Thursday, August 16, 2012


Drive north on I-87 from Albany toward Canada and you pass through a long stretch of the eastern Adirondacks.  Toward the north end of the park, between exits 32 and 33 (south of Keeseville), watch for the long, glacially-polished cliff of Poke-O-Moonshine Mountain on the left (west) side.  It's nearly a thousand feet high and a favorite of rock climbers.  You can get closer views by taking exit 33 and driving south on Route 9.  The mountain's name is derived from the Algonquin words for broken and smooth.

The north-south cliff is an up-thrown block of granitic gneiss along a fault line [a huge crack where there is enough pressure to shift the rocks out of alignment], while Route 9 traverses a down-dropped block of gneiss interfingered with anorthosite (very similar to Moon rocks) east of the cliff.  The term "block" may be misleading as these are large-scale structures with horizontal dimensions measured in miles. North-northeast of the mountain and continuing over halfway to Keeseville, Route 9 crosses the west edge of anorthosite mixed with metasedimentary rock (the green area on the geology map).

From the Geologic Map of New York - Adirondack Sheet
yellow is an extension of the Marcy massif, lavendar-pink is gneiss,
and green is a hybrid anorthosite mixed with metasedimentaries

Weathering of the cliff [a fault scarp, or escarpment] has obscured thick bands of darker rock, loosely classified as "metagabbro" (without knowing its original nature, either gabbro or basalt/diabase).  Although somewhat horizontal, the major rock layers have a gradual rise to the north (right).  In several locations, the bands are offset by vertical faults.  Also difficult to discern are thinner bands of dark diabase or basalt that intruded as molten sills between the bedded layers.

Like the core of the Adirondacks (actually a southeastward extension of the ancient Canadian Shield), the original sedimentary rocks of Poke-O-Moonshine were metamorphosed [transformed by intense heat and pressure just short of complete melting] during the Precambrian Grenville collision of pre-North America and pre-Europe/Africa around 1.1 billion years ago.  The name is taken from Grenville, Quebec.  The scarp's granitic gneiss is a strongly-banded and coarse-grained rock similar in composition to granite with feldspar, quartz and the dark minerals pyroxene, hornblende and biotite mica. The darker bands of "metagabbro" [a.k.a. amphibolite, a dark rock composed of hornblende and plagioclase with smaller amounts of biotite mica and/or dark-green pyroxene] were also metmorphosed by the end of the Grenville Orogeny [mountain-building episode] that completed the assembly of the supercontinent Rodinia at that time.  The thinner diabase sills were injected as hot, molten magma at a later date, likely after one of at least four collisions of North America and Europe when the continents were again separating and stretching the tectonic plates (which causes vertical cracks that allow magma to rise and then spread horizontally between bedded rock layers). Just south of the Adirondacks, in the Mohawk Valley, the diabase dikes [that cross-cut the older Precambrian rock layers] and the sills [that flowed between the layers] are also found in Cambrian and Ordivician rocks, but not the younger Devonian or Silurian deposits, so it's implied that they rose after the Taconic Orogeny when the continents began separating at the end of the Ordovician Period.

The anorthosite is related to the Marcy massif, a huge pool of crystalline rock under the High Peaks that entered (intruded) the surrounding Grenville rocks as a molten plume through cracks, spread laterally and then slowly cooled.  It can be likened to a geological breast implant.  Uplift and erosion has stripped more-recent rocks off the dome to expose the anorthosite.  A deeper hot spot is theorized to be fueling the continued rise in the Adirondack dome at the rate of three millimeters per year.  You may have heard the expression "new mountains from old rock."

The mineral composition of anorthosite rock is 90% plagioclase feldspar which forms under extreme heat and pressure at great depth (likely 15 miles, give or take a few) and can be termed Plutonic rock.  Plagioclase is a group of silicate minerals built around one silicon atom bonded with four oxygen atoms or three silicons plus eight oxygens.  These molecules combine with various amounts of sodium and/or calcium or potassium plus aluminum to form one of the minerals in the feldspar series.

Glacially-polished cliff face catches the morning sun, August 10, 2007.

The Grenville Province is a long band of rock stretching from the southern Appalachians north-northeastward through southern Ontario Province to the Laurentian Highlands of Quebec.  It's now moslty basement rock with narrow exposures in places like the Blue Ridge (NC/VA), South Mtn. (PA), the Hudson Highlands (NY), the Berkshires (MA), and the Green Mtns. (VT).  Also exposed are the Adirondack dome and the Frontenac Arc (across the Thousand Islands in the upper St. Lawrence River) which connects the Adirondacks to Ontario. The Grenville rocks were originally sediments, volcanics and plutonic (deep) intrusions that were metamorphosed (i.e., transformed or recycled by heat and pressure at great depth) during the Grenville Orogeny between 1.2 and 1.0 billion years ago in a process that resets the rock's "geological clock."  The Grenville collision completed the assembly of the Rodinia supercontinent.

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2 books by Bradford VanDiver, PhD: Rocks and Routes of the North Country (1976) and Roadside Geology of New York (1985/2003)

Geology of New York, NYS Museum (1966/1976)

Geology of the Adirondack High Peaks Region, Howard & Elizabeth Jaffe, 1986, is not directly relevant to Poke-O-Moonshine, but has a good overview with diagrams in the "Introduction and Geologic History."  Their dates for Grenville events are slightly older.

[also several general rock and mineral guides]

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