The Adirondack Mountains of New York State: Part IV - Climbing the Geology of a Fault-Bounded Valley

This is post IV of my four-part series on the geology of the Adirondack Mountains. Please visit:
Part I – What's so unique about their geology?
Part II – What do we know about their geological evolution?
Part III - Climbing the Geology of the High Peaks

This early 1900's postcard depicts Lake Colden from Avalanche Pass in the High Peaks Region of the Adirondacks of northern New York State. Lake Colden is the first of three spillover-lakes that reside in a post-Pleistocene, glacially-scoured, fault-bounded valley between sheer cliffs of Middle Proterozoic Grenville metanorthosite.



On the first day of our geological exploration in the High Peaks Region, my daughter and I ascended Mount Wright and Mount Algonquin, the latter being the highest peak in the MacIntyre Range and second highest in the state. To visit that post, click here. On this day, we explored the waterforms immediately east of the MacIntyres.

 

FINDING FAULT
During the assembly of the supercontinent of Rodinia, Middle Proterozoic Grenville orogenic events laid a foundation of metanorthosite and associated rocks throughout the region. Many of the NE-striking faults within that basement may have begun as normal faults in the extensional environment that existed during the closing stages of the orogeny.

 

The faults that persisted within the Adirondacks were likely reactivated (and new ones created) during Paleozoic Alleghenian tectonic collisions, specifically during overthrusting of the Taconic Orogeny. Late Cretaceous thermal doming of the region of the future Adirondacks subsequent to passage over the Great Meteor hotspot also may have reactivated the faults. These events, from the Middle Proterozoic through the present, are explained in greater detail in my post Part II here. Today, the myriad of NE-SW faults influence the orientation of many of the ranges and waterways within the High Peaks Region.  

 

THE BIG PICTURE
The following three-photo panorama faces south towards the High Peaks. It shows the relationship of the NE-trending MacIntyre Range to fault-bounded Avalanche Pass on the east and Indian Pass on the west. The open flat in the foreground is a small portion of a post-glacial, dry lakebed called South Meadows. Many large lakes such as this were breached when their ice and morainal dams catastrophically broke open. Our trek to the lakes via Avalanche Pass began at Heart Lake, at the base of Mount Jo. 

  

You can also see the fault-trending relationships aerially on this image captured from Google Earth.  



ASCENT TO MARCY DAM AND POND 
At sunrise, we departed from the Adirondack Loj (spelled correctly) at Heart Lake (elevation 2,169 feet). After a two-mile upward grade, we reached Marcy Dam and its impounded Marcy Pond (elevation 2,346 feet). In 1999 Hurricane Irene wiped out a footbridge that crossed the dam, which was rebuilt downstream across Marcy Brook. The dam replaces one first built for the logging industry at the turn of the previous century.



A wooden, weather-beaten Marcy Dam without its footbridge impounds brook trout-stocked Marcy Pond at low stage.

LEGACIES OF THE PLEISTOCENE
The waters of Marcy Pond are a small part of the St. Lawrence Watershed. They flow north to Lake Champlain and the St. Lawrence River, the widest river in the world and a major shipping lane between the Great Lakes and the Atlantic Ocean. The Adirondack’s watersheds and their respective waterforms formed subsequent to the most recent recession of the Laurentide continental ice sheet in the Pleistocene about 16,000 years ago.

Late Cretaceous uplift responsible for the creation of the Adirondack range initially established a radial stream pattern in the mountainous, elliptical dome that formed. With subsequent downcutting to erosion-resistant, anorthositic bedrock, Grenville-age faults began to dictate the pattern of flow. Today, still radial on a grand-scale, a trellis pattern follows the many linear fault zones with lakes, rivers and streams directing their waters ultimately to the Atlantic either via the St. Lawrence or Hudson Rivers.

We’re standing on Marcy Dam looking south across rain-depleted mudflats of Marcy Pond. The pond serves the watershed north of Avalanche Pass which is directly ahead. Landslide-scarred Mount Colden forms the east side of the pass, but the pass divides the watershed to the north and south.



ACID RAIN
The waters of the Adirondacks are sensitive to deposition in the form of acid rain due to their topography, low neutralizing capacity of the lakes and streams, and the relatively large amounts of annual rainfall. Sulfur dioxide and nitric oxide emissions from the burning of fossils fuels not only rains down from the atmosphere as diluted acids but falls to earth in the form of particles, gases and aerosols. As a result, the quality of the water has degraded over the past century.

The situation is compounded by sulfur dioxide emissions that provide the material for the growth of bacteria which in turn convert mercury into a form that is bio-available to fish. It gets worse in that acid soils become depleted of calcium and other nutrients, and release aluminum into the surface waters. Although pollution reduction measures from Clean Air Act legislation have shown improvements, the forests and aquatic life have suffered considerably. 

An informative paper on acid rain deposition in the Adirondacks is here.

 

MOUNTAIN STREAM DYNAMICS
Above Marcy Pond, the trail to Avalanche Pass roughly follows Marcy Brook, again dictated by the underlying bedrock. Seen here at low-flow stage at a bend in the brook, the distribution and size of anorthositic cobbles and boulders in the streambed, and the tangled mass of vegetation hint at the high volume and velocity of water during a spring melt or severe thundershower.

