whilst crossing i looked down and was able to discern strange movements within the oily smoky rings , the movement looked like a summer heat haze rising from a hot surface , looking through these areas it was difficult to focus upon the ground beyond , in fact this ground seemed to move and change , one second grass and foliage could clearly be seen and then a grey , barren , featureless place of what looked like course sand or gravel and then back to the grass again .