PM sent, the opening looks natural in shape ... looks L shaped in a way, maybe it started as a cave and got worked? Another couple of questions, do you have a ridge-top arch to the East or West of the ridge your mines are on? Is the ridge the mines are on run North to South or visa versa? Just indicators/landmarks from Swift Journals...these help verify.
My only reservation in the picture is the area doesn't look clifty as described in the journals. But, that could have been another way to deceive.