Just as mountains continually erode today, the Grenville Mountains eroded for the next few hundred million years after their formation. Over millions of years, weathering and erosion can reduce a mighty mountain range to low rolling hills. Rocks are constantly worn down and broken apart into finer and finer grains by wind, rivers, wave action, freezing and thawing, and chemical breakdown. Weathering and erosion are constants throughout the history of time. The sediments that eventually solidified into the rocks of the present Coastal Plain had not yet been deposited. All of New England east of the Berkshires and Green Mountains was not yet part of North America, and would not even be assembled for several million more years. At this point in geologic time, very little existed of the Northeast as we now know it. Orogenic compression folded (and even completely overturned) the metamorphosed sedimentary rocks and igneous intrusions of these mountains, forming the basement rock of today's Appalachians. The collisions created a tall (perhaps Himalaya-scale) mountain range, the Grenville Mountains, which stretched from Canada to Mexico. Intense heat and pressure associated with these continent-continent collisions produced molten rock that was injected into the crust, metamorphosing sediments that had eroded from the craton and thrusting them up onto the side of the continent. The ancient Grenville rocks tell a story of repeated collision-related mountain building on North America's east coast. The Grenville Orogeny was one of several Precambrian continental collisions that led to the assembly of the supercontinent Rodinia between about 1.4 billion and 900 million years ago. Compression from colliding plates, tension from plates pulling apart, the addition of land to North America, weathering, and erosion have all combined to slowly sculpt the form of the continent. The shape and position of North America has changed dramatically over the last billion years, and geologic processes continue these changes today. These rocks, dated at 1.1 billion years of age, were metamorphosed during a major period of mountain building called the Grenville Orogeny. The oldest rocks exposed in the Northeast are Precambrian gneisses, exemplified by the Baltimore Gneiss in Patapsco State Park, Maryland and the Fordham Gneiss in the Bronx of New York City. They formed and grew during pulses of magmatic activity, as bodies of molten rock deep in the Earth’s crust contributed to form new crust. Shields, or cratons, are the stable cores of all continents and are often covered by layers of younger sediments. These are part of the Canadian Shield, the ancient core of the North American continental landmass, which has experienced very little tectonic activity ( faulting and folding) for millions of years. The oldest rocks known on Earth are 4.3-billion-year-old rocks found along the eastern shore of Hudson Bay in northern Quebec. Rocks dating to around four billion years old are found on almost every continent, but they are not found at the Earth’s surface anywhere in the Northeast. The Earth is estimated to be approximately 4.6 billion years old-an age obtained by dating meteorites. Precambrian Mountain Building Part I: The Grenville Mountains Each of these regions has a different geological history and thus varies in terms of rocks, fossils, topography, mineral resources, soils, and other geological features. states are divided into five different physiographic provinces or regions: the Central Lowland (1), the Inland Basin (2), the Appalachians and Piedmont (3), the Coastal Plain (4), and New England (5). In the following discussion, the northeastern U.S. Repeated episodes of mountain building, sea level changes, and the erosion and deposition of sediment shaped the Northeast as we know it today. Millions of years ago, the Northeast was the site of multiple continent-continent collisions and the rifting of supercontinents. Today this part of North America is tectonically inactive, but this was not always the case. The Northeast is at the edge of a continent (North America), but in the middle of a plate (the North American plate), which extends from the mid-Atlantic ridge to the West Coast. The geologic history of the northeastern United States is a story of active mountain building and the quieter processes of weathering, erosion, and deposition of sediments.
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