reasons for dividing the Uplands into three terranes stems from dif- ferences in the age of the bedrock. Proto-North America and Avalonia are both dominantly Precambrian (i.e. older than 570 million years). Geologists know from world-wide studies that the closing of the Iapetos Ocean ran from about 500 to 250 million years ago, so Proto-North America and Avalonia both predate the closing. But the age of the rock between them - the Iapetos Terrane - is all younger and correlates with the time of the closing: 500 to 250 million years old. Geologists theorize that Proto-North America and Avalonia must have been separate land masses before the collision began. As they approached each other, the Iapetos Ocean filled with sediments eroded off the colliding land masses. When the collision finally occurred, the Iapetos sediments were caught and metamorphosed by the collision's grip, forming an intervening younger terrane between the older Proto-North American and Avalo- nian terranes.

Chemical analysis of the Uplands bedrock helps confirm the terrane divisions. Consistently, the metamorphic rocks of the Iapetos Terrane match the composition of shales, limey shales, dirty sandstones, volcanic ash, and other rocks typical of modern ocean environments. (Because of metamorphism, however, the deformed Iapetos rocks bear little outward resemblance to ocean sediments - only in chemistry do they match.) In contrast, the bedrock of Proto-North America and Avalonia is typical of the old rock found at the core of all continents.

Several other features of the Uplands terranes further confirm the collision story and, at the same time, demonstrate that the relationship between terranes, rocks, and landscapes runs deep. One of these features

is the marble of the Marble Valley. Marble results from the meta- morphism of limestone, and limestone forms when beds of shells, coral, and lime-rich muds become cemented together. Today, only in oceans are extensive beds of lime-rich material being deposited, usually in tropical or subtropical oceans. Florida and the Bahamas Bank are good examples and rank as the most likely analogues for the marble of the Marble Valley - a big shelf in warm water built by the slow pile-up of shells, coral, and lime-rich mud. Geologists call these lime-rich shelves "carbo- nate banks." Before the collision, a similar bank probably flanked the east

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