Iceberg scour marks
In the last issue of WAT ON EARTH I described ice-wedge polygons, - relict features left over from a short-lived regime of permafrost in southern Ontario in the period immediately after the ice retreat. As I explained, these features are virtually invisible at ground level but they can be clearly seen (under the right circumstances) from the air. The recent widespread media coverage of the floods affecting the southern part of Manitoba brought back some memories of traversing this region in the past, and it reminded me that other strange features can frequently be observed from the air.