In an unconformity between two layers of rock, how is the lower layer usually described? a. much older than the upper layer. b. much younger than the upper layer. c. the same age as the upper layer. d. a little younger than the upper layer

A. Much older than the upper layer.

An unconformity is a gap in age between two rock units. The upper unit is usually younger than the lower layer due to deposition of sediments in the lower layer due to erosion. Age gap between two layers of sediments or rocks depends upon how long the erosion took place and period in which  deposition of sediments stopped or ceased.