DEFINITION OF Adirondack Mountains The Adirondack Mountains occupythe core of a domal structure that brings deep-seated LateProterozoic rocks to the surface and represents a southernextension of the Grenville province of Canada. The Late Proterozoicrocks are unconformably overlain by the Upper Cambrian/Lower Ordovician Potsdam Sandstone, dipping awayfrom the Adirondack dome. The late Cenozoic uplift is shownby the anomalous elevations of the Adirondack Highlandscompared with the surrounding regions and the relativelyyoung (Tertiary) drainage patterns. Uplift is still occurring onthe order of few millimeters per year.Five periods of intrusion and two main periods of deformationare recognized in the Adirondacks. The earliest intrusionsare the tonalitic and calc-alkaline intrusions that areapproximately 1,350–1,250 million years old. These intrusionswere followed by the Elzevirian deformation at approximately1,210–1,160 million years ago. The largest and most significantmagmatic event was the emplacement of the anorthosites,mangerites, charnockites, and granites, commonly referred toas the AMCG suite. This suite is thought to have been intrudedabout 1,155–1,125 million years ago. This magmatism wasfollowed by two more magmatic events; hornblende granitesand leucogranites at approximately 1,100–1,090 million yearsago (Hawkeye suite) and 1,070–1,045 million years ago (LyonMountain granite), respectively. The most intense metamorphicevent was the Ottawan orogeny, which occurred1,100–1,000 million years ago, with “peak” metamorphismoccurring at about 1,050 million years ago.The Adirondacks are subdivided into two provinces: theNorthwest Lowlands and the Highlands, separated by theCarthage-Colton mylonite zone. Each province contains distinctrock types and geologic features, both of which haveclear affinities related to the Canadian Grenville province.
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