Why are there fossils of marine animals found in the sedimentary rocks of Nebraska, (or any other place far from today’s oceans where marine fossils are found)?
These fossils are evidence that ancient seas once covered the continents, leaving in their wake, the sediments and fossils deposited in the sea. Throughout Earth’s history, sea-level has risen and fallen, in response to glacial periods and changes in the rate of plate tectonic movement.
During glacial periods, water evaporated from the oceans falls as snow and is tied up in ice sheets on land, causing sea-level to fall—a regression in geologic terms. Melting ice returns water to the oceans, causing sea-level rise, and water from the deep ocean basins surrounding the continents laps up over the continents in what geologists term a transgression.
Illustration: North America during the Ordovician Period (about 440 million years ago). State boundaries are lightly dotted in; the light blue represents sea water overlapping--transgressing--the continent from the deep ocean basin (dark blue) From http://jan.ucc.nau.edu/~rcb7/namO450.jpg
No comments:
Post a Comment