Sea level is not stable. Sea level has risen and fallen many times in the
geologic past, flooding and exposing much of the land of the continents as
it did so.
On a very broad, shallow, marine shelf several types of sediments may be
deposited. On the beach and near shore, waves will deposit sand, which is
usually derived from land. Farther from shore, in deeper, quieter water, land-derived
silt and clay will be deposited. If the shelf is broad enough and covered
with warm water, corals and algae may form carbonate sediments still farther
seaward, beyond the reach of land-derived sediment. These sediments can lithify
to form a seaward sequence of sandstone, shale, and limestone.
If sea level rises or the land sinks (subsides), large areas of land will
be flooded and these three environments of rock deposition will migrate across
the land. This is a transgression of the sea as it moves across the land,
and it can result in a bed of sandstone overlain by shale, which in turn is
overlain by limestone. Note that different parts of a single rock bed are
deposited at different times-the seaward edge of the sandstone bed, for example,
is older than the landward edge.
In a regression the sea moves off the land and the three rock types are arranged
in a new vertical sequence-limestone is overlain by shale, and shale by sandstone.
A drop in sea level alone will not preserve this regression sequence. The
land must usually subside rapidly to preserve these rocks, so that they are
not destroyed by continental erosion.
The angles shown in the figure are exaggerated-the rocks often appear perfectly
horizontal. Geologists use these two contrasting vertical sequences of rock
to identify ancient transgressions and regressions.
