Joining the various barriers of the B-series geomorphs is a ruined entrance to some complex, perhaps a temple, a library, a crypt, or the living quarters of a leader. Part of the wall and ceiling has fallen, leaving boulders strewn about. The fancy Grecian columns that once flanked the entrance are now broken. The boulders and fallen column are a severe inconvenience, although they don't truly block passage completely; however, the forgotten portcullis will definitely halt explorers until they either open it or otherwise remove the obstacle.
There's also a passage running beneath the entryway that leads to a disconnected room.