Valley and Ridge Province
The erosional characteristics of the sedimentary rock formations exposed along great anticlines and synclines of the Appalachian Mountains are responsible for the characteristic Valley and Ridge topography. Durable layers of sandstone and conglomerate form ridges, whereas less resistant limestone and shale underlie the valleys in the region. Along the eastern margin of the Valley and Ridge is the Great Valley, a broad valley underlain by Cambrian and Ordovician shale and carbonate rocks that weather and erode faster that more durable sandstone and conglomerate that crop out in ridges and plateaus to the west (see Figure 52). It extends southward from the Adirondack Mountains region, encompassing the upper Hudson River Valley between the Taconic Mountains (to the east) and the Catskills (to the west). It gradually bends westward into northern New Jersey, forming a broad, low valley broken by long, low ridges. It is bordered by the Highlands of the Reading Prong on the south and east, and the high ridge of Kittatinny Mountain to the west. In New Jersey and eastern Pennsylvania, Kittatinny Mountain represents the eastern-most hogback ridge of Middle Paleozoic rocks of the Valley and Ridge. North of New Jersey the characteristic folds of the Valley and Ridge fade into the nearly flat-lying strata of the Catskills region and the Allegheny Plateau region of western New York and Pennsylvania. In the New York Bight region, the Allegheny Plateau and the Catskill Mountains of Pennsylvanian and New York are the northern extension of the greater Appalachian Plateau.
Precambrian age (Grenvillian) crystalline igneous and metamorphic rocks form the basement beneath the sedimentary rocks of the plateau regions and the Valley and Ridge Province. In general, the Paleozoic sedimentary cover above the Precambrian basement increases in thickness from several kilometers in the midcontinent region to nearly a dozen kilometers in portions of the Appalachian Basin region. Throughout Paleozoic time, the Appalachian Basin region was the site of accumulation of vast quantities of sediment derived from uplifts created by the Taconic Orogeny (Late Ordovician), the Acadian Orogeny (Late Devonian), and Alleghenian Orogeny (Late Mississippian to Permian). These three mountain building intervals each left a progressive tectonic impression on the rocks of the New York Bight region and beyond (generalized illustration of these events is shown in Figure 53). Between and following these mountain building episodes were extensive quiescent periods when weathering and erosion stripped away most topographic relief, allowing shallow marine seaways to episodically invade portions of the landscape in the New York Bight region. This is demonstrated by the thick sequence of sedimentary rock formations which crop out through the region extending from the Hudson Valley into the Appalachian Basin (including the Catskills, the Green Pond Outlier, and the Valley and Ridge regions [Figure 54]).
Aftermath of the Taconic Orogeny
As the Taconic Orogeny subsided in early Silurian time, uplifts and folds in the Hudson Valley region were beveled by erosion. Upon this surface sediments began to accumulate, derived from remaining uplifts in the New England region. The evidence for this is the Silurian Shawangunk Conglomerate, a massive, ridge-forming quartz sandstone and conglomerate formation, which rests unconformably on a surface of older gently- to steeply-dipping pre-Silurian age strata throughout the region. This ridge of Shawangunk Conglomerate extends southward from the Hudson Valley along the eastern front of the Catskills. It forms the impressive caprock ridge of the Shawangunk Mountains west of New Paltz, New York. To the south and west it becomes the prominent ridge-forming unit that crops out along the crest of Kittatinny Mountain in New Jersey.
Through Silurian time, the deposition of coarse alluvial sediments gave way to shallow marine fine-grained muds, and eventually to clear-water carbonate sediment accumulation with reefs formed from the accumulation of calcareous algae and the skeletal remains of coral, stromatoporoids, brachiopods, and other ancient marine fauna. The episodic eustatic rise and fall of sea level caused depositional environments to change or to shift laterally. As a result, the preserved faunal remains, and the character and composition of the sedimentary layers deposited in any particular location varied through time. The textural or compositional variations of the strata, as well as the changing fossil fauna preserved, are used to define the numerous sedimentary formations of Silurian through Devonian age preserved throughout the region.
The Acadian Orogeny
Uplifts and volcanic centers formed during the Acadian Orogeny in the New England Region shed fine-grained clastic material into an expansive inland seaway that covered most of the southern and central Appalachian region and much of the midcontinent during Middle Devonian time. As the Acadian Orogeny progressed, greater quantities of coarser clastic sediments migrated into shallow sea, building an extensive alluvial plain along the eastern margin of the seaway. The Catskills region was proximal to the Acadian Highlands, and therefore was the site of the greatest accumulation of sediment in the region. (The boundary between the two geologic regions is a line approximating the location of the modern Hudson River; the Acadian Highlands was to the east.) To the west, the marine strand line migrated back and forth through time as the supply of sediments fluctuated and as sea level rose and fell. Sediments of Late Devonian age accumulated as a sedimentary wedge to as much as 7,000 feet in the Catskills region; these sedimentary deposits are thickest in the east and grow progressively thinner westward and southward into the central Appalachian Basin region. Massive accumulations of conglomerate and sandstone exposed along the eastern edge of the Catskills plateau led to an early interpretation that the Catskills formed as a great delta-type deposit, similar to the modern greater Mississippi Delta. However, complexities in the sequence of the sedimentary formations throughout the greater Catskills have been revealed from more recent geological investigations. A new perspective of the Catskills sedimentary sequence is model of fluctuating shorelines and prograding alluvial environments along the western margin of the Acadian upland. Farther to the west massive quantities of organic-rich mud accumulated in a deeper restricted seaway basin. These organic-rich mud deposits represent the oil and gas shales that are abundant throughout Appalachian Basin and the Ohio Valley regions.
