curious ice rings

A recent trip to New York in mid February, 2010, followed the heavy blizzards that saw much of the eastern seaboard experiencing snow falls heavier than anything seen for the previous 10 years. While spared the worst of these blizzards the city of New York saw significant snow falls. Streets had thick coverings of snow, transport systems were seriously disrupted and the lakes and ponds of Central Park had extensive areas of thick surface ice. Central Park reservoir had large areas of ice thick enough that even 2 days later were still capable of supporting large flocks of gulls, but probably not skaters. It was in the vicinity of the edges of these sheets of lake ice that the ice-rings were found to have developed. Picture 1 (this is the first of the pictures above - I have yet to figure out how to format these more effectively) shows the location where these ice-rings were observed. One of these ice-rings can be seen near the left border of the ice sheet close to the eastern edge of the reservoir. Other areas exhibiting clusters of ice-rings are shown in Pictures 2 and 3.Those shown in Picture 2 were at the time of observation typified by central areas in which there was either no ice or ice that was relatively thin compared with the surrounding ice sheet. The circular edge ramparts ranged from 100 to 400mm in diameter and were comprised of heaped small grained particles of crushed ice. These ice-rings contrast with the similar sized edge accumulations of ice-rings shown by those of Picture 3.The ice-rings of Picture 3 can be seen to have ramparts that are composed of discrete areas of broken and rotated sheets of ice. Like the ice-rings type 1 the diameters were in the range of 100 to 400 mm. There appeared to be some evidence that these two distinct forms of ice-ring had been formed by two very different thermal-mechanical processes.
I am wondering if anyone else has observed these ice rings in New York? Or perhaps you have seen them somewhere else? Would be interested to know.
I would also be interested in your ideas as to how they might have formed? I have my own theories but will spare you these at the moment since i am interested to hear your views.