The upper angled layer of grey
dolomite is approximately 500 million years old. The reddish
rhyolite on which the dolomite rests is approximately 1.5
billion years old. There is a billion years of geologic history missing in this picture.
Missouri Ozarks.
An unconformity is a gap in the sequence of rocks in a geologic column. It is a buried erosion surface separating two rock strata of different ages. It shows that sediment deposition was broken for a time, usually because the land was above sea level at that time.
Usually, the older layer was exposed to erosion for some time before deposition of the younger. However, the term is used to describe any break in the sedimentary record.