The Adirondack Mountains of New York State: Part II – What do we know about their geological evolution?

 Yours truly atop Wright Peak in the High Peaks region of the Adirondacks

HUMAN HABITATION
The rugged and insular geomorphology of the Adirondack Mountains is attributed to their complex tectonic and glacial history. The mountains' geological past promoted a similarly colorful and varied history of human habitation. The word Adirondack is thought to be derived from a derisive Iroquois term toward the Algonquin tribe meaning “bark-eaters.” The phonetic spelling sounded similar to atiru’ taks. On old English maps the region was called “Deer Hunting Country” with “Adirondack” coming into usage around 1837.

Pleistocene deglaciation about 16,000 years ago opened the door to Native American hunting and fishing parties. During the eighteenth century, the Adirondack’s periphery saw the French and English struggle for control of North America. In the nineteenth century, the mountains enticed loggers and iron-miners, guides and hikers, dreamers and artists, and philosophers and poets. In the twentieth century, they witnessed titanium and magnetite-miners, climbers and naturalists, sportsmen and outdoorsmen, forest fires and logging-denudation followed by preservationists, environmentalists and tourists. 

Once blighted by logging and industry, the region has undergone a renaissance of woods and waters.” * Today, in the twenty-first century, the Adirondacks lives on as “a remarkable mix of wilderness and small towns in the midst of one of the most heavily developed regions in the world.” **

* Adirondack Park – Forever Wild by Verilyn Klinkenborg, National Geographic

** The Great Experiment in Conservation – Voices from the Adirondack Park by William F. Porter et al, 2009



BUILDING THE FOUNDATION OF A SUPERCONTINENT

“We now understand this ancient (Adirondack) terrain as a product of global tectonic processes that gave rise to the continents and ocean basins” of our planet. * In order to better understand how these processes formed the Adirondacks, we must look to some of the continent’s oldest rocks.

* The Great Experiment in Conservation:  Geology of the Adirondack Mountains by McLelland and Selleck

The ancient nucleus of the North American craton is the Canadian Shield (red) that formed during the Archaean and Early Proterozoic. It’s a two and a half to four billion year old, stable, igneous and metamorphic mosaic of accreted terranes and micro-plates that were progressively fused together by the process of plate tectonics. Shaped like a warrior’s shield, it was the first part of North America to remain permanently above sea level. One more massive terrane was needed to attach to the shield in order to finalize the supercontinent of Rodinia.

Today, the once-mountainous shield is a vast, gently-undulating, heavily-eroded and extensively-glaciated physiographic region of over three million square miles. From north to south, it extends from the islands of the Arctic Archipelago to the upper Midwestern states of Minnesota, Wisconsin and Michigan. From east to west, it extends from Greenland and Labrador of the Canadian Maritimes to the Canadian Northwest Territories. The Shield also exists in the subsurface beneath the Western Cordillera in the west and the Appalachians in the east.

Geologic bedrock map of North America with the Canadian Shield (red) embracing Hudson Bay.
The pointer is directed at Grenville bedrock (orange) and specifically the Adirondack outlier.
Notice the orange inliers in the Hudson Highlands, Reading Prong and within the Appalachians.
 (Modified from USGS)



ACCRETION OF THE GRENVILLE PROVINCE
During the Middle Proterozoic from ~1,300 to ~1,050 Ma, the Grenville Province (orange bedrock above) accreted to the Canadian Shield along its southeast boundary (contemporary coordinates). This was accomplished in a complex, long-lived, global-scale, tectonic collisional event called the Grenville Orogeny (after an exposure in a Canadian town in Quebec). The collision not only formed the Grenville orogen, an immense mountain belt, but it served to complete the final assembly of the supercontinent of Rodinia by bringing together most of the landmasses on the planet.

The ~3,000 kilometer-long and 600 kilometer-wide, supercontinent-spanning orogen was of Himalayan proportions that in North America extended from Labrador in eastern Canada to Mexico. Globally, the orogen reached as far as Australia, Antarctica and beyond in the west (contemporary coordinates), and in the east, Greenland, Scandinavia (Norway and Sweden), South America (Amazon) and Africa (Kalahari). This axis-sideways view of Middle Proterozoic Earth depicts the global extent of the orogen across Rodinia. The mountains in the region of the future Adirondacks (red ellipse) are Grenvillian NOT Adirondack, but the Grenville Province on which they would rise (orange blob at the arrow above) was in place!
 

