Concentrations of small circular ditches are among the most difficult to explain of the numerous remains shown by soil-marks and crop-marks in the Silt Fens. The writer first noticed a group south-east of Fifties Farm, March (Cambs.) when on a practice flight in early 1944, and then took it to be a group of barrow circles like those known in the Upper Thames country (1) and elsewhere (2). Later experience showed that there were so many circles—up to 30 or 40—crowded together in each group and the groups themselves so numerous, that this explanation no longer seemed adequate. Twenty-two groups of circles and two isolated examples were noted, but there must be very many more, as it was not possible to make a systematic search, nor even to keep particulars of all the sites observed. Most of the circles were shown by soil-marks.