Valley on the south to the St. 

 Lawrence lowland on the north. The northwestern part of the Adiron- 

 dacks is a rolling upland of gentle relief and a mean altitude of about 

 1000 feet above sea level, whereas the southeastern part is a rugged moun- 

 tain mass, individual ridges of which reach 3000 above the valley 

 floors, and the highest peak, Mount Marcy, stands 5344 feet above the sea. 



The Adirondacks consist mainly of Precambrian rocks. These are sur- 

 rounded by gently upturned Cambro-Ordovician sediments, except near 

 Kingston, Ontario, along the St. Lawrence, where a neck of the Pre- 

 cambrian rocks connects with the Precambrian of the Canadian Shield 

 ( the Frontenac axis ) and along Lake Champlain where highly deformed 

 strata of the Taconic system bound the dome. 



According to Balk ( Longwell, 1933 ) : 



The unconformity between pre-Cambrian and Paleozoic rocks is exposed in 

 numerous places, although in the southeast the primary relations are somewhat 

 blurred by post-Ordovician faults along which the Adirondacks have been 

 elevated with reference to the surrounding younger rocks. One of these faults 

 passes through Saratoga; another one forms the escarpment northwest of town 

 and is followed by the road from Saratoga to Glens Falls for many miles. 

 Escarpments near Lakes George and Champlain are due to additional border 

 faults along the eastern margin of the Adirondacks. 



The pre-Cambrian sedimentary rocks of the Adirondacks appear to be 

 identical with rocks of the same general age in the Provinces of Quebec and 

 Ontario, so that the whole region is to be regarded as an outlier of die Canadian 

 shield. 



Sedimentation in and around the Adirondack region in Cambrian and 

 Ordovician time is illustrated in the paleograpbic maps of Fig. 11.6 

 and by the cross section of Fig. 11.7. The Adirondack dome persisted with 

 some irregularities as an area of gentle uplift during the Cambrian and 

 Ordovician, and by late Cincinnatian time a broad domal structure was in 

 existence. Then the Taconic orogeny occurred along the east side and 

 following the orogeny closely the dome was broken by block faults. Figure 

 11.13 is a cross section that restores the Adirondack uplift and adjacent 