In spite of its shallow gradient, notice the scouring of the banks and bed at the widest point of the channel, also the deepest. Erosion occurs at the outside of the bend (the cutbank), while slower velocities at the inside of the bend causes point-bar deposition (the slip-off slope). At low-stage and at the inside of the bend the stream lacks the power to carry its load of suspended sediment and detritus.

Physical characteristics of the stream also influence water quality, and therein, the variety and type of habitat that is available to support life. Also notice how the riparian vegetation in the bank zone is affected by the stream’s hydraulic geometry. Evergreens are thriving on the inside of the bend; whereas, mixed deciduous, herbaceous growth has colonized the outside (i.e. willows), the damper more saturated soil region. 



Viewed within the context of the geological time frame, the contribution to landscape evolution of a small stream such as Marcy Brook, which might flood only a few times a year, should never be perceived as inconsequential. Significant change occurs over geological periods of a thousand or a million years especially accounting for the many companion streams that exist within the watershed!

 

AVALANCHE PASS
Continuing on our ascent, we reached Avalanche Pass between the confining mountains of Colden and Avalanche. The pass serves as the drainage divide for waters flowing north to Lake Champlain and south to the Hudson River within the Upper Hudson Watershed. As we entered the pass, its lichen-encrusted, sheer rock walls of anorthosite increasingly closed in, and the wind picked up as it accelerated from the confinement.

This is the narrowest section of Avalanche Pass looking south with opposing walls of anorthosite separated by 100 feet. 

 

Backpackers have taken the time to construct this mini-cairn city which my daughter felt obligated to contribute to.

In keeping with its namesake, an enormous, jumbled-mass of vegetation and bedrock avalanched downslope from Hurricane Floyd in 1999 along a 600 foot-long rock slide on Mount Colden. Volunteers and professionals have industriously cleared a trail directly through it. This photo looks up the slide on Colden’s western face past the mountain of cut foliage. In winter, hikers and skiers must be attuned to the inherent dangers that lurk within the pass from avalanching snow.

Within the pass is a spruce swamp with boardwalks over the boggy ground which supports the rare fern Dryopteris fragrans. Occasional, rust-colored stagnant pools of water suggest iron-rich pyroxenes that eroded from anorthosite underwent oxidation and imparted a dark-brown, rust color to the water.

 

AVALANCHE LAKE
Once through the pass you emerge into the uppermost reaches of the Upper Hudson River watershed. You're about to receive a visual reward for your climbing efforts. Before you is spectacular, sparkling Avalanche Lake, the first of three spillover-lakes that lie in a chain within a fault valley. Avalanche Lake is supposedly the highest lake in the United States east of the Rocky Mountains at an elevation of 2,864 feet. I’ll let my industrious readers research that one.

This is an incredibly special and beautiful place in the High Peaks that evokes strong emotions when first seen. Framed by the precipitous mountain flanks of Colden on the east and Avalanche Mountain on the west, their rock-walls plunge directly into the lake. This view is from the beach looking south from the end of the lake. Barely visible at the far end of the lake is its outlet and an active beaver dam. 

Avalanche Mountain’s vertical face plummets a thousand feet into the lake’s western side. Notice the parallel jointing on the rock wall, some of which is curvilinear, composed of metanorthosite and anorthositic gneiss. Alleghenian orogenic collisions of the Paleozoic, most likely the Taconic, are responsible for these deformational features along with the reactivation of faulting that contributed to the formation of the valley. Also notice the pattern of exfoliation in areas of the cliff with a reverse-step pattern. Anorthosite shares this property with granite by eroding from the surface in layers like an onion. Around the lake mixed conifers and deciduous hardwoods luxuriate in remnants of glacial till, a mixture of clay, sand, silt and stone.

The following USGS topographic map illustrates the NE-trending fault-valley that contains the lakes of Avalanche, Colden and Flowed Lands (not seen), all south of Avalanche Pass. The closeness of the contour lines on Avalanche Mountain’s eastern face reflects its thousand-foot verticality. Faults within the Grenville basement served to weaken the bedrock. Following Late Cretaceous uplift and unroofing, the bedrock was more readily eroded and excavated by Pleistocene Continental ice sheets that slowly bulldozed through the region.

The system of spillover lakes is a product of the post-glacial watershed that was established. Surface water elevations are primarily controlled by the underlying bedrock elevation rather than the type of bedrock. As mentioned, a trellis drainage pattern has developed in response to the system of faults and is superimposed upon an outward-from-the-center radial pattern dictated by the Adirondack’s uplifted dome (see my post Part I here).  

  

LAKE COLDEN AND AVALANCHE PASS FROM THE SOUTH
From the trail along the lake's western shore, this two-photo panorama looks back to the north at Avalanche Pass and its defining and confining cliffs of Avalanche Mountain and Mount Colden on the west and east, respectively. The lake is also notorious for its double echo at this spot which we tested successfully. 

