It matches the blueprint design of the 1740s which was carried on to the 1850s and beyond - it figures in an old Agricultural book I have. The concept is simple in that side friction holds each stone, rather than gravity alone; it means it can be built very high, is relatively slim, especially if built with proper concave surfaces..... but that takes skill. Every stone is properly prepared and properly laid. But, I digress .... ::15:
