Supposedly there is a legend regarding this hill:

" Saint Peter was visiting Wales in order to watch over the Faithful. Taking offence at the sudden appearance of the Devil, he picked up a large number of boulders and placed them in his apron so as to carry them more easily. He then gave chase to the Devil, both chaser and chased (having the stature of giants) leaping from mountain-top to mountain-top. As the Devil alighted on Mynydd Machen he paused to catch his breath, whereupon Saint Peter began hurling the rocks at him, leaving a considerable amount of debris around his adversary in the process. The area of rocks is known to this day as "The Devil's Apron Strings". "

The area which is referred to as "The Devil's Apron Strings"  is on a saddle just before the summit of Machen Mountain in a field of puddingstone boulders with the view looking up the Rhymney Valley towards Caerphilly.  I didn't spot them when I went up last but will look out for them the next time I do go.