The coast around Benagil, Portugal, is made up of Miocene limestone sitting on much older rocks created in the Mesozoic.

Over time, the limestone built up, with the layers that make up the Benagil Sea Cave forming about 20 million years ago.

https://video.nostr.build/1fe6f6d8c5adb0270a3cc40e26e6381be5c4c592b7b980c6d4a5c531951c0e48.mp4