Definition of "shallowing"

The main 200-feet basin is about 8 miles long; it includes three areas of over 300 feet, which, however, are only separated by very slight shallowings.

1907, Scottish Geographical Magazine, volume 23, page 349

And this shallowing, reinforced by the creation of mid-ocean ridges as the supercontinents split up, forced the world’s oceans to overflow, changing the outlines of the continents and all but sinking some of the European islands.

2018, Tim Flannery, Europe: A Natural History, page 28