The caption above links to the MSSS photo. This feature has 2 clear sides and one very faint.

Next I have drawn lines over the 2 clear sides. Note this is the recitified photo for an overhead view.

I have drawn a line where I think the third side is. If this is correct then the mesa is a right angled isoceles triangle, with sides of 45, 45, and 90 degrees.

Another possibility is to draw the line as below, which would give a triangle of 40,50,90 degrees. With the vague outline it is hard to be precise.