This Goggle Earth, northeast-facing, aerial-view of the fault helps to illustrate the orientation of Avalanche Pass and Lake to Avalanche Mountain on the west and Mount Colden on the east. Shear displacement along the fault on the western side is to the southwest. Notice the rock slides that have scarred the slopes of Mount Colden. Also notice the outlet-brook that flows from the lake's south end toward Lake Colden.

“HITCH-UP MATILDA!”
The trail south continues along the west side of the lake, but it’s nothing like you might expect. In two stretches where the cliff plummets straight into the lake the only way to construct a trail in the 1920’s was to bolt two, wooden catwalks to Avalanche Mountain’s rock-face. And that’s not all. Over a dozen wooden boardwalks and ladders lead you up, over and around massive boulders that have torn loose from above and littered the shoreline. It reminded me of the board game Chutes and Ladders from my youth.

As the 1868 story goes, a young woman named Matilda was being carried by a guide through the pass. As the water deepened, her sister repeatedly urged Matilda to “Hitch-up!” in order to remain dry. Such is the mountain lore of the Adirondacks.

  

THE TRAP DIKE OF COLDEN
Another surprise awaits the climber! Halfway down the lake on Hitch-up Matilda, Mount Colden’s famous and infamous “Trap Dike” comes into view across the lake. Dikes are conduits that transport molten, pressurized magma through fractures and weaknesses in the crust. Being less resistant to erosion than the anorthositic country rock through which it intruded, the dike has since weathered out upon its exhumation and exposure, leaving the gaping chasm that we see today. 

 

The dike appears as a deep, 80 foot-wide, vertical gash that extends from the lake to Colden’s summit, a distance of almost 2,800 feet. The dike's immense size is very deceiving with its length being twice the height of the Empire State Building. Look at the massive evergreens for scale. 

Although many hikers believe that you become ‘trapped’ once you enter the dike (which has some truth historically), the word is actually Swedish for stairs (trappa) referring to the “steps” that formed in the dike’s magma as it cooled and contracted. Geologically referred to as the Avalanche Dike, it was first explored by pioneering geologist Ebenezer Emmons in 1836 who described the experience of being within its vertical walls as of a "sublime grandeur.” Ebenezer made this sketch of "The Great Trap Dyke at Avalanche Lake" in 1883. 

DEFINING THE DIKE GEOLOGICALLY
Compositionally, the Trap Dike is a diabase of garnetiferrous metagabbro. The presence of garnet within the dike is a signature of high grade, granulite-facies metamorphism, and suggests that it intruded before the final metamorphic tectonic event to affect the Central Highlands Region.

The dike emplaced before anorthosite host-rock crystallization based on such characteristics such as sharp contacts with the anorthosite and its cross-cutting relationship. Its intrusion and metamorphism occurred during the protracted Grenville Orogeny, a billion years before the uplift of the Adirondacks into a mountain range in the Late Cretaceous. 

Although the Grenville Orogen was extremely complex, highly protracted and multi-phasic, for purposes of simplicity it can be subdivided into four major events: a subduction arc-collision, an episode of voluminous anorthosite and AMCG intrusion, a strong collisional event and an extensionally-dominated collapse of the orogen. The Trap Dike's gabbroic rock likely would have formed during anorthosite petrogenesis and its intrusion shortly after the host-anorthosite emplaced. For more information on anorthosite evolution, please visit my post Part III here.



A CLUE TO THE REGIONAL LANDFORM
The rock-type on opposite sides of Avalanche Pass is anorthosite, suggesting that the landform might simply have resulted from closely-spaced joints, of which there are many. The formative clue is revealed by Colden's Trap Dike which continues on the opposing side of the valley across the lake. Offset of the two dike-segments on either side confirms the landform is indeed a fault-valley, as is Indian Pass on the west of the MacIntyre Range between it and Wallface.

 

ROCK SLIDES OF COLDEN
Also of interest are the bare rock surfaces on Colden’s granite-like face. They are in fact landslide scars and are typical of many high peaks in the Adirondacks. The thin cover of soil that tentatively clings to the anorthosite 's rough surface is stabilized by vegetation but can become destabilized on steep slopes when saturated by heavy, unrelenting rains.

The Trap Dike serves as a natural funnel for runoff and slide material by channeling everything down to the lake. At the dike’s outlet a large debris fan extends out into the lake. So massive was the debris-flow from Hurricane Floyd in 1999 that it instantly raised the height of the lake ten feet, and in 2007 an avalanche on Colden blasted debris to the opposite shore. Historically, the earliest documented slides are from the hurricanes of 1869 and 1942.

This view of Colden’s west face was taken from Mount Algonquin on our climb the previous day. Old slides are distinguished from new by the color of freshly exposed plagioclase gleaming in the sunlight. The slides' funneling of material into the dike have deforested its lower half. The dike's vegetated upper portion appears as a depression and continues over the crest of Colden and beyond, as seen on the topo map (above). Remember that the dike continues to the west as well into the body of Avalanche Mountain, offset by faulting regionally. A second, smaller dike also has been found on Colden near its summit parallel to the foliation of the anorthosite.   

  

Aside from its prominence and topographic expression, Mount Colden's dike is not unique in these high peaks of the Adirondacks. For example, a billion years after the emplacement of the Trap Dike, swarms of gabbroic dikes formed during the Mesozoic rifting of the Atlantic Ocean, but they occur with greater abundance in the eastern Adirondacks, southeastern New York and throughout New England.

