English: Gwys yr Ychen Bannog. Gwys yr Ychen Bannog is a 2km long linear ditch thought to be a pre-historic boundary demarcation (Coflein[1]: http://www.coflein.gov.uk/en/site/303652/ ). It radiates northwestward from Garn Gron's west top 1214522 across bog (where it isn't visible on the ground) towards and across y Bryn, the minor hill on the left in the picture. It can be seen as a linear, slightly lighter brown line crossing y Bryn just to the left of the centre of the picture. With a little imagination, it can be traced back towards the camera in the dark brown part of the bog ahead. Not much has changed since pre-historic times: the modern day political boundary (between Ystrad Fflur and Tregaron Communities) runs parallel 200m to the south along the fence that stands out on the left, crossing the top of y Bryn.

A series of images showing different sections of Gwys yr Ychen Bannog, from east to west: https://www.geograph.org.uk/photo/1214526 https://www.geograph.org.uk/photo/1214578 https://www.geograph.org.uk/photo/1214592 https://www.geograph.org.uk/photo/1214617 https://www.geograph.org.uk/photo/1214623 https://www.geograph.org.uk/photo/1214638 . The name Gwys yr Ychen Bannog means 'furrow of the horned oxen'. There is a pair of Ychen Bannog in Welsh mythology which are thought to have been extraordinarily strong beasts. Tradition has it that they dragged a large rock used when building the church at Llanddewi Brefi some 8km southwest, engraving the furrow in the landscape in the process[2]. Clearly, the Ychen Bannog didn't have OS maps - Llanddewi Brefi lies on a bearing at right angles with the structure!

[2] R Gwyndaf, MD Jones; Welsh Folk Tales, National Museum of Wales (1989); Google Books link: http://tinyurl.com/cazzft