Ebbie

I want to visit that lovely place you described with all the river rocks. Did google image search and looks like they could be glacial deposit. The pressure and friction during glacio-fluvial transportation causes egg shape stones found in some river beds.

The next time you visit a river with such stones, if the water is fast flowing look for large stones that have bowls worn into them. It is caused by smaller stones and water roiling around and around in a trapped place, until divot then larger hole is created. If you are really lucky you may see bowls with the stones still churning away in them.

Woot!