 

CLIMBERS BEWARE!
Since the first documented climb in 1850, the Trap Dike has become a classic and dangerous mountaineering route in the High Peaks, renowned for its steepness and difficulty especially in wet weather. It contains many large boulders and ledges to negotiate, and even a waterfall or two in the spring. Wet anorthosite, even with its rough texture, can be extremely slippery when wet.

Climbers heading for Colden's summit that bail out of the dike too early find themselves on a precipitously-steep, exposed-slope of 45º. “Stay in the dike, where the climbing becomes easier!” warn online climbing journals where the best spot to exit the dike is marked by a cairn. Overall it’s a non-technical, Class 3-4 climb, but many rescues take place when climbers get stuck or "trapped" in the dike, and many have lost their lives by literally falling off the mountain. Check out YouTube.com for climbing videos, but don't forget to look at the geology! 

 

AVALANCHE LAKE TO LAKE COLDEN
Continuing further on the trail, Avalanche Lake’s outlet at its south end provides a close look at a beaver dam and a tremendous view north toward Avalanche Pass. We’re looking directly up the fault!

Having departed from Avalanche Lake, we followed its outlet stream down to Lake Colden at an elevation of 2,766 feet. Colden is the second of three lakes in the chain within the fault-valley and the end of our journey into the High Peaks Wilderness. Beyond Lake Colden’s beaver-marsh, we’re looking north toward Colden with its rock slides. The trail continues on below Lake Colden along another outlet brook to the curiously named lake of Flowed Lands which drains south to the Hudson River and down to the Atlantic Ocean.  

Lake Colden ended our exploration of the chain of lakes within Avalanche "fault." A curious stream enters the Flowed Lands from the east called Opalescent Brook. The stream bed is renowned for its anorthosite filled with beautiful, blue-green iridescent labradorite that shimmers in the water. We were hoping to reach that point, but the trek got the best of us. The round trip was almost 12 miles and 8 hours, returning via the same route. Perhaps next summer!

 

 

"FOREVER WILD"
In spite of the fact that the Adirondack Park and Forest Preserve was established with the catchy phrase “Forever Wild” in 1885, the logging industry managed to denude vast areas that left the region susceptible to wildfires. In 1903 an estimated 600,000 acres of land burned in the Adirondacks including vast tracts of this High Peaks Watershed. Various conflagrations continued for an additional decade. Between rampant logging, forest fires and disruption to wildlife, much of the Adirondack wilderness laid decimated.

  

 

 

 

The Adirondack region was the "crucible of the American conservation ethic" at the turn of the twentieth century (The Great Experiment in Conservation by Porter et al, 2009). These days, tourism, timber and mining are the mainstays of the modern Adirondack economy. Yet, significant change is likely to be in the future of the Adirondacks as it continues to grapple with a shared vision of sustainability.

 

The mountains are actually wilder and more pristine now than they were a century ago. Today, the park's 2.6 million acres are heavily protected and well-managed. Its size is 6 million acres, larger than Yellowstone, Yosemite, Grand Canyon, Great Smoky and Everglades National Park COMBINED! 

 

“We do not inherit the earth from our ancestors;
We borrow it from our children.”
Native American proverb

The Adirondack Mountains of New York State: Part III - Climbing the Geology of the High Peaks

We’re facing north from the summit of Algonquin Peak, the second highest mountain in the State of New York (5,114 feet). In the foreground, Wright Peak (4,580 feet) displays two Holocene rock slides, typical of the Adirondack’s higher peaks. Just to the left of Wright, lowly Mount Jo stands reign over glacial Heart Lake, the base for our climbs. Lake Placid Basin is in the left, middle distance. In the August haze, Whiteface Mountain (4,865 feet) is perched on the horizon (left of center) with the Sentinel Range sprawling off to the right. Another 45 miles and you reach the end of the Adirondack’s elliptical, uplifted dome. There you’ll find the lowlands of the mighty St. Lawrence River flowing to the Atlantic Ocean from Lake Ontario of the Great Lakes. 

 
How did the Adirondack Mountains form? Please visit my post Part II here.

VESTIGES OF A SUPERCONTINENT
Virtually all of the bedrock in this Adirondack Mountain vista is Middle Proterozoic Grenville in origin. The last billion years were witness to the formation of the supercontinent-spanning Grenville mountain belt culminating with the assembly of Rodinia, to its fragmentation, to the Iapetus Ocean’s formation and eventual closure, to the supercontinent of Pangaea’s unification and rifting apart, and to the birth of the Atlantic Ocean. Blanketing Early Paleozoic marine assemblages have been unroofed by thermal doming of the Early Cretaceous. A hundred million years later, Pleistocene continental glaciation bulldozed the region at least four times, likely more, leaving its erosive signature everywhere. The story of the Adirondacks is indeed “Written in Stone.”


