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 will require at this rate thirty million years.
They were, I think, harder than metal, yet they had been here long enough for the elements to erode them into featureless shards.
When Erode saw this, he said: "I think it is hard to tug at a rope with a strong man."
The area up-stream from the culvert will not erode below the level of the top of the box at the inlet end.
It would take a lot of time to erode away that much massive stone.
As a result, there is a substantial defense imbalance that will erode fighting power.