Giddy: Geology of the Hudson River

A Violent Past
The history of the North East makes for a land of great interest to students of geology. It is one of violent continental collisions, upwellings of lava, and enormous forces which caused the land to rise up and up into huge mountain ranges rivalling the Himalayas. They were then worn down steadily over eons, by rain, ice and wind, at a rate of 2 inches per 1000 years. Flooding by the sea followed, then the flattened land was subjected to the same geological processes as before and the cycle continued. The area has been tectonically quiet since the Triassic era, though continental drift still continues, with the Atlantic widening every year. Later upwellings of lava from turmoil in the earth's mantle have covered the earlier layers of rock, but here and there, the older rocks have persisted on the surface, resisting erosion. These mountain ranges tend to run north-south as Europe pushed up against North America, and the rivers carved the valleys between them deeper.

Forces of Nature
The major geologic forces on this area were:
One billion years ago, in the Precambrian period, the Grenville orogeny (the name for a mountain-building event). This event turned the rocks into the gneisses (sedimentary rock that has been subjected to great heat and pressure and has metamorphosed into a very hard rock) and granites that we see today, in the Hudson Highlands. The only other New York sites for this ancient rock, which has successfully resisted erosion, are in the Adirondacks.

Further mountain building events, or orogenies, followed - the Taconic, 450 million years ago. From Albany to Beacon (mile 53E) the rock is brittle, mostly sedimentary shale, limestone and slate, with contorted strata resulting from the "Taconic revolution". If you climb on the outcropping rocks in New York's Central Park, you will be on rocks formed in that period.

Two more orogenies, the Acadian, 410 to 380 million years ago, and the Alleghanian, 320-280 million years ago continued to shape the building of the continent. Each of these was the result of a continental collision and again caused lifting and faulting of the layers of rocks, sometimes subducting faults so that older layers appear above younger layers.

Between 300 million and 200 million years ago, a supercontinent began splitting apart as Africa, Europe, North and South America fractured into separate pieces and migrated as part of continental drift. It was during this period of land splitting apart, in the Mesozoic, that the Palisades were formed and it left a gap the Hudson would eventually fill. At this period, the North East USA was much nearer the equator - 15 degrees, or 1,000 miles south of where it is today. The climate was warmer and plants and creatures suited to these sub-tropical climes flourished.

The Last Ice Age
The most recent ice age, about 10-25,000 years ago, with its bulldozing mile-thick glaciers, shaped the surface as we see it today. Huge tongues of ice moved inexorably south to where present-day New York city is, gouging out valleys, turning V-shaped valleys into U-shapes. Four separate advances and retreats ground away the soil and old surface features of the landscape. Mighty rivers carved out deep river beds. It was about this time that the present configuration of the river was cut. Near Storm King, there is a place on the Hudson River bed that is nearly 200 feet below sea level. When the glaciers melted and the sea level rose back to normal, these streams and rivers flooded with sea water. The Hudson is in certain places like Norwegian fjords, drowned valleys. The Hudson as an estuary had its origins in the last glaciation, when the Wisconsin ice-sheet deepened the river through the scouring action of advancing ice, and by water flow from the combined Great Lakes and Champlain basins. The Lower Hudson Valley and Long Island Sound became a freshwater lake, hemmed in by large glacial moraines left behind by the waning ice age. The estuary was created when the moraine-dam at Verrazano Narrows was breached nearly 13,000 years ago, allowing ocean waters to flood the deep valley, more so as the sea level rose when the glaciers melted.

Glacial till (the sediment of rocks and boulders deposited by the retreating glaciers as they melted northwards) may have been helpful for brickmaking, sand and gravel mining, but it certainly made agriculture difficult, and farming is no longer a major feature of the area.

