The collision and "Great Crack" terranes: the most basic geologic subdivision of Connecticut.

Iapetos Ocean for flux. The supercontinent held together for only a relatively short time, and about 200 million years ago Pangaea began breaking apart. Many great cracks opened in the weld zone, vying for the right to form a new ocean. Eventually, one crack won out over the others and widened into Atlas's Atlantic Ocean, separating once again Europe, Africa, and North America. Remnants of the weld remain stuck to the edges of the renewed continents. Connecticut is one such remnant; what we now call Connecticut reaches from the edge of the early North American continent, across some sorry scrinched remains of the Iapetos Ocean floor, and over to a chunk of Avalonia. Depending on how far offshore our particular Avalonian Japan was, Connecticut may once have been 500 to 3000 miles across. Today it spans about 100 miles.

That, in one paragraph, is the geologic history of Connecticut.

For two centuries geologists worked on unraveling Connecticut's geologic history from its twisted and knotted bedrock. But until recently, geologists did not have a sword sharp enough to cut apart this Gordian knot. This new tool for understanding earth dynamics - plate tectonics - has revealed the story just told of continents on the move and disappearing oceans. In Connecticut, plate tectonic theory's keenest edge has been the identification of terranes.

At the most basic level, Connecticut can be divided into just two terranes: a Collision terrane and a Great Crack terrane. This division corresponds precisely with the major landscape regions of Connecticut. The Collision terrane, rock metamorphosed by the heat and pressure of the weld, correlates with the Eastern Uplands and Western Uplands. The Great Crack, one of many such cracks formed during Pangaea's break-up, corresponds precisely to the Central Valley. This match of landscape and terrane stems from the influence of plate tectonic history on land's great sculptor - erosion. The Uplands Collision terrane suffered metamorphism, hardening the heart of the rock to erosion. But the Great Crack came too late for the Continental Collision Circus (the true greatest show on Earth), and the soft sediments that filled the crack remain unmetamorphosed. Thus, the harder metamorphosed rock of the collision stands up high as uplands, and the softer unmetamorphosed crack rocks are relegated to the status of lowlands. The logic lines go like

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