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The Hadar fossil record is made up of, from oldest to youngest, the Basal Member, Sidi Hakoma Member, Denen Dora Member, and Kada Hadar Member.
Are these regions geographically distinct, or, are they layers on top of another on the same geographical region?
Here is a map of the region where they found the Lucy skeleton.
The researchers speak of the Hadar formation at Dikika, so they are giving it an expanse of 10-15 kilometers at least.
it's quite flat as well. 
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