Sort of interesting

The other day we were at Crowley's Ridge SP walking a trail. The trail bridged a small creek, and when we looked down on the creek we saw something sort of interesting. The creek at that point was perhaps four feet across, maybe four or five inches deep. The rocks under the surface were dotted with dark spots that at first I thought might be pond snails, but when we looked with our binoculars, we could see were mayfly naiads gathering in numbers, presumably preparing to emerge that evening.