The Adirondack Mountains of New York State: Part I – What's so unique about their geology?

EVERY PICTURE TELLS A GEOLOGICAL STORY

This mountaineous vista is reminiscent of a real western scene with a big sky filled with billowy white clouds, wide open spaces, over forty summits and grassy fields blanketed with flowers. The Adirondack Mountains in northern New York State record one-third of the geological history of the Earth from Middle Proterozoic Grenville orogenesis through Pleistocene glaciation. The geological scenario that is depicted here is repeated throughout the region, that of an ancient, peneplaned, mountainous terrane that has been faulted, regionally uplifted and glacially-sculpted.  

THE HIGH PEAKS OF THE ADIRONDACKS
Barely ten miles from the historic ski and Olympic town of Lake Placid, we’re looking south at the High Peaks region of the Adirondacks. We’re standing just off the road to the Adirondack Loj (their spelling) only a few miles away, the base for our upcoming geological climbs. On the left is the landslide-scarred, glacially-cirqued edifice of Mount Colden. On center stage is the eight-mile run of the MacIntyre Range that includes the mountains of Wright, Algonquin, Iroquois and Marshall. Colden and the MacIntyres are separated by Avalanche Pass, a NE-SW fault that contains a glacier-sculpted valley filled with a chain of pristine lakes.

To the right, the geologically-equivalent cleft of Indian Pass defines the precipitous, 800 foot cliff of aptly-named Wallface. The flat grassy field in the foreground is a portion of the highest glacial meltwater lakebed in the Adirondacks called North Meadow at 2,200 feet above sea level. The depth of the valley-fill has been estimated at 300 feet with unmistakable beaches on its shoulders.

REMNANT WATERFORMS OF THE ICE AGE

Just around the bend facing east, North Meadow Brook flows west to join the West Branch of the Ausable River, a meandering post-glacial stream that has established drainage through the sand plain seen above. The High Peaks region is off to the right beyond the stand of evergreens. Repeated waxing and waning of Pleistocene continental glaciers scoured the landscape, but eventually warming trends and deglaciation are thought to have stranded enumerable alpine or valley glaciers to finish the job of cirque-formation on the higher summits. A final glacial advance is recorded during the Wisconsinan Stage from 35,000 to 10,000 ka, the last pulse of the Laurentide ice sheet. Vast quantities of meltwater spilling from the glaciers became impounded by ice, resistant bedrock and glacial debris giving rise to numerous lakes such as the modern drybed at North Meadow.

RIVERS FLOW TO THE SEA, EVENTUALLY

The waters of the West Branch will, in due course, empty into the Atlantic Ocean via glacial Lake Champlain to the east and then the St. Lawrence River. Also called the Seaway due to its heavy shipping traffic, it is the widest river in the world and connects the five Great Lakes upriver with the Atlantic Ocean. The entire drainage system was established by the close of the Wisconsinan glacial episode. On the west side of the Adirondack divide, waters flow to the Atlantic via Lake Ontario and then the St. Lawrence, while on the south side of the divide, to the Mohawk and Hudson Rivers and then to the Atlantic. Our geological excursion into the High Peaks (in post Part III) crosses Avalanche Pass that directs waters either north to the Seaway or south to the Hudson. 

River patterns are determined by slope and structure, and once established, tend to persist. The fluvial architecture in the Adirondack Mountains provides clues to its unique geomorphology and the chronology of the events of its formation. Underlying structure, tectonism and glaciation have all played a role in establishing the spatial arrangement of channels in the landscape.


OBSERVATIONS OF AN UNTRAINED EYE
In my early, pre-geology years, I frequently visited the Adirondacks and the various ranges of New England. Even to my untrained eye their rocks seemed to differ: the bluish, granite-like anorthosites of the Adirondacks, the gnarled schists of the Greens of Vermont and the Berkshires of Massachusetts, and the infamous, mica-laden granites of the Granite State, New Hampshire. The shapes of the ranges varied noticeably as well. Some were treeless, exposed, lofty and angular, and others verdant, stout and rounded. In whatever manner they were "manufactured" I suspected that must have differed as well. That was the extent of my geological knowledge.

