This chalky bluff is on the bank of the Tombigbee River in west Alabama at a place called Moscow Landing. Geologists come here from all over the world to look at the line between the brownish ground in the lower part of the bluff and the grayish white ground higher up. The black backpack in the picture pretty much marks the line, which is known as the K-T boundary. Below it are fossils of oceanic critters that lived in the age of the dinosaurs, more than 63 million years ago. Above the boundary are more recent fossils--oysters and snails and suchlike. The line itself in some parts of the world has a chemical irregularity associated with meteorites, but not at Moscow Landing, which is just a few hundred miles from the probable impact site of the meteorite that is believed to have extinguished the dinosaurs. Here, the K-T boundary shows evidence of scouring and faulting from ... a tsunami. The thinking is that when the meteorite smashed into the Yucatan, which was then mostly covered by a shallow sea, it caused earthquakes, which just might have caused tsunamis.
The tsunami evidence is controversial. But the boundary itself is obvious--you don't need any training in geology to see a difference between the dinosaur-age fossils in the brown chalk and the puny little oyster fossils in the white chalk.
Best thing about Moscow Landing: the chalk. The fossils are easy to dig out--no hammer and chisel needed.
What is chalk? Fossils of teeny tiny critters that live in anaerobic seas. The oceans have gone back and forth a few times between aerobic and anaerobic conditions--they were anaerobic in the dinosaur age, but they're aerobic now. Why the switch? I don't know, and I'm told that nobody does.
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