Evidence can still be seen of the origin of the rocks in certain areas. As the region was flooded by seas during long periods of its history, tiny marine creatures lived and died in the waters; their skeletal remains settled and were compressed into layers of sedimentary rock. An impressive example of this exposed history can be seen in layers at the Kaaterskill Falls in the Catskills, where the river tumbles over 260 ft of strata deposited over millions of years. On the man-made cuts through rocks for road building, such as on the hill leading to the Kingston-Rhinecliff bridge, one can also observe the folded strata of sediment. Fossils of animals that lived during the Pleistocene Epoch, or Ice Age, are found in sediments throughout most of New York. They are most abundant in the Hudson-Champlain Valley. This valley was probably an important migration route for large Ice Age mammals such as mammoths.

In some places the Hudson's banks consist of low rolling hills, but in others mountains and cliffs rise directly above the river. It's interesting to know about the different causes of these formations. Surprisingly, the two banks of the Hudson River are composed of rocks from different eons. On the east side, the rocks known as the Taconic Orogeny, date from the Cambrian period about 530 million years ago. Their formation took place over a time span of about 65 million years and was caused by continents colliding over a period of 65 million years.

On the opposite bank, the west side rocks are about 100 million years younger, dating from the late Silurian and early Devonian periods. From Haverstraw down to the George Washington Bridge and beyond are the majestic Palisades. They consist of columnar basalt, remains of a lava upwelling that once flooded a Jurassic rift valley, and formed by solid stone, not stratified: evidence that it was forced up through a great fissure in the strata.The Palisades actually continue all the way to the ocean, running underground for some distance until they re-emerge at Staten Island. The cliffs' reddish tint reflects the rock's iron content, rusted with exposure to oxygen. Paler rock suggests a relatively recent rockfall.

Cliffs of the Hudson Highlands
The spectacular Hudson Highlands, an unbroken mountain range of granite, stretching from northern Massachusetts, crossing the Hudson at Cold Spring, and ending in northern New Jersey provides a unique vista in North America. From Beacon to Stony Point (mile 35W), the rocks are composed primarily of granite and gneiss with various intrusions. Around World's End and Storm King the river runs very deep, suggesting water erosion in past eons when the continent was at a higher level, probably a valley of great antiquity.

“The Hudson River has cut a narrow, 15-mile long gorge through the range between Peekskill and Newburgh that served as a channelway for ice erosion during Pleistocene glaciation. The gorge is a true fjord, like those of the Norwegian coast, a glacially-gouged valley now invaded by the sea, and through which daily tides reach 160 miles inland to Troy!"
- Bradford Van Diver, Roadside Geology of New York

The tides rise and fall an average of six feet daily through the Highlands as fresh water flows above the salt water on its way tothe Atlantic Ocean.

The Hudson did not always follow its present course. Before merging with the Croton River, it cut west throught the Sparkill Gap, in the region of present-day Nyack, going south through New Jersey and emerging near Sandy Hook. All that remains of that course is the Raritan River.

The valley formed by the river does not stop at its mouth. It continues underwater for a distance more than halfway to Bermuda. The ice ages 1 to 2 million years ago lowered the sea level, and as recently as eleven thousand years ago, before the last glacier retreated, the Continental Shelf was above sea level and the Hudson ran all the way through the "Hudson Canyon" to the shelf's end, 200 miles from the present Atlantic shoreline. Echo soundings suggest the Canyon to be 9,000 feet deep near the rim of the shelf.

References:

Van Diver, Bradford. Roadside Geology of New York. Mountain Press Publishing Co., 1985,
Dunwell, Frances F. The Hudson River Highlands. New York : Columbia University Press, 1991.
Raymo, Chet and Maureen E. Raymo. Written in stone; a geological history of the northeastern United States. Hensonville, NY : Black Dome Press, 2001.

Websites:

The Geologic History of Harriman and Bear Mountain State Park, NY
http://geology2.rutgers.edu/harriman2/history.htm

The Hudson River Chronicles; flowing through a gap in time, by Paul Grondahl.
http://www.timesunion.com/news/special/hudsonriver/geo.asp

Geology of the Hudson Valley