Perhaps the lack of stone balls found near me in South Lanarkshire (just two - one at Biggar and one at Carnwath) goes some way to explain the lack of big megaliths in the area.
They just didn't have enough ball bearings.
The ancient South Lanarkshirians did make a metal one though. The only one if its kind ever found anywhere. A unique and beautiful item from just over the back from me at Walston.

http://nms.scran.ac.uk/database/media.php?file=postcard/image/0098/00987615.jpg&PHPSESSID