Rocks in river ice
 

I was drilling a hole yesterday on the South Saskatchewan and hit a stone at about a foot. I grabbed a chisel and chipped it out. The thing was larger than a looney and 1/2" thick. The ice was about 28"

Hitting pea size gravel on the river with your auger is common and it sure wrecks the blades. My question: How is it gravel freezes in the ice? I was fishing in 12' of water.

Any ideas? Thanks

Fell off of a sled maybe?

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Maybe...
Before river ice freezes to a solid thick state the ice is freezing up and it can still be moving down the current and picking up rocks when it moves over shallows.

YUP - exactly this.

Rocks, Sand and other debris, as the ice starts to freeze picks up all that stuff as it scrapes and tumbles downstream before coming to rest in place, often times frozen in the middle of the ice as it forms around it. Because ice floats, that dirty chunk can end up suspended over deep water quite commonly.

Drilling ice on many rivers is hard on the blades, because quite often, even sand, dirt or silt are (that you can't even see) in that ice and just kill the edge on your blades.