(Modified from Scotese.com)



This cartoonish representation (~700 Ma) shows the extent of the Grenville orogen (reddish-brown) running through Rodinia’s building blocks. After Rodinia’s final assembly, it would fragment (rift) apart. Smaller cratonic blocks would be sent tectonically adrift along with the Grenville rocks they acquired. After the craton of Amazonia fragmented from Rodinia, the region of the future Adirondack’s (white dot) would assume a coastal locale. Geologists are studying the Grenvillian rocks on ancient continents far-adrift in an attempt to piece together the collisional events that formed Rodinia, and the details and timing of its fragmentation.

(Modified after Callan Bentley, 1991)

VESTIGES OF RODINIA

The fate of all orogens is their eventual reduction to a low-lying peneplain. Thus, the mountain belt’s long and complex history of igneous intrusion, metamorphism and deformation is represented today by ongoing degradation (erosion) and exhumation (exposure). In North America, the Grenville Province’s presence in the subsurface of the Appalachians (diagonal lines) is extensive, having been overprinted subsequently by the Appalachian Orogeny (although recently the southern and central Appalachian basement crust appears to be exotic). Surficially, it extends into southeastern Canada (yellow) and outliers of the Adirondack Mountains (green AD). It surfaces again in the Hudson Highlands, the Manhattan Prong of New York and inliers of the Appalachians (black blobs), and down south in Texas and Mexico. Globally, vestiges of Rodinia are present in the cratons of rifted landmasses that once formed the supercontinent.

Allochthonous (yellow and green) Grenville rocks thrust upon autochthonous (indigenous) rocks,
making much of the Grenville Province “reworked” older continental crust.
The Grenville Front separates the Grenville Province from the Canadian Shield.  
 (Modified after Rivers et al, 1989)

 

DEMYSTIFYING THE GRENVILLE OROGENY

Lay descriptions of the orogeny depict it as a singular, protracted mountain-building event. In reality, it consisted of a multitude of events spanning perhaps 300 million years and is best viewed as a collection of collisional and magmatic phases separated from each other by 50 to 80 million years. The scenario is somewhat analogous to the more recent long-lived Appalachian Orogeny that includes Taconic, Acadian and Alleghenian phases or episodes.

Although dates and details vary considerably and are controversial, the phases of the collective Grenville event are: the Elzevirian orogeny (1350 to 1220 Ma), the Shawinigan orogeny (1180 to 1170 Ma), magmatism of the enigmatic AMCG (anorthosite-mangerite-charnockite-granite) suite (1160 to 1150 Ma), the Ottawan orogeny (1090 to 1050 Ma) and the Rigolet orogeny (1010 to 980 Ma). The Grenville timeline might look something like this.

A-F coincides with panels below
(Timeline by Doctor Jack)

DEMYSTIFYING THE PHASES OF THE GRENVILLE

To gain a sense of how the Adirondack’s bedrock was derived, here’s a VERY abbreviated synopsis of the Grenville’s phases assimilated from numerous sources most notably from McLelland et al.* Importantly, the proposed terrane of Adirondis (red letters) is thought to have formed the basement of portions of Quebec to New Jersey (MC, VT, NY, NJ) and includes the Adirondack region!

The Canadian Shield (light gray) experienced rifting (gray arrows), opening and closing (black arrows) of the Central Metasedimentary Belt (CMB) of the Grenville Province in the Middle Proterozoic. This allochthonous belt was thrust to its location in the ensuing arc-collision. Adirondis is thought to have rifted from the North American craton and then reattached (A-D). The Elzevirian (B) and Shawinigan (D) orogenies and the enigmatic, mantle-derived AMCG suite magmatism (E) provided additional metamorphism, deformation, and further contributed to the formation of the Adirondacks. Note that the AMCG suites formed anorogenically due to lithospheric delamination and tectonic transportation in large thrust slices and nappes, and were emplaced in two intervals (1160-1130  and 1080-1040 Ma). 

