Explain the difference between a sea-level transgression and regression.

Sea level regression is also known as marine regression and it describes a process where sea floor that was once covered by the sea is now exposed. Ice ages are known to cause regression or falls in the sea level as much of the ocean's water become locked up at the poles and on vast ice sheets on land. Sea level transgression is the opposite and occurs when the sea covers land that was exposed. The melting of ice in the last ice age flooded land that was once exposed.