Assume that one cannot climb on the rocks, e.g., the rocks are too slippery or are dangerously sharp, or it is taboo (thought to displease vindictive local gods), or the Russians have smeared them with Novichok. . . but it is possible and acceptable to pass a line over the rocks without touching them.
Two people take a 50 ft line (or a measuring tape) with a plumb bob hanging from the middle and hold it taut so the plumb bob is just over a spot on the rocks that the two people agree to center the circle on. Each person uses a plumb bob to mark the spot under their end of the rope and drives in a stake. The specified circle is now determined and it just remains to fill it out. It can be filled out with only the measuring tape.
There is a trig formula that gives the length of the sides of a regular polygon inscribed in a circle of a given radius*. For a circle of radius 25 ft this gives:
Number of sides, Length of sides (ft)
So to locate the two other vertices of the square use the tape to inscribe two arcs of length 35.36 ft from each of the original two stakes and place two stakes at the two intersection points of the arcs. There are now 4 stakes on the circle.
Next, from each of the four stakes inscribe two arcs of length 19.13 ft which generate 4 intersection points which are on the circle and put a stake at each of the 4. There are now 8 equally spaced stakes on the circle.
Next, from each of these 8 stakes inscribe two arcs of length 9.75 ft which intersect at 8 points on the circle and put a stake at each of the 8. There are now 16 equally spaced stakes on the circle.
Next, from each of these 16 stakes inscribe two arcs of length 4.90 ft which intersect at 16 points on the circle and put a stake in each of the 16. There are now 32 equally spaced stakes on the circle.
And so forth.
*s = sqrt(2) r sqrt(1 - cos(360/n))