Lake McDonald's Rainbow Rocks

The rocks on the shore of Lake McDonald look painted. They are red, and green, and purple, and cream, stacked and tumbled together along the waterline, and on a clear morning with the peaks of the Livingston Range reflected in the water behind them, they look less like a geological formation and more like something someone arranged. They were not arranged. They have been here for roughly one and a half billion years.

The rocks belong to a formation called the Precambrian Belt Supergroup, sediment deposited in a shallow sea that covered this region before there were fish, before there were complex animals of any kind, before the Rocky Mountains existed. The sea compacted the sediment into rock over hundreds of millions of years. Iron oxide in the sediment accounts for the reds and purples. Mineral content in different layers accounts for the greens and creams. The variation you are seeing, the color differences from rock to rock and layer to layer, records different chemical conditions in that ancient shallow sea at different moments in time.

The glaciers that carved Lake McDonald, the same glaciers whose descendants are now retreating from the park, polished these rocks as they moved through. The water of Lake McDonald, cold and clear enough that on a calm day you can see the bottom at twenty feet of depth, has been rounding and smoothing the surfaces for the ten thousand years since the last major glaciation ended.

When you pick one up, you are holding something that was being formed while the oxygen in the air we breathe was still a geological novelty.