Swedish icy mystery

My friend and colleague, Tom, was visiting family in Sweden recently and on a forest walk he came across this remarkable and so far unexplained icy feature and photographed it.

Surprising sharp edge around a patch of ice. 

At first glance it just looks like an icy puddle, but when you look more carefully it has some very peculiar and surprising features.

Closer view of the upstanding edge - Swedish icy mystery!

Tom described it as follows:

I have never seen or heard of anything even remotely like it before. I probably saw about a thousand other frozen puddles that week, and none of them had any unusual features at all.

Stalks of grass through the mysterious sharp ice feature.

It had snowed about 10-15cm some days before we got there and this had largely thawed by the time of this forest walk, after several days of the temperature being about -2°C overnight and 0 to +2°C during the day.

There are many deer in the forest, and a few dog walkers. Several other puddles showed evidence of the water level having gone down under the ice crust before refreezing. I now see that where some larger grass stalks had lain through the developing ice wall, presumably wiggling in the breeze, they have made round holes in the wall. The little ones are just frozen in position as I described the other day.

Another view - including a reed passing through a melted hole.

Perhaps the explanation for the large green reed having melted a hole is more related to it being a dark colour, and thus collecting more warmth from the sun, but how this sharp edge was formed is still a mystery.

So . . . are there any arctic explorers out there who have an explanation please?