Cairn I
This was built partly over the premegalithic mound in a northerly direction. The extension of both structures is parallel, in a SW-NE direction.
The plan of the first cairn is trapezoidal with rounded corners. On the ground, it measures 30m in length by 20m in width in the NE end, and only 12m in the SW end.
Only the eastern corner has been cleared to its base and displays three faces. The external faces, which are quite plain, seem to be only meant to reinforce the more carefully worked internal face.
The walls are subvertical at the base and gradually slope inwards. Originally, this cairn may have ended with a "hull" edge of about 3000m3 that was about 9m high at its highest point (present-day height: just over 6m).
Although the mass of this cairn has been widely explored, only stone rubble was found inside. It was probably a solid mass intended to support superstructures which have since disappeared.
The double face of cairn I in the SE corner : the slope increases towards the top, as if the monument had been built as a reversed hull.