How do geologists know a landscape was once covered by a sea?
Quick Answer
They look for marine fossils, sedimentary rocks laid down in water, and chemical clues such as evaporites or marine isotopes. Together, these signs show that the area was once underwater.
The Full Story
Ancient seas leave behind several kinds of evidence. Marine fossils such as shells, corals, and microfossils can be preserved in sedimentary rocks far inland. Layered limestones, shales, and sandstones may contain features like ripple marks, mud cracks, or burrows that reveal shoreline or shallow-water settings. Chemical signatures in certain minerals can also indicate saltwater conditions or changing sea levels. By combining these clues with plate reconstructions, geologists can reconstruct vanished oceans and seaways, such as the Western Interior Seaway of North America.
Key Facts
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"A desert can hide the memory of an ancient sea—if you know what to look for."
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