THE ADIRONDACK LOJ
In August, my daughter and I drove from Boston to the Adirondack Loj (correct spelling), a few miles south of Lake Placid, New York. The lodge is efficiently run by the Adirondack Mountain Club and served as our base for two days of geological exploration within the High Peaks region. The lodge is replete with home-cooked meals and bagged lunches for hikers. It is immaculately clean with private and family bunk-rooms, and a communal great room for relaxing beside a stone hearth. There’s even swimming and canoeing in crystal clear Heart Lake. Built in 1927, this idyllic “gem-in-the-woods” has it all: mountain hospitality, Wi-Fi access, education classes in geology, botany and mountain lore, and easy access to the high peaks. Go there (shameless plug)! For their website click here.

My daughter (and climbing partner) enjoys the night air outside the lodge.

And yes, that IS a moose head above the hearth!

GLACIAL HEART LAKE
The lodge is situated on the edge of most pristine Heart Lake in the shadow of Mount Jo at 2,340 feet. It’s diminutive by Adirondack standards, but after a short hike above the glacial talus that litters the region, anorthosite bedrock quickly crops out. Go a little further, and the gabbroic anorthosite becomes gneissic as its constituent labradorite feldspar crystals begin to align. Still further, the trail crosses a fine-grained, black camptonite dike. All that geology within a mile of the lodge!

Taken from the summit of Mount Jo above Heart Lake with Mount Colden (left), the MacIntyre Range including the Peaks of Wright and Algonquin (center), and precipitous Wallface (right of center) are separated by the NE-SW fault valleys of Avalanche and Indian Pass, respectively. From a wonderful National Geographic article entitled “Adirondack Park-Forever Wild” at www.ngm.national geographic.com and photographer Michael Melford at www.michaelmelford.com)

The geological verdict on the lake is still out. Some believe it's a kettle lake that formed when ice calved from the front of a receding glacier. In this scenario the lake would have become established in the glacial outwash when the ice melted. An alternative origin depicts its formation in a glacially-scoured basin replenished by melting glaciers and eventually mountain streams. That would lend credence to the thought that Heart Lake and the adjacent drybeds with unmistakable beaches were once one large glacial lake. The outlet of Heart Lake flows north into the lake basin of South Meadow. We’re looking south at the foothills of the MacIntyre range just before sunset, tomorrow’s destination.

Tranquility will have a new meaning!

ADIRONDACK MOUNTAIN HIGH
After a restful night in the lodge (2,174 feet), we began our sunrise-ascent to Wright Peak (4,580 feet) which was a warm up for Algonquin Peak (5,114 feet) to follow. Both mountains are within the MacIntyre Range, named after the owner of the Tahawus open pit, iron mining operation in the 1800’s and titanium dioxide in the early 1900’s.

The MacIntyre Range stands apart from the surrounding peaks and extends for eight miles running NE and SW along the trend of the faults that confine it. Its steep SW slope forms Indian Pass, while the NE side defines spectacular Avalanche Pass. Our two-day plan was to climb the range from Wright to Algonquin on the first day and investigate the system of lakes within the fault-valley to the east of the range on the second day.

The Adirondacks have a distinctive look and feel right down to the moss-covered, gnarled tree-roots that seem to imprison boulders of glacial talus.

The rough and rocky trail starts out in unconsolidated glacial talus and till, and transitions to anorthosite bedrock. The verdant slopes and valleys of the Adirondacks contain a deciduous mix of aspen, ash, cherry, beech, maple and birch at lower levels and hardy evergreens at higher elevations that includes pine, spruce, hemlock and cedar.

A TRAIL OF ANORTHOSITE
It wasn’t until about 2,340 feet that we encountered our first outcrop of anorthosite bedrock as the going steepened. From then on, the trail was entirely on exposures of metanorthosite and anorthositic gneiss requiring lots of scrambling and more planning for each step. We’re looking uptrail at one such steep exposure. The pitch is very deceiving at about 40-45º. My daughter is actually sitting upright. What a place to traverse in a downpour! The bedrock has been stripped of 30 km (give or take) of Grenville overburden by erosion, exhumation and uplift.

Notice the intrusion of a wide dike through the anorthosite with a small apophysis (offshoot) from the main channel mid-way up to the right. I suspect this dike to be of pyroxenite in composition. It lacks the chilled margin of fine crystalline growth indicative of most regional dikes which would indicate rapid cooling; therefore, the magma contacted the anorthosite while it was still hot. However, notice the cracks perpendicular to the path of dike-emplacement. The dike had already cooled enough to contract.

There are many dikes in the Adirondacks of various tectonic causations and time frames. Examples include: Late Proterozoic dikes of alkaline basalts (meta-diabasic) that intruded Grenvillian crust during orogenesis; late- to post-orogenic dikes associated with extensional collapse of the Grenville orogen; dikes associated with the rifting of Rodinia and the opening of the Iapetus Ocean in the latest Proterozoic and Early Cambrian; Mesozoic tholeiitic dikes associated with the rifting of Pangaea and the opening of the Atlantic Ocean; and dikes associated with passage over the Great Meteor hotspot (more so eastern Adirondacks). Dikes are of significance in studying such processes as continental breakup, and the composition of the lithosphere and asthenosphere.