The Phases of the Grenville Orogeny
 (A) Adirondis rifting; (B) Elzevirian east-directed subduction zone;
(C) Back-arc basin closure and Adirondis accretion; (D) Shawinigan CMB thrusting;
(E) AMCG suite intrusions; (F) Ottawan thrusting of Grenville rocks over the shield’s foreland.
MA, Marcy Anorthosite of the High Peaks region.  
(Modified from McLelland et al, 2010)

The Grenville Orogeny ended with deformation and metamorphism during the Ottawan phase (F) which is considered the main orogen-wide, continent-continent collision and the culminating event in the evolution of the Grenville Province. Convergence is thought to have occurred when one or more continental blocks (likely including the South American craton of Amazonia although collisions with Baltica and the Kalahari have been implicated) collided with Adirondis and the previously accreted Grenville terranes. The orogeny is comparable to the convergence of India with Asia that created the Himalayan Mountains and the Tibetan Plateau in terms of magnitude, crustal thickness, metamorphic fabric and tectonic design.

* Review of the Proterozoic Evolution of the Grenville Province, its Adirondack Outlier, and the Mesoproterozoic Inliers of the Appalachians  by McLelland, Selleck and Bickford, GSA, Memoir 206, 2010.

THREE GEOLOGIC SUBDIVISIONS OF THE ADIRONDACKS
The final outcome of the multi-phasic orogeny was the Grenville Province that includes a southern extension or outlier in northern New York, the locale of the future Adirondack Mountains. The tectonic and magmatic history of the Adirondacks is extremely complex. The timing of deformation, the identification of sutures, and the clarification of phases responsible for structural features remain unclear due to overprinting, metamorphic obscuring of boundaries and bedrock inaccessibility.

Today, the Adirondacks are divided into three terranes based on metamorphic grade, rock type and structure. Their rocks are metamorphic almost without exception, having been subjected to high temperatures and pressures at depths of 19-25 miles (30-40 km).

The three recognized subdivisions are:
 

1.) The Central Highlands (red HL) is a mountainous terrain underlain by erosion-resistant igneous rocks that were metamorphosed under granulite facies conditions (high temperature and pressure during the Shawinigan and Ottawan orogenies). Its meta-plutonic rocks include orthogneisses, meta-anorthosite, a voluminous AMCG suite and olivine meta-gabbro. The High Peaks region is located within the center of the Highlands with the Marcy Massif as its centerpiece. The red ellipse denotes the region of our geologic ascent in post Part III.

The three subdivisions of the Adirondacks in northern New York State
(Modified from Huemann et al, 2006)

2.) The Northwest Lowlands (red LL), a smaller, topographically-subdued region. Its varied rocks include metamorphosed sedimentary rocks of shallow-marine origin (notably marble, quartzite and gneiss) that are folded, faulted, and then intruded by metamorphosed volcanic rocks. These supracrustal rocks were metamorphosed to amphibolite facies (intermediate temperatures and pressures) during the Shawinigan orogeny. The Lowlands are contiguous with the main Grenville Province in Canada via the Frontenac Arch which extends across the St. Lawrence River in the region of the Thousand Islands. It is a terrane that is lithologically similar to the Lowlands, and many consider the Lowlands to be part of it.

3.) The Carthage-Colton Mylonite Shear Zone (red CCZ) is a kilometers-wide, major northeast-trending, ~45º northwest-dipping fault and terrane boundary that separates the two above domains. Its shear zone is a major Ottawan Orogeny extensional feature. The Lowlands were thrust over the Highlands along a  suture zone coincident with the present Carthage-Colton Zone.


WHAT GOES UP MUST COME DOWN

With the orogen and mountain-building complete, and the removal of convergent tectonic driving-forces, compression changed to extension. The constructive phase of mountain building was succeeded by a late-stage, destructive phase as erosion and sediment transport overwhelmed the orogen. The orogen’s over-thickened crust gave way under its own weight spreading laterally. Syn- (at the time of) through post-orogenic collapse is a fundamental process in the tectonic evolution of mountain belts.

