This is the remains of a burial chamber near Hay-on-Wye, it is in a field near a big bend in the road, there is nowhere to park except a mud patch, and then a short climb up to the field where the 4 stones are. They form the shape of the chamber, and are around 5000 years old. The view of the river in the field below the stones is also worth a look.
Image credit: Jake Zennaman