Geology and Physiography
The Adirondack Mountains are a physiographic province of the larger Appalachian physiographic division.
The mountains consist primarily of metamorphic rocks, mainly gneiss, surrounding a central core of intrusive igneous rocks, most notably anorthosite, in the high peaks region. These crystalline rocks are a lobe of the Precambrian Grenville Basement rock complex and represent the southernmost extent of the Canadian Shield, a cratonic expression of igneous and metamorphic rock 880 million to 1 billion years in age that covers most of eastern and northern Canada and all of Greenland. Although the rocks are ancient, the uplift that formed the Adirondack dome has occurred within the last 5 million years — relatively recent in geologic time — and is ongoing. The dome itself is roughly circular, approximately 160 miles (260 km) in diameter and about one mile (1600 m) high. The uplift is almost completely surrounded by Palaeozoic strata which lap up on the sides of the underlying basement rocks.
The rate of uplift in the Adirondack dome is the subject of some debate, but in order to have the rocks which constitute the Adirondacks rise from the depth where they were formed to their present height, within the last 20 million years, an uplift rate of 1-3mm a year is required. This rate is greater than the rate of erosion in the region today and is considered a fairly high rate of movement. Earthquakes in the region have exceeded 5 on the Richter scale.
The mountains form the drainage divide between the Hudson watershed and the Great Lakes Basin/Saint Lawrence River watershed. On the south and south-west the waters flow either directly into the Hudson, which rises in the center of the group, or else reach it through the Mohawk River. On the north and east the waters reach the Saint Lawrence by way of Lakes George(32 miles long) and Champlain, and on the west they flow directly into that stream or reach it through Lake Ontario. The tiny Lake Tear-of-the-Clouds, nestled in the heart of the High Peaks area between Mt. Marcy and Skylight, is considered to be the source of the mighty Hudson. The most important streams within the area are the Hudson, Black, Oswegatchie, Grasse, Raquette, Saranac, Schroon and Ausable rivers.
The region was once covered by the Laurentian Glacier, whose erosion, while perhaps having little effect on the larger features of the country, has greatly modified it in detail, producing lakes and ponds, whose number is said to exceed 1300, and causing many falls and rapids in the streams. Among the larger lakes are Lake George, The Fulton Chain, the Upper and Lower Saranac, Big and Little Tupper, Schroon, Placid, Long, Raquette and Blue Mountain. The region known as the Adirondack Wilderness, or the Great North Woods, embraces between 5000 and 6000 square miles (13,000 km² and 16,000 km²) of mountain, lake, plateau and forest.
Mining was once a significant industry in the Adirondacks. The region is rich in magnetic iron ores, which were mined for many years. The Benson Mines was an open pit iron mine extracting magnetite and hematite ores from the Grenville gneiss in St. Lawrence County on the northwestern portion of the Adirondack uplift. Other mineral products are graphite, garnet used as an abrasive, pyrite, wollastonite, and zinc ore. The Balmat-Edwards district on the northwest flank of the massif also in St. Lawrence County was a major zinc ore deposit within Grenville age marbles worked during the mid twentieth century. There is also a great quantity of titanium, which was mined extensively. The Sanford Lake district was a significant titanium ore producer during the 20th century. It is in Essex County within the anorthosite bodies on the east flank of the range.
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