Cluster of Bees

My wife discovered this cluster of bees on the side of the large ravine that runs across our property. The dirt wall was vertical and the cluster was about 14 inches long from top to bottom. The bees are stacked one on top of another to a thickness of about three inches.

The bees were very passive with only a few of them flying around, but the ones on the outside were moving slightly. I thought possibly they were trying to keep a queen warm or something, except that it wasn't a particularly chilly day. They were gone the next day and left no trace that they had been there.