Mt. Marcy (Tahawus)
       7 ½’ X 15’ quad: Mt. Marcy, NY
       Essex County
       Adirondack State Park
       Adirondack Mountains

Igneous Intrusive
5,344 ft (1,629 m)

Mount Marcy, at west (left) end of "The Range", from Dix. (1987)

Bedrock: Marcy Anorthosite

Late Proterozoic (1160 to 1145 Ma)

Foliated, white-weathering, blue-gray coarse anorthosite, with large inclusions or "rafts" of finer-grained gabbroic anorthosite and at 4420 feet, gabbro (Grenvillian country rock?). Many of the large plagioclase crystals are concentrically zoned white to blue (due to dusty inclusions) and some show the characteristic iridescence of labradorite, thus the name for Opalescent Brook west of Marcy. The anorthosite consists of more than 80% labradorite feldspar, minor pyroxene, and garnet surrounding ilmenite, whereas the gabbro, which is a brown-weathering, dark gray rock, contains only 50% feldspar and the remainder pyroxene and opaques.

All these rocks reached granulite metamorphic conditions at around 1080 to 1030 Ma during the Ottawan Orogeny (collision of Laurentia with Amazonia?), but being dry and competent, they resisted strong deformation. (Technically they are metamorphic, but they retain much of their original igneous character.)

Despite the great age of the rocks, the Adirondack dome itself is relatively young, having started rising about five million years ago. Earthquakes in the region, for example at Cornwall in 1944, Blue Mountain Lake in 1983, and Plattsburgh in 2002, suggest that the uplift may not have ceased.

Surficial Geology: Erratics have been found in the Adirondacks as high as 4600 feet, but no striae have been reported on the high summits, suggesting that they may have been nunataks during Pleistocene glaciation. The summit of Marcy is mostly bare rock, with pockets of alpine vegetation in weathered grus-like soil between outcrops.

Selected References:

• Hamilton, Michael A., James McLelland and Bruce Selleck, 2004, SHRIMP U-Pb zircon geochronology of the anorthosite-mangerite-charnockite-granite suite, Adirondack mountains, New York: Age of emplacement and metamorphism in Tollo, R.P. and others, Proterozoic tectonic evolution of the Grenville Orogen in North America: Geological Society of America Memoir 197, p.337-355.
• Jaffe, Howard W. and Elizabeth B. Jaffe, 1986, Geology of the Adirondacks High Peaks Region : Appalachian Mountain Club, Lake George, N.Y.

Other suggested sources of information:

• Glossary: http://www2.nature.nps.gov/geology/usgsnps/misc/glossaryAtoC.html
• Time scale: http://www.ucmp.berkeley.edu/help/timeform.html
• Adirondack Park geology: http://www.apa.state.ny.us/About_Park/geology.htm