(From wikipedia.com)



AND OF A TRAINED EYE

The Adirondacks are nothing like other major mountain systems in North America. Unlike the Rockies and Appalachians that are long, continuous mountain chains, the Adirondacks form a 160 mile-wide and one mile-high, slightly-elliptical, dome consisting of seemingly random peaks. And while the trend of the Rockies and Appalachians roughly parallels their respective continental margins (although the Rockies are set inland considerably), the Adirondacks possess no such apparent coastal association.

Furthermore, the range doesn't bear the telltale geological signatures of converging plates such as a subduction zone, an orogenic belt or surface volcanism. And, its mountains uniquely expose a crystalline Precambrian metamorphic basement. What’s more, the range is located within an enigmatic intraplate setting, well inboard of the passive plate margin of North America's east coast (which was active when the Appalachians were formed). Are the Adirondacks not part of the Appalachians? What forces of nature conspired to create such a singular landform appearing so isolated from the other ranges of the Northeast and suggesting a completely independent tectonic genesis?

(From earthobservatory.nasa.gov)

NEW MOUNTAINS FROM OLD ROCKS

Back in the 60's in ninth-grade Earth Science, I was taught that the Adirondacks were "ancient mountains," actually "some of the oldest on Earth” and were “part of the Appalachians.” Since then, detrital zircon geochronologic dating of its rocks has confirmed that they are indeed old, from the Middle Proterozoic. But more recent apatite fission-track thermochronology indicates its mountains were uplifted during the Late Cretaceous and considered young. A geological conundrum!

My school lesson was accurate but only in part. The Adirondacks ARE indeed ancient; however, they are NOT old mountains but NEW (geologically speaking). They’re actually some of the youngest on Earth, and according to some accounts, they’re still rising! They are, therefore, "new mountains from old rocks." *

Lastly, they are NOT part of the Appalachians, a common misconception, having formed during separate geological eras and under totally unrelated tectonic circumstances. In fact, they are the only mountains in the eastern United States that are not geographically Appalachian.

* The Geology of New York: A Simplified Account by the NYS Geological Survey 


OLD ROCK VESTIGES OF AN ANCIENT SUPERCONTINENT

The unusual mountain-beauty of the jagged Great Range of the High Peaks region is portrayed in its entirety in this three-photo panorama. A continuous 10.65 mile-trail extends from lowly Hedgehog, Rooster Comb and the Wolfjaws on the left, across the cols and tops of a half-dozen peaks to the angular summit of Mount Marcy on the right, the highest peak in the state. The valley in the foreground is called Johns Brook and is filled with glacially-generated rock debris including massive glacial erratics. Bedrock is exposed in stream beds, avalanched-flanks and mountain-summits. Johns Brook Valley is a fantastic gateway for the exploration of this unspoiled wilderness.  

The billion year “old rocks” of these peaks are metamorphosed volcanic strata called anorthosites, and their gabbroic and gneissic iterations. Anorthosite is a large-grained, intrusive igneous rock possessing a predominance of the mineral plagioclase feldspar (90-100%) and a minimal mafic component variably with pyroxene, ilmenite, magnetite and olivine. The precise origins of the Adirondack’s Proterozoic anorthosites have been a subject of debate for decades and referred to in older literature as the “anorthosite problem.” Curiously, anorthosite was a component of rock samples brought back from the moon.

 
The emplacement of the High Peak's meta-anorthosites did not occur on our contemporary North American continent, a late Mesozoic and Cenozoic landmass. Neither did it form on Pangaea, the late Paleozoic and early Mesozoic supercontinent that existed before it. Instead, it originated in Rodinia, the supercontinent of the Middle and Late Proterozoic. The anorthosites that comprise the High Peaks were generated deep within the Earth's mantle in tectonic collisions of the Middle Proterozoic called the Grenville Orogeny, over a billion years ago, and are thus vestiges of an ancient supercontinent. We’ll return to the region's anorthosites on our next post.