Tectonically in brief, the over-thickened lithosphere of the orogen is removed either by delamination or convection which allows asthenosphere to well upward. The buoyant asthenosphere undergoes compression melting forming ponded gabbroic magmas that further fractionate, and exerting upward (POP UP) and outward (Fb), extensional vectors. In this manner, it is thought that the plagioclase-rich anorthosite (black squares) and the enigmatic AMCG suite (MCG) typical of the anorthositic massifs of the High Peaks may have developed. Obviously over-simplified, but we can see how orogenic collapse contributes to the formation of the Adirondack’s magmas. The genesis of the magmas is referred to as “anorogenic” emplacement (versus orogenic emplacement). 

Overthickened collisional orogen undergoing lithospheric delamination, consequent orogen rebound
and collapse along low-angle, normal faults during late phases of orogenesis.
(From McLelland, 2010)

In addition, many of the NE-striking faults found throughout the region may have originated as normal faults during this period of Late Proterozoic extension. These faults and additional from the Paleozoic were re-activated at various times and are responsible for much of the Adirondack’s contemporary landscape!

Cartoon of orogen collapse after asthenospheric upwelling has produced orogen rise,
lateral spreading and extensional faulting.
(Modified from Selverstone, 2005)

By ~1,020 Ma, the orogen's broad, elevated topography began to gravitationally collapse (the destructive phase). The Rigolet Orogeny (1,010 to 980 Ma) was an independent, final phase involving renewed orogen-wide contraction and additional collapse. Over 30 km of rock was stripped away as the majestic Grenville range was reduced to a peneplain of low relief, exposing the deep core of the mountain belt at the surface. The Adirondack Mountains still had not yet formed, but their basement rocks, the very core of the Grenville orogen, were now in place!

BREAKING UP IS HARD TO DO
Rifting typically follows the final consolidation of a supercontinent and ultimately results in its demise. Its continental crust is both thick and brittle, and becomes a trap for the buildup of heat. Tectonic movements generate stresses greater than the crust can sustain causing the supercontinent to rift apart, often along inherently-weak convergent boundaries. Following Rodinia’s breakup, fragmented cratonic blocks as newly-formed continents were sent tectonically adrift throughout the globe taking along their share of the Grenville.

Traditional Rodinia models argue that breakup on Rodinia’s west coast commenced with the opening of the Panthalassic Ocean (Paleo-Pacific) at 800 to 700 Ma between the conjugates of Australia and East Antarctica, while on the east coast, the Iapetus Ocean (Paleo-Atlantic) opened by 600 to 535 Ma. With the cessation of ongoing tectonic activity both coasts were converted from an active rift-margin into a passive rifted-margin.

(Modified from Dalziel, 1997 and Torsvik et al, 1996)

This Mollweide Projection (note the equator for orientation) shows the postulated position of Rodinia (~750 Ma) shortly after breakup with South American terrane of Amazonia beginning to disengage. The newly-formed continents of Laurentia (~550 Ma) and Western Gondwana are separated by the nascent southern Iapetus Ocean. Black shaded areas are Grenville mobile belts. Red arrow points to the region of the future Adirondack Mountains.

 (Modified from Cocks and Torsvik, 2005)

RIFTING TO DRIFTING > ACTIVE TO PASSIVE > SUBSIDENCE AND SEDIMENTATION

As the developing rift widened into the expanding Iapetus Ocean on the east (south using Cambrian coordinates), Laurentia’s passive margin was characterized by subsidence and sedimentation. Low-lying coastal regions including the region of the future Adirondacks were flooded by rising global seas (possibly caused by the many shallow ocean-basins following Rodinia’s fragmentation, rapid seafloor rift-spreading and/or thermal subsidence of passive margins). As mentioned, many of the NE-striking faults found in the region of the Adirondacks and throughout the state may have originated as normal faults during this rifting-period of Late Proterozoic extension.

