You know what it looks like… but what is it called?
TAKE THE QUIZ“bottomset”
We have one entry that includes the term bottomset.
bottomset beds
plural noun: layers of sedimentary material lying along the bottom of a body of water near the point of entry of a stream, the material having been carried to the area by the entering stream and being subsequently covered by foreset beds and topset beds in the formation of a delta
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Merriam-Webster unabridged