Geologic History

The Hudson Valley region has experienced three mountain-building episodes that punctuated prolonged intervals of subaerial erosion and periodic invasion by epicontinental seas (Seyfert and Sirkin, 1979). Late in this history, glacial erosion reshaped the peaks and ridges, and deepened valleys.

Pleistocene Glacial Deposits, Cretaceous, Tertiary

Figure 2.2. Generalized bedrock geology map of the New York State region. Modified after Geological Survey, New York State Museum, geological map, 1989.

Pleistocene Glacial Deposits, Cretaceous, Tertiary

Figure 2.2. Generalized bedrock geology map of the New York State region. Modified after Geological Survey, New York State Museum, geological map, 1989.

The oldest bedrock in the Adirondack headwaters of the Hudson is an anorthosite of mid-Proterozoic age, dated at about 1.4 billion years (Fig. 2.2). Anorthosite originated as igneous rock intruded into sedimentary deposits, mainly sandstone and limestone. After mountain-building episodes, the sedimentary rock units were folded, faulted, and metamorphosed to quartzite, gneiss, and marble. The first major mountain building episode, the Grenville Orogeny, began around 1.2 billion years ago. This event affected abroad region along the margin of ancestral North America, from maritime Canada to northwestern Mexico. The mountain system created by the Grenville Orogeny is believed to have rivaled the Himalayas, driven by a collision in which Laurentia (North America) was overridden by Gondwana (Africa). The deep burial of Laurentia resulted in the first episode of metamorphism, partial melting of rock, and separation of light and dark minerals of the Adirondack gneisses. As the continents subsequently rifted in the late Proterozoic Period, basaltic volcanic rocks were intruded into the mountains, cutting across the anorthosites and gneisses.

The Hudson Highlands gneisses and the lowest unit, the Fordham gneiss, of the New York Group rocks of the Manhattan Prong in southern New York (Fig. 2.2) are late Proterozoic in age. In both cases, the gneisses were probably derived from sedimentary rocks during the Grenville event. These gneisses have been dated at around one billion years, although the Highlands gneisses may be somewhat older and the Fordham somewhat younger. Long episodes of erosion of the Grenville mountains and subsequent crustal uplift have brought this once-deeply buried crust to the surface. Late in the Proterozoic, erosion of the Grenville mountains provided a source of thick sedimentary deposits that partly engulfed the upland, but while these deposits are found elsewhere in the Appalachians, almost all were removed from the Hudson Valley.

In the early Paleozoic, sand and gravel eroded from the mountains became basal sandstone and conglomerate (e.g., the Potsdam Sandstone of northern New York and the Lowerre Quartzite of the Hudson Highlands). As the epicontinental sea inundated the mountain region, a thick cover of marine limestone and shale was laid down in an elongated trough that formed on the continental margin where the mountains had once prevailed. Limestone was deposited in shallow water along the continental margin, and shale solidified from muds carried into the deeper, seaward part of the basin. The shale bedrock between Glens Falls and the Highlands is what remains of thousands of feet of sediment deposited in the trough. Limestone strata found north and west of the mid-Hudson valley represents the carbonate, platform deposits thought to be similar to the Modern Bahama Banks (Isachsen, Fisher, and Rickard, 1970).

In the late Cambrian Period (ca. 500 millionyears ago), Laurentia collided with the ancestrial core of Europe, Baltica and a large fragment of continental crust known as Avalonia. This mountainbuilding event, known as the Taconic Orogeny, lasted throughout the Ordovician Period and resulted in the new supercontinent called Laurasia. While much of the subduction, metamorphism, and volcanism took place well to the east, island arc volcanic structures (such as the Cortlandt Complex) have been identified in the vicinity of the Hudson Highlands. To the north and west in the mid-Hudson Valley, the sedimentaryrocks were folded and faulted, with the trend of the folds parallel to the southwestto northeastAppalachianstruc-tures. Closer to the margin, thin sheets of rock were thrust dozens of kilometers westward, known as the Taconic thrusts. Fine-grained shales were crumpled and thrust into the narrow seaway west of the mountains. Blocks of limestone slid into the trough and were incorporated in the melange of jumbled, shale masses. Today the river flows past the western edge of the thrusts and cuts into the melange deposits.

