An indication of the source of the river comes from Ordnance Survey maps which show, a short distance east of Redstone Cross (a junction of the
A40 and the B4313 roads), a place called Blaenmarlais in the
community of
Narberth at . In its first kilometre, the Marlais, flowing in a roughly easterly direction, then southeasterly, is joined by an unnamed left bank tributary before running under the
A478 at Pant-y-gorphwys, then another left bank tributary and two right bank tributaries before turning east, then northeast meandering along Lampeter Vale where it is joined by numerous further tributaries, to pass under the A40 again at Pont Fadog. Approximately 500m further, it joins the Tâf at . For all of its length after Pant-y-gorphwys, the river marks the boundary between the communities of
Lampeter Velfrey and
Llanddewi Velfrey in
Pembrokeshire. Ordnance Survey names one of the tributaries (right bank) as Afon Cwm at . ==Geology==