THE ENIGMATIC ADIRONDACK DOME

Seen from space, the Adirondack Mountains have a curious domal configuration that encourages its rivers and streams to radiate outward like the spokes of a wheel. Guided by the prevailing slope, its rivers flow to all quarters of the compass, but many of their courses have been modified. As uplift of the region progressed, outward radiating-rivers carved deeper into resistant bedrock with a prevailing NE-SW trend which then dictated their courses into the beginnings of a trellis pattern. Some stream patterns seem to ignore fractures in the bedrock. This suggests that the resistant bedrock in their path was uncovered only recently.

(Modified from stevekluge.com)

Notice the prevailing orientation of Adirondack’s waterforms, over 3,000 lakes and ponds, and 30,000 miles of rivers and streams. Their centrifugal drainage patterns bear witness to the domal uplift and the NE-SW prevailing fault trends. Even the geometry of the roads that emanate from the mountains reflects the region’s geological evolution. 

   

The blue line delineates the six million-acre Adirondack Park established in 1892.
The red dot depicts our location in the High Peaks region in post Part III.

(From adirondack.net)

FINDING FAULT

During the Pleistocene, the shattered rocks of the NE-SW fault zones were readily excavated by continental glaciers many of which contain a chain of interconnecting lakes. A perfect example is stellar Avalanche Lake seen below which we’ll visit in post Part II. We’re looking north from the beaver dam and flooded-wetland at its south outlet. It’s the highest of three spillover lakes connected by mountain-brooks that lie in the glaciated-fault zone between the extreme verticality of Avalanche Mountain on the left and Mount Colden on the right. Notice the debris flow that has built an apron out into the lake. Its associated rock slide is famous throughout the Adirondacks. Learn why in my post Part III.

A solitary park ranger plies pristine Avalanche Lake

The waters of Avalanche Lake spillover south to Lake Colden, then to the lake of Flowed Lands, and eventually merge with rivers that comprise the headwaters of the great Hudson River. The divide at Avalanche Pass at the opposite end of the lake sends its waters to the north toward the St. Lawrence River.


A SCARRED PAST

A great many of the high peaks in the region bear the scars of rock slides. Over the millennia, a score of immense slides have gradually exposed barren rock on Mount Colden’s slopes, and in so doing, raised the height of the lake by many feet. The thin, post-glacial cover of soil is weakly adherent to the slick anorthosite of the steep slopes barely stabilized by vegetation. The origin of many slides coincides with hurricanes and nor’easters that made their way to the High Peaks region. Rotating counter-clockwise, they send their soil-drenching bands throughout New England and New York from the northeast. Water-saturated soils are too much for the slopes to retain.

This is Mount Colden seen from atop Wright Peak looking east. With lowly Avalanche Mountain in between, just beyond its blanketed summit an 800-foot, shear cliff plummets straight down to Lake Colden (seen above) that is nestled snuggly within the fault scarp. You can easily tell the old slides on Colden from the new by the color of the anorthosite. The large, gray slide in the center was created during the hurricane of August 20, 1869. In 1942, a September hurricane catastrophically raised the height of the lake by ten feet. The gleaming white scar in the center is from hurricane Irene in 2011. Beyond Colden is Mount Marcy, the tallest peak in the state. All the summits in this photo are part of the Marcy massif of the High Peaks region whose bedrock was formed during the Grenville Orogeny. And as we will see in post Part II, possess a highly complex tectonic genesis.

COMING FULL CIRCLE

The geological singularity of the Adirondack Mountains makes them distinctively unique and accounts for their incredible beauty. Their story is of an ancient supercontinent long gone, the formation of an immense mountain belt ravaged by collapse and erosion, enigmatic uplift into an elliptical dome and scores of Ice Age ice sheets that bulldozed the region. What are the details of the Adirondack’s geological evolution? Please visit my forthcoming post Part II, and in my post Part III, we’ll climb the unique geology of the High Peaks region and explore the lakes within the fault scarp.