Many of the waterfalls in the Adirondacks are associated with dikes that succumb more readily to erosion than the surrounding resistant anorthositic country rock. Such is the case with this waterfall of MacIntyre Brook associated with several diabase dikes that crosscut the bedrock. At an elevation of 3,255 feet, it only had a trickle of water. One can imagine the raging fury during a summer thunderstorm.

 

 



Along the trail, we encountered frequent veins, likely quartz, cross-cutting the bedrock where tension-cracks in the rock admitted the injection of erosion-resistant, mineral-bearing solutions.

 

 

 

ANORTHOSITES OF THE HIGH PEAKS
“Proterozoic massif-type anorthosites” (Ashwal, 1993) were emplaced along the southeastern aspect of the Canadian Shield within the Grenville Province during the waning stages of the Grenville Orogeny. The Adirondack Mountains of northern New York State represent a southern extension of the Grenville Province (visit my post Part II for details here). Separated by the Carthage-Colton Shear Zone, they are topographically divided into Central Highlands and Western Lowlands. Our climb in the High Peaks region of the Highlands was entirely within the Marcy massif (orange) and surrounded by associated granitoids of the AMCG suite (stripes), a tongue-twisting, felsic and intermediate complex of anorthosite, mangerite, charnockite and granite.





Anorthosite and AMCG series distribution in the Central Highlands of the Adirondacks
(Modified from Chiarenzelli and Valentino, 2008)

THE “ANORTHOSITE PROBLEM”
Anorthosite is the most difficult igneous rock to explain. Its unique geochemical nature and puzzling tectonogenesis have intrigued geologists for almost a hundred years. Enigmatic are its: near mono-mineralic composition and large crystals of over 90% plagioclase feldspar (fractional crystallization in Bowen’s Reaction Series is generally 40-50%); its gabbroic parental magma (the precursor of any igneous rock); its enigmatic association with bimodal granitoid-suites (the AMCG suite); its low (less than 10%) mafic to intermediate (diorite and gabbro) rock composition; its restrictive occurrence as plutonic rocks; its presence with layered mafic intrusions; its emplacement largely confined to the Middle Proterozoic; and its unique tectonic setting (“anorogenic”).

Many of these petrological problems have been resolved, but their genesis has remained elusive. Clearly, they formed by igneous processes, but they can not have formed from a magma of their own bulk composition. The problem with anorthosite is its geochemical composition and begins with the generation of magma, the necessary precursor of any igneous rock. Magma that is generated by small amounts of partial melting of the mantle is generally of basaltic composition, which has the opposite composition found in anorthosite, lower plagioclase and no ultramafic rocks.


BOWEN’S REACTION SERIES
The series (delineated by a petrologist in the early 1900’s) indicates the temperature at which minerals melt or crystallize in magma. It also explains why some minerals are always found together and why others are almost never associated. Magma generated by partial melting of the mantle is generally of basaltic composition. On the series under normal conditions, the composition of basaltic magma requires it to crystallize between 50 to 70% plagioclase with the bulk of the remaining magma crystallizing as mafic minerals such as pyroxene. Thus, basaltic magmas are typically plagioclase- AND pyroxene-rich. Basaltic magmas of anorthosite, however, are defined by a much higher plagioclase content and much lower mafic content. In petrology, this is known as the “anorthosite problem.” 
 

Gabbroic anorthosites are plagioclase-rich and mafic-poor in content unlike conventional intermediate basaltic igneous rocks.
Note that granite, somewhat similar in appearance to anorthosite, is derived lower in Bowen’s Series and chemically unrelated.
(From ck12.org)

For a more detailed explanation of the Bowen Reaction Series click here.


AN ANORTHOSITE (THEORETICAL) SOLUTION
Although controversial for many decades, a consensus has developed to provide an anorthosite solution. Simply stated, anorthosites are considered to be the product of basaltic magma and that the removal of mafic minerals has occurred at a deeper level. A key point is the ascending asthenosphere that provides thermal energy to melt gabbroic magma that has underplated the lower crust. And also uniquely Adirondack is the intense deformation during or after crystallization that occurred which generated the re-crystallized parent liquids of anorthosite.

The following is a chronological model of how anorthosite, plagioclase-rich and mafic-poor, may have formed along with its associated AMCG suite. Note that the process is “anorogenic” in that ponded magmas evolved in an extensional and regional event not directly derived from normal mantle melting rather than in an “orogenic” convergent tectonic event. Although the suite represents a small percentage of the Adirondacks, the AMCG's are crucial in understanding the petrogenesis of massif anorthosite. For clarification of events related to extension within the Grenville Orogeny, please visit my post Part II here.


