Viking photo.

Note the straight cut through the mounds formation.

     Joining together the mounds on this plain gives 2 isosceles triangles, one with the significant 60 degrees (BDE), and the other with 35 degrees (DAB), often found in other photos. DEH and BEH are 30,60, 90 degrees formed when the isosceles triangle is bisected.
The right angled triangle of 15,75, and 90 degrees (ABC) is also common in other areas. The right angle at F is divided into 40 and 50 degrees, 2 other common angles in other areas.