Page "Cretaceous–Paleogene extinction event" Paragraph 51

There is clear evidence that sea levels fell in the final stage of the Cretaceous by more than at any other time in the Mesozoic era.

In some Maastrichtian stage rock layers from various parts of the world, the later layers are terrestrial ; earlier layers represent shorelines and the earliest layers represent seabeds.

These layers do not show the tilting and distortion associated with mountain building, therefore, the likeliest explanation is a " regression ", that is, a drop in sea level.

There is no direct evidence for the cause of the regression, but the explanation currently accepted as most likely is that the mid-ocean ridges became less active and therefore sank under their own weight .< ref >