The pattern and extent of Devonian age outcrops that exist in the New York Bight region provide information about even more extensive Devonian age deposits that existed in the past. The eastern edge of outcrop belt of Late Devonian rock shown in Figure 52 roughly outlines the extent of Catskills. The southern extent of the Devonian outcrop belt is part of the folded strata along the western Delaware River valley along the New Jersey-/Pennsylvania border. Devonian sedimentary rocks also crop out closer to New York City in the Green Pond Outlier, a complex synclinal trough that trends northeastward through the heart of the Highlands region in northern New Jersey and southern New York. Based on the occurrence of marine sedimentary units in the Green Pond Outlier it can be assumed that Devonian sedimentary units were continuous across much of the New York Bight region prior to the Acadian Orogeny. Devonian sedimentary rocks are also preserved in a complex synclinal area in northeastern Connecticut and extending northward into central Massachusetts. Igneous intrusions of Late Devonian age occur in small portions of Westchester County, New York (the Peekskill Granite just east of Peekskill, and the Bedford Augen Gneiss which crops out along the New York/Connecticut Boarder near the Mianus River Gorge). Several massive intrusions of Devonian age occur in the central Western Connecticut Uplands. Some of these intrusions may have contributed to episodes of volcanism in the region.
Regional metamorphism during the Acadian Orogeny affected the rocks throughout New England, including the bedrock of the New York City area. Heating and annealing during metamorphism "reset" the geologic ages of most older rocks in the eastern Highlands Province (including the rocks throughout Manhattan and the Bronx) to Late Devonian age. The influence of regional metamorphism associated with the Acadian Orogeny diminishes significantly west of the Hudson River
The Acadian Orogeny lasted from Late Devonian into early Mississippian time. This is inferred, in part, by the abundance of igneous intrusions of these ages throughout the Appalachian region. By Late Mississippian time, mountain building in throughout Appalachian region had drastically subsided. This can be inferred from the extensive sequence of marine limestones formed from clear water marine sedimentation preserved as strata of late Early Mississippian age (Meramecian, around 350 million years ago) throughout the western Appalachian Basin region and the midcontinent. By the end of Mississippian time, mountain building was once again proceeding. This is represented in the sedimentary record as the flood of clastic material preserved in association with the Pennsylvanian coal measures throughout the Appalachian Basin region. These coal measures formed in association with alluvial flood plains and inland coastal swamplands that developed along the western margin that of the Appalachian Mountains and in Late Paleozoic sedimentary basin throughout the midcontinent.
The Alleghenian Orogeny
During late Paleozoic time the ancient Iapetus Ocean (also called Proto-Atlantic Ocean) continued to vanish as the North America continent (Laurentia) collided with Africa (which was part of a larger collection of continents called Gondwanaland). During this time all of the Earth's continents were coalescing to form a single, great supercontinent, Pangaea (beginning roughly 320 million years ago during the Pennsylvanian Period [see Figures 8, 53C, and 83]). In eastern North America the formation of Pangaea corresponded to the Alleghenian Orogeny, the mountain-building episode associated with the formation of great folds and thrust faults throughout the central Appalachian Mountains region.
As the continents collided, the rock material trapped in-between was crushed and forced upward into a great mountain range, probably similar in size and character of the modern Alps. With nowhere to go, rocks along the eastern margin of the North American continent were shoved far inland (the same occurred in the opposite direction along the margin of the African continent, forming the Atlas Mountains of Morocco and the western Sahara). The sedimentary rock in the eastern Appalachian Basin region was squeezed into great folds that ran perpendicular to the direction of forces. The greatest amount of deformation associated with the Alleghenian orogeny occurred in the Southern Appalachians (North Carolina, Tennessee, Virginia, and West Virginia). In that region a series of great fault developed in addition to the folds. As the two continents collided, large belts of rock bounded by thrust faults piled one on top another, shortening of the crust along the eastern edge of North America in the North Carolina and Tennessee region by as much as 200 miles. The relative amount of deformation gradually diminishes northward. The fold belt extends northward through Pennsylvania and gradually peters in the vicinity of the New York border. The Kittatinny Mountains in northwestern New Jersey mark the northeastern-most extension of the high ridges of the Valley and Ridge Province. The influence of Alleghenian deformation on the regions east of the Valley and Ridge Province must have be even more intense, however, there is little evidence preserved. Rocks of Mississippian, Pennsylvanian, and Permian age are missing in the New York Bight region.
A great unconformity beneath the Triassic sedimentary rocks of the Newark Basin series represents an extensive period of erosion of uplifted rocks and sediments during and after the Alleghenian Orogeny. In the New York Bight region, this unconformable surface is flooded beneath the lower Hudson River below the Palisades, and in New Jersey it is covered by younger sediments of the Coastal Plain.
Field Trips Destinations in New York, New Jersey, and eastern Pennsylvania:
Kittatinny Mountain, New Jersey and Pennsylvania
The URL is: https://gotbooks.miracosta.edu/gonp/nyc/valleyandridge/valleyandridge.htm
Last modified: 3/11/2019