A THEORETICAL MODEL
(A) After accretion of the Grenville Province in the late- to post-tectonic setting of the Grenville Orogeny, delamination of over-thickened lithosphere (from the Grenville contractional orogeny) and post-collisional extension (during orogen-collapse) promoted an influx of gabbroic magma from the asthenosphere yielded by decompression melting. Having left its mantle source, the picritic magma (olivine-rich and plagioclase-poor) underplated the crust, ponded there and differentiated into a magma chamber.
(B) Crystallization of olivine and pyroxene (aka Bowen) occurred with these dense mafic (ferro-magnesium) phases sinking back into the mantle.
(C) The remaining crystal mush became enriched in plagioclase, Al and Fe/Mg. This lower-density, buoyant basaltic melt (now a plagioclase-rich anorthosite) began to diapirically (hotspot plume-like) ascend into the crust.
(D) Anorthosite further ascended as plutons.
(E) The plutons coalesced to form massive anorthosite. The rising, hot asthenosphere (a key point) provides heat to partially melt the lower crust resulting in the formation of granitoids which, along with anorthosite magmas, formed the AMCG suites coevally (at the same time) but not co-magmatically (from separate magma chambers).

Model of Anorthosite and AMCG Suite Petrogenesis
 (Modified from Ashwal, 1993)

 

Why is this massif-type of anorthosite largely Proterozoic? At the early stage of Earth’s history, the emplacement of anorthosites was likely fueled by the Proterozoic crust, still sufficiently hot from the post-Archean age, yet sufficiently cool and rigid to support the intrusion of mafic magma and yet hot enough to allow the downward draining of dense magma residua.

 

METANORTHOSITE
The end result is our anorthosite, a phaneritic (coarse-grained), plutonic (magma chamber), intrusive (formed under the surface), mantle-derived (but not from mantle-melting), igneous rock that is enriched with plagioclase feldspar (usually labradorite, andesine or sometimes bytownite related to Bowen's Series) and depleted mafic derivatives (such as ilmenite, olivine, magnetite or pyroxene). The formation of anorthosite and associated granitoids are thought to have occurred late in the Shawinigan Orogeny and metamorphically imprinted during the Ottawan Orogeny (see Part II).

 

Plagioclase imparts a gray to bluish-black color to anorthosite due to Fe-Ti oxide inclusions. Anorthosite boulders and cobbles typically bed the brooks in the High Peaks region. Notice its distinctive blue-gray, granite-like, speckled-appearance and its characteristic eroded cobble-form.
 

 

After anorthosite crystallized, tectonic collisions toward the end of the multi-phasic Grenville event metamorphosed the rocks. This close-up of Marcy-type anorthositic gabbro shows metamorphic reaction-rims with coronas of garnet (C) surrounding mafic pyroxene megacrysts (B) within the plagioclase feldspar's interlocking-matrix (A). After initial metamorphism, an influx of fluids, garnet and hornblende growth, and textural modifications occurred. Garnets are indicative of the high temperature and pressure of granulite-facies metamorphism that occurred during the Ottawan Orogenic phase of the Grenville Orogeny. Garnets, whose formation is not completely understood, are useful in interpreting the genesis of many igneous and metamorphic rocks and in particular the temperature-time histories of the rocks in which they grew and in defining metamorphic facies of rocks.

By the way, garnet has been designated as the official New York State gemstone. It's used in coated abrasives, glass and metal grinding and polishing, and even to remove the red hulls of peanuts. The Barton mine in the Adirondacks sells up to 12,000 tons annually harvested from an amphibolite. Chances are if you're using red sandpaper, it's from the Barton mine.

Referring to the Bowen Reaction Series above, the plagioclase family of feldspars displays numerous mineral phases as it cools and migrates from calcium- to sodium-rich. One of the minerals, labradorite, is a principal constituent in anorthosite and is responsible for its blue-gray color, actually attributable to black ilmenite within its crystalline framework. Another interesting feature is labradorite’s blue-green iridescence (also called Schiller effect, labradorescence, opalescence and chatoyancy) especially under water. In fact, Opalescent River, that flows into the lake of Flowed Lands (see post Part IV coming next) contains a preponderance of iridescent anorthosite. The bluish optical phenomenon is related to light diffraction and reflection within submicroscopic layering or exsolution lamellae of the labradorite.

And lastly, the ‘zebra-stripes’ or ‘record-groove’ effect that plagioclase, particularly labradorite, exhibits is related to twinning during crystal growth. Symmetrical ingrowth of crystals enables plagioclase’s identification in the field. 

Photomicrograph of plagioclase crystal under cross-polarized light
showing distinct banding effect called twinning
(From Wikipedia.com)

 

ASCENDING WRIGHT PEAK
The spectacular view from Wright’s treeless summit captivated my daughter’s attention with Pitchoff, Cascade and Porter Mountains off to the northeast. Cloaked in low, ominous, swirling, gray clouds, the temps plummeted 30 degrees with wind gusting 25-35 mph. Instantly cooling down, out came the fleece and windbreakers on this otherwise hot August day. The threatening skies had us wondering about the conditions on adjacent Algonquin and if there’d be a view at all. We would be duly surprised!

On Wright, two sets of prominent vertical joints in the anorthosite intersect at right angles. Jointing is actually widespread throughout the massif and is a manifestation of forces of compression that resulted in the NE-SW faults. In some cases jointing has slight offsets indicative of faulting. Faults are responsible for the formation of the NE-SW valleys, as well as the subordinate NW-SE valleys. We seldom see faults on the surface but are aware of their presence by the landforms they create: belts of high mountains separated by narrow, swamp or lake-filled valleys. Deformational folds exist in the anorthosite as well, but because of its nearly mono-mineralic composition, they are difficult to identify.

