The headwaters include two sites near
Pentir. From here the Cegin flows to the east, before turning towards the north and passing through the village of
Glasinfryn. Just north of here it is crossed by the
A55 Expressway. Immediately after this, the
Lôn Las Ogwen joins the line of the river, and the two run close together until the mouth of the river. The river is forded by a minor road near
Llandygai, and is crossed by the Cegin Viaduct, which carries the main Chester to Holyhead trainline. The river then passes between
Maesgeirchen housing estate to the west, and Llandygai Industrial Estate to the east. The river is also crossed by the
A5 closer to
Bangor. The most northerly stretch is crossed by a further two bridges, which carried the former L. & N.W.R. branchline. The river passes through the arches of the Penrhyn viaduct, before entering the Cegin pool, an area of tidal saltmarsh. The final crossings are: Pont Penrhyn, carrying a road way accessing the west gatehouse of Penrhyn Castle, and a lower bridge now carrying road traffic to Porth Penrhyn. North of these bridges stone walls leading the river to its entry into Bae Hirael. ==History==