Middle Cambrian (500 Ma) Laurentia with flooded coastal and cratonic regions
inlcuding the region of the future Adirondack Mountains.
(From Ron Blakey, Colorado Plateau Geosystems, Inc. and courtesy of Wayne Ranney)

As the rising Cambrian Sauk seas flooded the landscape, a thick wedge-shaped blanket of siliciclastic sand and mud covered the surface of the Grenville basement followed by an overlying carbonate system in deeper waters. The sandstone-shale-limestone assemblage transgressed with the rising seas advancing landward and drowning most of Laurentia’s craton. For the record (and everyone that thrives on names and details), the entire sedimentary package is referred to as a Sauk (the first global high-water of the Phanerozoic of which there are six) Supersequence (a conformable, time-orderly succession of strata) of Sloss (the proposing sedimentary geologist).


ADIRONDACK REGION IN THE EARLY PALEOZOIC

Thus, in the region of the future Adirondacks, the eroded Middle Proterozoic Grenville basement rocks were overlain by Late Cambrian to Early Ordovician Potsdam Sandstone (yellow) followed by an overlying limestone-dolostone sequence of the Theresa Formation and the Beekmantown Group (light gray). The contact between the two rock layers represents a billion-year-plus gap in time called an unconformity. It formed due to a prolonged interruption in deposition and/or protracted erosion, likely both. The amount of missing time (and strata) is so massive that it has achieved capital letter status in the geological literature called the Great Unconformity. And, it’s global in its extent, found wherever a Paleozoic sequence overlies a Precambrian basement.

(Modified from the Geology of New York, 2000)

The Potsdam Sandstone is the geological and temporal equivalent of the Tapeats Sandstone, the basalmost strata of the classic-textbook, time-transgressive Tonto Group within the Grand Canyon. The Great Unconformity between Middle Proterozoic Vishnu Schist and the overlying Middle Cambrian Tapeats formed on Laurentia’s west coast. It is the same time-gap that we see on the periphery of the Adirondacks!


ADIRONDACK REGION IN THE MIDDLE TO LATE PALEOZOIC AND MESOZOIC

From the Devonian through the Mesozoic, the Adirondack region remains poorly constrained. With the arrival of the Taconic Orogeny in the Middle Ordovician, loading and subsidence due to Taconic Allochthon overthrusting resulted in the creation of additional normal faults within the Grenville basement and the reactivation of pre-existing Grenville ones, as well as burial of much of the eastern Adirondacks. Like the Taconic, the subsequent Acadian Orogeny during the Middle to Late Devonian further subsided and buried portions of the Adirondack region.

The final event of the Appalachian orogenic cycle in late Pennsylvanian to Permian time brought the Alleghenian phase to the northeast, this time with the eastern Adirondacks experiencing slow uplift and exhumation. Mesozoic continental rifting of Pangaea likely prolonged regional exhumation. Still, no mountains existed in the region of the Adirondacks, but the geological stage was set with a Grenville basement covered by a Sauk sequence, exposed and fault-scarred!

The following map displays known faults and lineaments within the State of New York. The strike pattern is the cumulative result of Grenville and Appalachian orogenesis, Rodinian and Pangaean rifting. The scars within the basement structure will serve to dictate the presentation of landforms in the Holocene.

(Modified from Fakundiny et al, 2002)

ADIRONDACK REGION IN THE EARLY CRETACEOUS

As the North American plate tectonically drifted northwest, it passed over the stationary Great Meteor hotspot (also called the New England hotspot). A hotspot is a hypothetical region of mantle-derived, voluminous volcanism in the form of a thermal plume that upwells to the surface. The plate’s passage produced a somewhat linear track or age progression of igneous intrusions of various compositions on the surface.

The hotspot track can be traced by a line of kimberlite dikes in the Laurentian Uplands of Quebec to Mont Royal in Montreal, the Monteregian Hills magmatic complex east of Montreal, into northern New York and New England with intrusions of hypabyssal dikes, and off the coast of Massachusetts with the New England Seamounts (e.g. Corner, Nashville, Gosnold and Bear). The seamounts are a line of extinct, submarine volcanoes that extend over 1,000 km along the track. At about 80 million years, the Mid-Atlantic oceanic spreading center migrated to the west over the hotspot. The track of the hotspot continues on the African Plate at the Great Meteor Seamounts off the coast of West Africa from which the hotspot gets its name.