Notice the prominent vertical joints in the anorthosite that decorate the entire summit. Two sets of them intersect at right angles. Vertical jointing is common throughout the Adirondack massif and is a manifestation of the forces of compression that resulted in the NE-SW faults. In some cases the jointing has slight offsets indicative of faulting. Folds exist in the anorthosite as well, but because of its nearly mono-mineralic composition, they are difficult to identify.

On January 16, 1962, a jet-powered strategic bomber, 30 miles off course in bad weather, clipped the top of Wright during a training mission killing four men on board. Parts of the plane still litter the crash site. Coincidentally, earlier this summer I climbed Mount Humphreys, the tallest peak in Arizona. It too was struck by a bomber on September 15, 1944 killing 8 airmen. A bronze plaque on Wright memorializes the airmen who lost their lives in service to their country. 


THE ARCTIC-ALPINE ZONE
The Adirondack timberline is about 4,000 feet, where the sub-alpine forest transitions into treeless alpine tundra. Timberline is not simply a matter of elevation. After all, timberline in the Rockies is nearly 12,000 feet. Even elevation and latitude together do not tell the entire story. In fact, timberline can be substantially lower on a cooler north-facing slope versus a sun-exposed southern slope. Timberline is determined by a combination of conditions that include low temperatures, frequent frosts, high winds, thick snow pack, inadequate precipitation and poor soils, all of which diminish seed production and viability.  

The Arctic-Alpine Plant Zone is the rarest habitat in New York State on 11 of the highest peaks of only 85 acres in the entire state! Its plants are identical to those found in tundra arctic regions at high latitudes, being equivocal to extreme elevation. Alpine low mean annual temperatures, frost-free periods (only two months a year), exposure to wind and ultraviolet radiation, lack of sufficient and nutritious soils, and wind speeds are comparable to that of the arctic. The Alpine Zone in the High Peaks Region is restricted to the meadows of 14 summits and are relics of the Ice Age, common throughout the region as the last glaciers made their retreat about 12,000 years ago. The plant communities were forced upslope by warming trends and the expansion of the forests in order to sustain their optimal growing conditions. The vegetation faces extinction similar to the threats facing arctic plants as the climate slowly warms.

The tundra vegetation is very fragile and slow-growing confined to isolated patches on thin remnants of soil that tenuously cling to the anorthosite. This Deer’s Hair Sedge is a densely tufted grass-like perennial that grows in large, windswept patches. The vegetated region seen here is on the leeward side of the summit from the wind. Can you tell the direction of the prevailing winds from the twisted balsam fir? Small stones were brought to the summit (over four tons!) by hikers and placed as barriers to protect the plants from inadvertent human trampling. For the last twenty years, many of the higher peaks have Summit Stewards that camp down below and spend their days educating the public about everything Adirondack especially the rare and fragile alpine ecosystems.


ALGONQUIN PEAK
Compared to the windy, cold and overcast summit of Wright, Algonquin, 536 feet higher, was semi-tropical in the upper 70’s with bright sun and a gentle breeze. It’s a lesson in Adirondack weather on the summits. Even in summer conditions can change in a flash. Being prepared is essential to survival.

Our view to the east takes in massive Mount Colden (4,714 feet), scarred with landslides that look like huge vertical stripes. A veneer of thin soil, often less than a meter thick, tentatively mantles the slopes of many of the high peaks. Held in place by tangles of trees, shrubs, grassy roots and the coarse texture of anorthosite, soils on steep slopes can easily be destabilized by heavy, saturating rains.

Such was the case with Mount Colden during Hurricane Floyd in 1999 that delivered 10% of the annual regional precipitation in one day. In fact, Floyd’s was the single largest precipitation event recorded in the previous 71 years. The slide completely blocked Avalanche Pass with rock debris and a tangled mass of vegetation. More recently, Hurricane Irene in 2011 created the highly noticeable clean white slide. In all, I counted over 15 separate slides on Colden’s western face! Snow avalanches are a major threat to skiers and winter hikers as well in the pass. Mount Marcy is in the background to the left. At the base of Colden and out of view is a magnificent faulted-valley that contains a string of glacially-derived spillover lakes. We’ll visit those lakes tomorrow.

My daughter took this panoramic video with her iPhone. It begins and ends facing to the west.

 

 

 

Grass-like Deer’s Hair Sedge, the threatened rich-blue, close-mouthed Bottle Gentian and the deciduous, round-leafed alpine bilberry are prominent members of the alpine tundra community on Algonquin’s summit.

 



 

 

 

The elevation gain on our steadily-upward trek from the lodge to Algonquin’s summit including the side excursion to Wright was almost 3,000 feet! The elevation of the Adirondack “Forty-Six” High Peaks averages between 4,000 and 5,344 feet. Compared to other mountain ranges the summits might seem diminutive, but with an average ascent of 2,500 to 4,500 feet, the climbs are significant not to mention the geology. Leaving Algonquin, we returned along the same trail of our ascent to the lodge at Heart Lake. The total excursion for the day was almost 12 miles. Tomorrow, we investigate the geology of the lakes in the fault-bounded valley (post Part IV).