For over 250 years, the Valley has been exploited for earth materials, dating from the incipient iron industry of the early 1700s in the Hudson Highlands to the present need for aggregate and building materials (Hartnagel, 1927).
Of the metallic minerals, iron was the first to be mined from early scrapings of bog iron in wetlands to exploiting concentrations of magnetite, found mainly in veins cutting through the Proterozoic metamorphic rocks of the Hudson Highlands (Hurlibut, 1965). Iron was important to the Colonial economy as early as the 1750s. During the Revolutionary War, iron mines like the Sterling mine in Orange County supplied the forges of the American Army with the raw material for cannon and ball, as well as the links for a chain to span the Hudson at West Point and block British naval advances (Isachsen, 1980). The emery is used for abrasives, and pyrite, an important source of sulphur, came from mines in the lenses and veins in the mafic Cortlandt complex rocks near Peekskill.
Magnetite mines opened in the Adirondacks in the early 1800s (Schneider, 1997). The northern mines became major sources of iron and titanium derived from magnetite and illmenite deposits. While magnetite supplied the steel for heavy industry, illmenite, limonite and other iron compounds were used mainly for paint pigment. Limonite, found as an iron oxide crust, was mined in Dutchess and Columbia counties early in the 1700s.
Other minerals associated with metamorphic rocks are graphite, garnet, and zircon. Graphite is mined in the Adirondacks near Ticonderoga and is used in making 'lead' pencils. Real lead is found in disseminated masses in sedimentary rocks in the Shawangunk Mountains southeast of the Catskills.
Garnethas beenminedsince the nineteenth century from the mountains bordering the Hudson gorge near North Creek in the Adirondack Town of Johnsburg (The Editorial Committee, 1994). It occurs as large, attractive, dark reddish-purple crystals accented by a halo of white feldspar in a matrix of black hornblende schist. While enticing in their size and color, the crystals are generally fractured, and only occasionally are gem-caliber specimens encountered. Garnet has been quarried for use in industrial abrasives. Zircon of gem quality is found in Orange County mines.
A great variety of whole rock products have been exploited in the Hudson watershed, taken from the earth where found or wherever convenient. Granite and gneiss have been quarried in the Highlands and Adirondacks, and anorthosite in the northern mountains. Quarried blocks were used as riprap on steep slopes, and glacial boulders support earthen walls and line roadways. Stonewalls were built of cobbles hauled from cornfields, while crushed rock is used for road fill and in gabions. Large-sized crystals of feldspar and mica have been taken from coarse-grained pegmatite dikes that cut across the metamorphic rocks of the Highlands: the feldspar is used in insulators and the mica formed the 'isinglass' windows in furnace doors. Crushed basaltic rock, mainly from quarries in the Palisades and the
Cortlandt Complex rocks, is the 'trap rock' in most railroad beds, selected for its crushing strength and durability. The product continues to be in demand.
Of the metamorphic rocks, the Inwood Marble and the Fordham Gneiss were quarried in Westchester County for facing stone to adorn high-rise buildings in Manhattan. White marble also comes from the marble belt east of Albany. Lower grades of marble wind up in sacks of ground and slaked lime for lawns and agriculture soil enrichment. The slate belt of eastern New York's Taconic range parallels the marble trend in Washington County. European slate miners crafted an enduring industry in slate products for roofing, walkways, and floor tiles.
Sandstone slabs and blocks are derived from beds of the Cambrian Period, Potsdam Sandstone, that border the Adirondacks. Taking advantage of the natural rectangular jointing of the bedrock, sandstone could be readily worked into facing and foundation stone. Red sandstone from the Newark Basin also shows up in older structures in the southern part of the valley, and crushed red sandstone gravel decorates creative gardens and driveways. Devonian-age flagstone, a uniformly fine-grained gray sandstone (a variety of which is the Catskill 'bluestone') was quarried and split into thin slabs for the sidewalks of northeastern cities.
While many mineral mines have closed, the sand and gravel and limestone-marble quarries prevail. The limestone formations, the Ordovician Trenton limestone, Silurian-Devonian Helderberg Formation, and the Devonian Onondaga limestone of the escarpments of the mid-Hudson region (Isachsen et al., 1970) became the walls of many colonial homes, such as the Huguenot cottages near New Paltz. The chemical nature of the Silurian-Devonian rock provided a basis for the Portland cement industry that flourished in the valley. Coal was not available as a local commodity. Colonial iron had been concentrated with charcoal because the thin coal seams in the Catskill delta, evidence of small Devonian swamps, were not enough to sustain smelting. Similarly, peat was not a major energy source.
Special use sediment of different grades supplied other building industries. Fine sands were used for molds, pure quartz for glass, and clay for bricks and ceramics. The brick industry throve in scores of factories turning clay, from the Cretaceous-age
Raritan clays of Staten Island to Glacial Lake Hudson and Albany clays all the way to Glens Falls, into stone-hard building blocks.
All of these rocks and minerals, are, or have been, essential to the economy, but mining of earth materials has created a number of environmental hazards ranging from the variety of excavations - gaping and hidden holes in the ground, hollowed-out mountains, and forgotten subsurface rooms - to waste products, such as mine tailings, slag dumps, acid waters, acid rain, and air pollution, as well as sediment clogged streams. It was not until the 1970s that State and Federal environmental agencies began to require mine restoration and clean up and closure plans. But by then, many mines were grandfathered or abandoned, went out of business, or were converted into landfills. Today, the New York State Department of Environmental Conservation issues mining permits and monitors development, changing use, and closure plans.
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