To sum up the solar alignments from the circle.
We have the May sunrise marked by the top of Burrien Hill, from 600 to 700 feet high, Az. 59° 30′.
We have the November sunset marked by a standing stone on the other side of the Loch of Stenness, Az. 53° 30′.
June rising, Line from Barnstone over Maeshowe tumulus.
December rising, tumulus (Az. 41°) on Ward Hill.
December setting, tumulus Onston 36° 30′.
It is not a little remarkable that the summer solstice rising and the winter solstice rising and setting seem to have been provided for at the Stenness circle by alignment on the centres of tumuli, two of them, across the Loch, one the Onston tumulus to the S.W. (Az. 36° 30′), the other tumulus being on Ward Hill to the S.E., Az. 41° (rough measurement).
If the Maeshowe tumulus was a structure erected at the time I have suggested to use the Barnstone for the summer solstice rising; then these two other tumuli, to deal with the winter solstice at Stenness circle, may have been built at the same time. All these provided for a new cult.
There are also tumuli near the line (which cannot be exactly determined because the heights of the hills are unknown) of the summer solstice setting; none was required for the sunrise at this date, as the line passes over the highest point of Hindera fiold, a natural tumulus more than 500 feet high, and on that account a triangulation station.
Another argument in favour of the tumuli being additions to the original design is that the place of the November setting from the Stenness circle is marked, not by a tumulus, but by a standing stone. As this stone, near Deepdale, and the tumulus at Onston are only about 1200 yards apart, the suggestion may be made that under certain unknown conditions and possibly in later times tumuli in some cases replaced stones as collimation marks.