Sandstone, limestone, and shale, similar in age to the mid-Hudson strata, and Proterozoic bedrock from the Highlands south and east in the Manhattan Prong, were deeply buried as the continent's margin was subducted near the zone of plate convergence. The rocks were partially melted and metamorphosed to gneiss, marble, and schist, and folded into the typical Appalachian alignment.

(The New York Group consists of the Proterozoic Fordham Gneiss and the early Paleozoic Lowerre Quartzite, Inwood Marble and Manhattan Schist; Isachsen and Fisher, 1970,Isachsen, 1980).Streams in the metamorphic lowlands follow valleys formed along fault lines or on the softer, more soluble marble layers. Metamorphosed oceanic crust borders the rocks of the New York Group to the east. Deep, ocean-basin volcanic and sedimentary sequences, that is, ophiolites, have been metamorphosed to greenstone schists, that is, serpentinites. Mafic mineral-rich metamorphic rock of the Hartland-Harrison Group represents the oceanic deposits.

Following the Taconic Orogeny, a long interval of erosion began the process of stripping away crustal overburden as the new continent was slowly uplifted by plate compression. As the upland was eroded, the epicontinental sea gradually inundated the Hudson Valley region from the low-lying continental interior to the west. During the Silurian Period and into the early Devonian, shallow seas covered the region, and tropical calcium carbonate-rich sediments were deposited. In the early Devonian, rivers flowed from the eastern uplands, carrying sediment westward into the sea to form layers of marine sandstone. At the shoreline, a large coastal delta formed over the marine beds. By the mid-Devonian an alluvial plan extended westward across the Catskill region; the shoreline had shifted to the west. At this time, thousands of meters of mid-Paleozoic sediment were piled over the Hudson Valley; continental red sandstones from the east interfingered with gray, marine sandstone to the west. The compressive force overturned folds to the northwest (SchunemunkMountain along the New York State Thruway near Highland Mills is an example of folded early Devonian limestone and sandstone).

Renewed plate compression, and the resulting uplift of the eastern ranges, marks the beginning of the Acadian Orogeny. This mountain building episode was associated with collision of the North American continent, Laurasia, and the southern supercontinent, Gondwana. Acadian volcanic arcs and granitic intrusions of Devonian age were located east of the Hudson Valley near the continental margin. One granitic pluton, a possible volcanic arc remnant, was intruded just east of Peekskill (Isachsen, 1980).

By this time, the sea was retreating from east to west, exposing great thicknesses of sedimentary rocks from the Acadian Mountains across the Catskill Delta. The final compression of the converging Paleozoic Era continents, the Alleghenian Orogeny, began late in the Permian Period. All of earth's landmasses were now joined to form the supercontinent, Pangaea. Pulses of this orogeny had folded and uplifted the Paleozoic rocks of the Appalachians, forcing the epicontinental sea from the Catskills to the Pocono Plateau in northeastern Pennsylvania to western Pennsylvania. In the east, only relict marine embayments, like that in Rhode Island, persisted into late Paleozoic time when the sandstone, conglomerate, and coal deposits were metamorphosed.

Once above sea level, the Devonian strata of eastern New York were subjected to over 250 million years of subaerial erosion. At some point during this time span the drainage reoriented from west to southward aligning the ancestral Hudson River along a north-south trend. Perhaps this redirection of the drainage took place as the upslope edge of the deltaic beds on the east side were eroded from the mountain front during the late Paleozoic and early Mesozoic. Streams would have followed the tilt of the land and the resistant edge of the strata, both to the south, gradually capturing the headwaters of the west-flowing streams. As the bedrock was worn away, the boundary of the Paleozoic strata migrated westward so that only small outliers of mid-Paleozoic rock units would remain east of the Catskill Front. With the more resistant, meta-morphic Taconic Mountains to the east and the Catskill Mountains to the west (Fig. 2.1), the river system in the mid-Hudson Valley worked its way down through softer sedimentary layers, leaving behind the slopes of the mid-Devonian Hamilton Shales and the limestone benches of the Onondaga and Helderberg formations, east of the Catskill Mountains, before reaching the Ordovician age Canajoharie Shale of the current bedrock surface (Isachsen and Fisher, 1970).