Generalized map of the Great Meteor hotspot track
(Modified from Duncan, 1984)

This topographic map demonstrates the Great Meteor’s surficial features. Trace the track from the Monteregian Hills (M) through New England (NEM) including the Adirondacks (red arrow) and past the Great Stone Dome (GSD), an intrusion into passive margin sediments domed by pressure-release melting. The track follows the submarine New England Seamounts across the Dynamic Gap and to the Cormer Seamounts (offset due to seafloor spreading). It then crosses the mid-Atlantic ridge to the African plate and continues as the Great Meteor Seamounts off the African coast.

(Modified from Smith and Sandwell, 1997)

THE ADIRONDACKS GET THE LIFT THEY NEEDED

The hotspot is thought to have induced regional heating between ~125 and 100 Ma in the vicinity of the Adirondack Highlands, as the North American plate on which it rides migrated over it. The scenario is analogous to the Hawaiian Island chain and Yellowstone magmatism. Mantle lithosphere under the hotspot is suspected to have delaminated thereby producing dynamic uplift as the buoyant asthenosphere welled up to replace the mantle lithosphere.

The result is ~1 km of domal uplift of the Grenville basement of rocks giving rise to the Adirondack Mountains forming “new mountains from old rocks.” In addition to re-activated normal faults in the Adirondacks during the orogenies of the Paleozoic, it is plausible that thermal doming may have contributed to additional re-activation in the region.

(Modified from Geology of New York)

 

 

THE GREAT UNCONFORMITY OF THE ADIRONDACKS
The thermal doming of the Adirondacks unroofed the Early Paleozoic Sauk sequence that once covered the region and re-exposed the Middle Proterozoic Grenville basement. On the periphery of the dome where uplift is minimal, the sedimentary cover and the intervening time gap of the Great Unconformity can be found.

(Modified from Geology of New York)

ADIRONDACK GRAVES

How do we know that the region of the Adirondacks was once covered by sandstones and limestones, if the sediments were unroofed and now missing from the dome? Because the transgressive sequence surrounds the periphery of the range and from down-dropped grabens that contain Cambrian and Ordovician rocks in the southern Adirondacks. These geological “graves” that formed in the extensional Grenville regime protected the landscape from erosion while uplifted horst-blocks were eroded during regional uplift. We are reminded of the preservation of the Grand Canyon Supergroup within erosion-protected, down-dropped grabens.




(Modified from Artemis at MIT)

ENIGMATIC UPLIFT *
Q.  Why did doming occur in the Adirondack region and not elsewhere along the hotspot track? Why is there not a train of Adirondack-like mountains along the track?
A.  The lack of an uplifted-track may be due to a failure of the plume to penetrate the Canadian Shield or a strengthening of the plume as it tracked eastward. The answer likely lies in the structure of the lithosphere and mantle under the Adirondacks relating to dynamic support.

An alternative interpretation of the hotspot model relates to the inferred hotspot as it encountered a progressively thinning lithosphere due to the motion of the overriding plate. Notice the path of the earthquake epicenters (black line) along the hotspot track in Quebec and New England. Earthquakes can be used as an indirect measure of magmatism and to measure its track out to sea. The track crosses two large orogenic belts that cut across the region, that of the Grenville and Appalachian orogenies. The heavy lines are failed rift arms (characterized by normal faults and mafic dikes) emplaced subsequent to the rifting of Rodinia and the opening of the Iapetus Ocean. A comparison of the track with pre-existing crustal structures suggests that a reactivation of structural features may have occurred. The emplacement of buoyant asthenosphere may account for the systemic evolution on the surface of kimberlite dikes to more voluminous crustal magmatism and Adirondack doming.

Earthquake epicenters align with the Great Meteor hotspot track (dashed line),
while Grenville and Appalachian orogenic belts transect the region.
Adirondack region at red arrow.
(Modified from Shutian and Eaton, 2007)

Q.  Why are there seamounts in the Atlantic basin along the track?
A.  Seamounts occur along hotspot tracks in oceanic lithosphere which is thinner than continental crust. Hotspots readily melt material at the base of the crust generating submarine magmatism.

