Strictly speaking this is one lake, originally called Llyn Mymbyr, with a maximum depth of 30 feet.

About 3/4 mile long in total, a delta has built up midway along the north shore, and this has to all purposes cut the lake in two - hence the plural name.

The river feeding the lakes is the Nantygwryd (or Nant-y-gwryd), which has its source at Llyn Cwm-y-ffynnon, also marked on maps as the Nant Gwryd, which is a tributary of the Afon Llugwy.

The lake was used for fly-fishing and sub-aqua courses in the 1960s and is still used today as a canoe training resource by Plas y Brenin and other local education authorities.

The lakes lie in a depression formed by a ring of sandstone called the Capel Curig Volcanic Formation which can be vaguely seen in the panorama below which was taken from the north side.