What is transgression in geology?

In geology, transgression is the process of a coastline migrating inland due to a rising sea level. Regression, in contrast, occurs when a coastline moves oceanward as a sea level falls.

Some geographic areas endure cycles between these two processes called transgressive-regressive sequences. The rocks of western Pennsylvania are one example. Sandy beaches often leave observable records of transgression by covering marsh sediments that were once behind it as it moves inland. The original sediments are then covered by even deeper water sediments, which geologists can trace and record. It is generally believed that transgression will increase in accordance with rising sea levels worldwide.