Cat Canyon has a limestone "slot" and is only about 10 feet wide in places. It's been on my "to-do" list for a long time. There used to be a picture online that I can't seem to find anymore, looked pretty dope. I hear rumors of other limestone "slots" in the area. Here is a link to some hiking beta:http://www.utah.com/hike/cat_canyon.htm
A quote from the link:
"The canyon meanders, and before two miles are hiked, narrows at times to no more than 10 feet or so."