The summit cairn has unfortunately been badly disturbed by the erecting of an OS trig station.... hence Coflein is unforthcoming about possible origin. However my own opinion, that of GAT, together with a likeness to the not-so-distant monument crowning Foel-y-Geifr.... not to mention the OS themselves... strongly suggest prehistoric. Arenig Fawr can be seen top right, needless to say with its own monument.
Image credit: Robert Gladstone
Nice light. That's a very sturdy looking trig!