'Geirfa' suggests this is a place or river-name. In CLlH p.222 I Wms proposes emending to 'amaruy' GPC '(a) ring (b) circle, bond, girdle, (c) purfle, fringe, border? He suggests the meaning 'border country' here, parallel to 68a 'ar eluydeu'. The meaning 'circle', however, is ironic since the information given in 67 + 68 makes those rivers behave like the circular rivers of paradise!

438 Canu Heledd: Amhaual ar auaerwy. /yd aa tren yn trydonwy / ac yd aa twrch ym marchnwy.

5.  Similarly on the border /does Tren go into Roden and Twrch into Marchnwy.