Ice formation is typically a complex DYNAMIC phenomenon. Pond ice is not a static creation, but rather goes through many freezes and partial thaws during the period where net ice is increasing.
In the simplest version, ice forms at the edges of the pond, and the center freezes last. This mostly due to the to the densest water at 39F sinking down the pond slope and going to the deep spots with the remaining water at the edges cooling down to 32F and freezing.
However, at a smaller scale you can frequently observe open water right at the very edge of the pond while there is a band of ice growing towards the middle. This is for two reasons.
First, the soil of your pond banks below the frost line is MUCH warmer than the water in your pond. The soil at depth will be approximately the average annual temperature of your geographic location. There will then be a decreasing temperature gradient from that point to the soil at the air interface. This small bit of geothermal heat does help warm the water at the very edge of your pond.
Secondly, even on a very cold day, but with bright sunshine, the dark materials at the edge of the pond can absorb a lot of incoming solar radiation. It can be 15F for the air temperature, yet the soil at noon may have heated up to 40F on a calm day.
Finally, right at the edges where the water is only a 1/4" deep, the maximum ice thickness is also only a 1/4". If the water level in your pond goes down a 1/4" or more during a month-long freeze, then the edge ice can melt on a warm day and run back into the pond in patches. Other patches may also melt, but the deeper ice blocks the meltwater from leaving the area, so that area subsequently re-freezes.
If you observe your ice holes and consider them in the light of a complex, dynamic environment do all of those mechanisms make possible the actual structures you are observing?
(If not, then please put out same game cameras. I am sure we would all love to see the pics of aliens you post to the forum.
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