The break-up of Pangaea followed in the Triassic Period. Large rifts and grabens stretched from northeast to southwest. In the lower Hudson region, a Mesozoic rift basin known as the Newark Basin of the Triassic Lowlands (Fig. 2.2) covers much of southern New York south of the Hudson

Highlands, west of the river and continuing into east central New Jersey. This basin received thousands of meters of Hudson Valley sediment, much of it colored red by oxidized iron minerals from Proterozic and Paleozoic metamorphic rocks or re-deposited from the Catskill red beds. The Mesozoic red beds show flow patterns emanating from the Highlands as indicators of north to south drainage.

Concurrent with graben formation, basaltic magmas were intruded along fault lines and into the red beds of the basin between 200 and 190 million years ago. The magmas formed the Palisades Sill. Today, the more resistant Palisades stand as ridges above the softer red beds of the Newark Basin. The tabular Palisades Sill slopes to the west, and the eastern edge forms the escarpment, or 'palisade' of vertically jointed basaltic rock so recognizable from the New York side of the Hudson.

In late Mesozoic times, igneous intrusions were emplaced along a northwest to southwest trend across southern Canada and northern New England, and the mountains were uplifted. The linear trend of the intrusions aligns with a chain of younger seamounts, or subsea volcanoes, across the continental shelf and into the ocean basin, reaching as far as the mid-Atlantic rift. As the North American continent moved away the midocean ridge and over a source of high heat flow embedded in the earth's mantle, the hot spot generated intrusions and volcanoes. It may also have been responsible for uplifting the northern, or higher section of the Appalachian Mountains, thereby reactivating erosion in the mountains and doming up the Adirondack anorthosites. The lower-lying mountains of southern New York experienced uplift to a lesser degree, but the thick, overlying sedimentary cover was eroded to expose the deep-seated, high-temperature metamorphics of the Highlands and the New York groups.

Deposition in the Newark Basin ended in the early Jurassic Period. The Hudson became entrenched into its flood plain and began carving its gorge into the resistant gneisses of the Highlands and southern New York. Relict meanders of the channel may date from this time. With the widening of the Atlantic, river sediment was carried to the new continental margin to form the coastal plain and continental shelf. By late Cretaceous time, the eastern rivers were depositing alluvial and deltaic sediment over marine strata on the continent's margin from Long Island to Virginia. The Hudson drainage carried upland sediment to a new sea level close to the edge of the metamorphic upland, about twenty kilometers inland from the present shoreline.

Uplift of the Long Island platform and embay-ment in the Raritan region to the south, coupled with lower sea level, allowed deposition of the younger, Tertiary-age sediments on the seaward margin of the Cretaceous delta. Lower sea level may also have enabled the river to begin excavating the Hudson Canyon into the continental shelf both by subaerial erosion and turbidity currents below sea level (Shepard, 1963). In the late Tertiary Period, the river turned toward the southwest as a tributary to the Delaware River in central New Jersey (Stanford, 2000). This drainage carried fluvial sediments along the inner margin of the coastal plain, over Cretaceous strata in southern New Jersey, and into the Delmarva region (Owens and Minard, 1979; Owens and Denny, 1979). Tertiary fluvial sediments interfingered with marine strata in the coastal plains and the offshore shelf of NewJersey and the Delmarva Peninsula.

pleistocene glaciation

Although there is no definitive evidence of earlier Pleistocene glaciation, the HudsonRiver Valley was the arena for the last two advances of Laurentide glaciers, the older during the Illinoian glacial stage and between 140,000 and 200,000 years ago and the younger in the later part of the Wisconsinan stage ending 22,000 years ago. Regional topography enabled the glacier to form a lobate ice margin, as the ice thinned over the Catskill and Taconic uplands (Fig. 2.1) and thickened and expanded southward down the valley. The older drift on western Long Island appears to contain more rock debris from Highlands and Hartland gneisses and less material form the northwest side of the Valley. The lower, U-shaped tributary valleys and bedrock gorges may be related to the last ice advance and postglacial rivers, while more open upland topography might have originated during the earlier advance.

