Ordnance Survey maps show a place called Blaenmarlais a short distance east of Redstone Cross, where the A40 meets the B4313. The Welsh 'blaen' means head or source, and that is where this river starts: in low pasture in the community of Narberth, not far from the Landsker line that divides Welsh-speaking north Pembrokeshire from the English-speaking south. The first kilometre runs east, then southeast, accepting one unnamed left-bank tributary before ducking under the A478 at a spot called Pant-y-gorphwys - 'Hollow of Rest', which is the kind of name landscape gives itself when nothing much has ever happened there. Two more tributaries join. The river bends east, then northeast, and starts wandering through Lampeter Vale.

For most of its length after Pant-y-gorphwys the Marlais serves as the boundary between two communities - Lampeter Velfrey to the north and Llanddewi Velfrey to the south. Welsh community boundaries are old, often older than the historic county divisions that overlay them, and a river marking such a boundary is older still. Local farmers know the river by which side of it they stand on. A tributary called the Afon Cwm joins from the right bank, and by then the Marlais has stopped being any one thing - it is a knot of small streams pulling rainwater off the surrounding hills toward the Taf. Pont Fadog, where the Marlais passes under the A40 again, is a Royal Commission recorded historic place name. So is the bridge at Pant-y-gorphwys. These are working names for working crossings.

The geology of the catchment is shaped by limestones of the Llandeilo Group - Ordovician rocks roughly 460 million years old, named after the small Carmarthenshire town where they were first described. Limestone weathers fast in Welsh rain. The water that flows in the Marlais picks up calcium carbonate, becomes mildly alkaline, and supports the kind of stream life - mayflies, caddisflies, brown trout - that limestone water is famous for. The valley sides are gentle. The fields are good for dairy. None of this is dramatic countryside, and that is its own merit; Pembrokeshire has the Coast Path for drama, and the Vale of Lampeter for the slow agricultural pulse that has kept the rest of the county fed for centuries.

The British Geological Survey report on the catchment notes the gold grains. They are angular - their edges not yet rounded by long river transport - which strongly suggests that the gold has not travelled far from its source. Somewhere in the Marlais catchment, or in a tributary feeding it, there is a small body of mineralised rock that has been releasing flecks of gold into the water for as long as anyone has bothered to look. Wales has a long and largely vanished gold-mining history. The Roman mines at Dolaucothi in Carmarthenshire are the most famous example; the small Welsh gold industry of the 19th and 20th centuries produced the metal used for some royal wedding rings. Whether the Marlais grains are worth pursuing commercially is doubtful. Whether they are interesting? Yes.

On a 1907 map - and on current Ordnance Survey sheets - a building labelled White Mill (Flour) sits beside the river. Mills like this one once stood every few kilometres along Welsh rivers: small grist mills where local farmers brought their wheat, oats, or barley to be ground by water power. Most have vanished. A few survive as private homes, often with the millpond intact. The Marlais White Mill is what the river ultimately does: turn calm pasture water into mechanical advantage, then into bread, then into the slow accumulation of Pembrokeshire farming life. The river has no scenic destination, no waterfall, no famous beauty spot. It just goes east, finds the Taf, and disappears. Most rivers, when you look honestly, are doing exactly that.