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.
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 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.
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.|
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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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!
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.
We borrow it from our children.”
Native American proverb