Ledges & Doglegs In Drilling Risks & Stuck Mitigation
ledges at formation changes

Ledge in Drilling: The wellbore passes through the rock of varying types and ledges develop at the interfaces between layers of differing hardness.

Closely spaced ledges decrease the effective diameter of the hole and produce a "shouldered" hole surface. Any type of casing string requires a smooth, more in-gauge hole diameter for free passage and not to have a pipe sticking problem.
https://www.drillingmanual.com/ledges-doglegs-stuck-pipe/