Q.  If cooling is occurring in the Adirondack region with the passage of the hotspot, could uplift still be taking place other than from glacial isostatic rebound?
A.  If uplift is indeed present, it would be related to dynamic support within the lower crust and mantle.

Q.  Why are there no extrusive volcanics in the Adirondacks as in hotspot-related Yellowstone and the Hawaiian Islands?
A.  The possibility exists that magmatism may have occurred in places within the mountains and has since eroded away. Perhaps the intrusive stocks in Canada are erosive remnants that fed long-extinct volcanoes. Projecting the track to the west in Canada where it appears devoid of surficial volcanic activity, intrusives may not have reached the surface. Unconfirmed seismic reflectors in the middle and lower crust under the eastern Adirondacks do imply the presence of a mafic intrusion of the same age at depth. Again, we must look to the mantle for an answer.

* Personal communication, name withheld

ICING ON THE CAKE

With incipient accumulations in the Middle Pliocene and in earnest by the Pleistocene, the two-mile thick North American Laurentide continental ice sheet covered hundreds of thousands of square miles throughout the majority of Canada and northern United States a multitude of times. Better known as the Ice Ages, the furthest southern extent of the continental glaciations surpassed New York City and Chicago with a mid-continent terminus of approximately 38º latitude. The ice sheet created much of the surface geology of southern Canada and northern United States by gradually bulldozing its way through the landscape.




The northeast extent of the Laurentide Ice Sheet during the Late Wisconsinan Stage.
Blue, 14,000-18,000 ky; Turquoise, 10,000-14,000 ky; Dark blue, 6,000-10,000 ky.
Red line is the end moraine. Red arrow points to the Adirondack region.
(Modified from Geographie Physique et Quaternaire from erudite.com)

After some two million years of glaciation, about 10,000 years ago the ice had fully retreated from the Northeast including the Adirondacks. With the coming of interglacial warming trends alpine glaciers continued the work of scouring the upper reaches of the Adirondack’s now-elevated landscape and are responsible for the distinctive, sculpted and scoured appearance of the region today. The eroded, domal architecture of the Adirondacks has dictated the configuration of its landforms and the path of drainage that its waterforms have chosen to take. Once radial in design, the Adirondack’s lakes, rivers and streams have begun to adapt a trellis pattern as they eroded into resistant Grenville bedrock and followed the NE-trending faults in the landscape. This NASA satellite photo of the Adirondack Mountains shows the ranges, valleys and waterways that orient with the strike of the prevailing bedrock structures within the Adirondack Mountains. 




(From earthobservatory.nasa.gov)

Some workers have proposed that the Adirondacks are still experiencing uplift at a rate of ~1 to 3 mm/yr due to prolonged thermal doming; however, this hypothesis remains controversial. Other hypotheses explain contemporary uplift, if truly active, by an isostatic response to crustal thickening relating to Great Meteor Mesozoic magmatism or post-glacial isostatic rebound.


THE ADIRONDACKS OF TODAY

We’ve witnessed the emplacement of the Adirondack’s crystalline basement via Middle Proterozoic Grenville orogenesis well over a billion years ago. After Late Proterozoic mountain belt collapse and erosion, exhumation brought the deep roots of the orogen to the Earth’s surface. Latest Proterozoic rifting fragmented Rodinia, and Early Paleozoic high seas flooded the region with the Sauk sequence of deposits. Multi-phasic Appalachian orogenesis further exhumed and scored the region with faults and fracture zones. Late Cretaceous passage near the Great Meteor hotspot uplifted the Grenville foundation into the Adirondack range followed by Pleistocene glaciation that sculpted the region. Voila!

The Adirondack’s complex geological history explains their enigmatic intraplate locale at a considerable distance from the Appalachian passive margin of the continent. We now understand how the Adirondack Mountains appear to be part of the Appalachian chain but are uniquely independent geographically, tectonically and temporally. And finally, having derived their structure from ancient Precambrian rocks, we see they are truly “new mountains from old rocks.”

Please visit my upcoming post on the Adirondacks entitled Part III "Climbing the Geology."