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After a rock layer is eroded, which additional event must occur to create an unconformity, or a gap in the geologic record?
a. an earthquake or motion along a fault.
b. new deposits of sediment must build up on the eroded area.
c. folding of sedimentary rock layers.
d. a volcanic eruption.
Is this A? My lesson gives a definition for unconformity as "a rock or sediment surface separating two layers of different ages."