This limit of the level of the sea beneath which they cannot erode is known as baselevel.
Secure in its grip, these are used as graving-tools to erode its bed.
There has not been time to erode them away since the Pleistocene glaciation.
It has been, and is being, created by sediments from the many torrents that erode the interior mountains.
To erode a stratum 5000 feet thick ...