This amazing case is surprisingly little documented, and the information we have is scant. The event was first mentioned in the journal UFO Investigator, May 1975, when Robert Cornett and Kevin Randle wrote a report.
In essence, the circles were found during an investigation into a calf found dead and mutilated in a field in Meeker County, in central Minnesota. (We are unable to identify the precise location.) The calf was laying in a patch of bare grass, with a covering of snow in the rest of the field, and was discovered on December 1, 1974.
Subsequently a fly-over was organised and the photograph to the right was obtained. The farmer had himself seen flattened circles in the immediate surrounding area, and this photograph revealed no fewer than forty seven in a single field. (The snow had by then largely melted, but residual patches can be seen near the field edges.)
The circles are approximately equal in size, although those to the left as we look may be slightly larger. (Our diagram shows them identical.) The circles are not scattered randomly, but grouped in chains and pairs, lining up in linear rows. They also seem to be 'aware' of the field boundaries, running generally parallel to most of the edges.
Below are comments from Marcia Smith in The UFO Enigma, a paper prepared for the Congressional Research Service. Once again the main interest is not the circles, but the mutilation, which is attributed to an unnamed Satanic cult.
She does however state that the area the calf was initially found in was not circular, but irregular. She refers to it as "compacted snow", and states (unconvincingly) that it was created by other cattle treading the snow down. She had presumably not seen the aerial photography, since she leaves her comments there.