Pollen analysis and radiocarbon dating indicates much warmer conditions than the present during the last interglacial following the Illinoian glaciation. At that time, forests like those of the present, southeastern coastal plain grew in the Adirondacks and as far north as Toronto (Muller et al., 1993). Sea level rose several meters higher than today's sea level. During the last advance glaciers expanded in the early Wisconsinan (60,000 years ago) as far as the St. Lawrence valley, and cold conditions, along with boreal forests, persisted in the northeast prior to 34,000 years ago when a warming trend began. This warm interval, called the Portwashing-tonian warm interval, peaked around 28,000 years ago, at which time oak and hickory forests prevailed and sea levels rose from glacial lows around 100 meters below to within 20 meters of the present level (Sirkin and Stuckenrath, 1980; Sirkin, 1986). As cooling resumed, boreal forests migrated back into the region. By 26,000 years ago, the Laurentide Glacier covered the Ontario and St. Lawrence lowlands. Subsequently, an ice lobe advanced into the Champlain Valley and over the Adirondacks and Green Mountains. At the height of this glacial advance, the ice may have overtopped the High Peaks region by as much as 300 meters (Flint, 1971). The south-flowing glacier deepened the Hudson gorge guided by the softer, metamorphic rock, such as the marble on the west side of the Crane Mountain near Warrensburg.

South of Glens Falls, the ice deepened the channel of the river, and, at the Highlands, cut the fjord, leaving Storm King, Beacon and Bear mountains over 400 m high above the present water level and the river's thalweg over 250 m below sea level (Flint, 1971). With glacial sea level depressed over 100 m, the bedrockwas lowered to over 80 m below present sea level near Manhattan and 60 m near the Verrazano Narrows as the river cut its way down to the lower base level. The pre-existing Hudson Canyon was also more deeply entrenched in its new subaerial reach by glacial meltwater flow, and, possibly, eroded below the glacial sea level to the continental rise by turbidity currents.

The Hudson-Champlain Lobe of the Laurentide Glacier reached its southerly boundary 22,000 years ago. The position is marked by the terminal moraine, which stretches from Long Island across Staten Island and New Jersey to Pennsylvania (Sirkin, 1986; Stanford, 2000). The terminal moraine of this lobe, known as the Harbor Hill Moraine (Fig. 2.3, after Long Island's Harbor Hill in Roslyn), impounded glacial meltwater resulting in large proglacial lakes, such as Lake Hackensack inNew Jersey, Glacial Lake Hudson and Glacial Lake

Figure 2.3. Major recessional ice margins. (1) Manetto Hills, (2) Harbor Hill Moraine, (3) Sands Point, (4) Pellets Island, (5) Shenandoah, (6) Poughkeepsie, (7) Hyde Park, (8) Wallkill, (9) Rosendale, (10) White Plains, (11) Red Hook, (12) Middleburg, and (13) Del-mar. (After Cadwell, 1986; and Connally and Sirkin, 1986).

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Figure 2.3. Major recessional ice margins. (1) Manetto Hills, (2) Harbor Hill Moraine, (3) Sands Point, (4) Pellets Island, (5) Shenandoah, (6) Poughkeepsie, (7) Hyde Park, (8) Wallkill, (9) Rosendale, (10) White Plains, (11) Red Hook, (12) Middleburg, and (13) Del-mar. (After Cadwell, 1986; and Connally and Sirkin, 1986).

Connecticut whose basin is now occupied by Long Island Sound. Thick deposits of lake clay overlap bedrock along the Staten Island and Manhattan shorelines and into the low topography in mid-Manhattan (Cadwell and Pair, 1989). Meltwa-ter drainage blocked by the moraine flowed eastward into the Glacial Lake Connecticut, in the current basin of the Long Island Sound.

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