When I spent time with Margaret Curtis on Lewis earlier in the year, she started by going over the different shaped 'circles' in the UK by means of sand bins and and string. Just checked my notes, apparently there are 600 real circles, 100 elliptical circles and 150 'flattened' stone circles. Being an adherent of Alexander Thom, she naturally also talked about the megalithic yard and astronomic alignments in relation to Callanish (which is a stone row, a circle and an avenue).