Preface, and Introduction
To the Geologic History of Virginia
To the Geologic History of Virginia
PREFACE
"No rock is accidental."
What we are talking about here are rocks and fossils. These are the only records of the geological past. Without them there would be no record, no history to recover. We construct geologic history from the study of rocks and fossils. How do geologists and paleontologists do it?No rock is accidental. No idea in geology is more important than this. Every rock forms under a specific and unique set of conditions. Find the rock and you know what conditions existed at that spot at the time the rock formed. Change the conditions and a different rock forms. If the temperature is raised enough the rock will either bake (a metamorphic rock) or melt (an igneous rock). But expose them to conditions comfortable for us at the Earth's surface and they are just as unstable and begin to weather and form new minerals that are stable under these conditions (forming sediments). The adjustment may be slow by human standards but in time a new rock stable under the new conditions results.
Much of the training of a geologist is coming to understand how the hundreds of different kinds of rocks form. It may seem baffling when a geologist picks up a rock, that just looks like an old stone, and tells a tale about where it came from and how it formed. But to the trained eye and mind every rock is like a book waiting for its history to be read. And the story it tells is of the conditions that existed at the time it formed.
The work of a paleontologist is similar. Every fossil was once a living organism which was born, fed, grew up, interacted with other organisms and the environment, reproduced, and died. A paleontologist studies to understand how these petrified remains once lived.
What then must you know to read this history? We do not expect you to have much knowledge of geology and paleontology. Rather, as the history progresses the knowledge you need to understand the events will be explained. It is our hope this history will interest you enough you will want to learn more about geology and paleontology, perhaps even take some courses.
More than anything we want to capture your imagination, and sit in wonder at the teeming events that make up the rocks of Virginia, and the Mid-Atlantic Region.
INTRODUCTION
The spot where you are has not always been as it is now, may never have been as it is now. It strains the imagination to think that at one time this spot may have been deep in the earth and red hot with molten rock, and at another time a broad expanse of tidal flat baking in a tropical sun, and at another time a deep sea, and at another time a high mountain. The geologist is driven by curiosity to wonder, "What was it like right here 10, 100, 1000 million years ago...and how do I know that?"The state of Virginia and the surrounding Mid-Atlantic states have a long and fascinating geologic history. The oldest rocks, in the Blue Ridge, are as much as 1.8 billion years old. At 1.1 billion years the Blue Ridge was molten rock deep inside a mountain range, and connected directly to Africa. At 600 million years ago Virginia and Africa rifted apart and an ocean basin (the Protoatlantic) opened along the east coast. At this time the land on which Richmond, Washington, D.C., Virginia Beach, and Norfolk now lie had not yet appeared.
To create the now geologically quiet real estate of the eastern half of Virginia required, in the past 600 million years, a first rifting event (opening of the Protoatlantic), a geologically quiet continental margin, a first orogeny (mountain building; Taconic), a second orogeny (Acadian), a third orogeny (Alleghenian) which brought Africa back to North America and closed the Protoatlantic ocean, a second rifting event (opening of the Atlantic ocean), and creation of the present continental margin.
And in that span of time the life populating the state has ranged from a time when only bacterial cells existed, to shallow seas filled with invertebrate life, to flood plains covered with tropical jungles of strange plants, to dinosaurs tromping across the state, to mammoths, mastodons and giant ground sloths, and eventually humans.
The record of this history lies in the rocks under our feet, and in the mountains that form our scenery. Unraveling this history is one of the most fascinating adventures we can pursue.
Continue to: Origin of the History, Request for Contributions, Technical Depth of theHistory, Theoretical Models Behind the History
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