Ice on pond

There were two layers of ice on the pond this morning.  The lower (black-looking) layer is still intact in the photo, but the upper layer has retreated to the white-coloured thin ice round the edges.  Pondcam (see Extra) showed that the white layer covered most of the pond at around 8 am.  I think that the double layer must be connected with the water level in the pond dropping since yesterday, because the water table would have dropped once the heavy rain stopped.