I hope all is well. Good to hear from you.
The reason is...the planks they used to slide the stones were thick, say 4-6", and once they reached the end, the front potion of the stone drop off 4-6", causing a minor rotation. That drop exposed an opening. At that point, they levered over the stone, turning it once.
If you are only referring to the vertical ends, they were not critical to sliding the stones. Why care when they had a thick plaster, filled with small aggregate. It was just easier.
Edited 1 time(s). Last edit at 13-Sep-16 17:36 by Steve Clayton.