At first I thought that this was the work of hikers but the lack of tracks in the snow ruled out human intervention. A quick search revealed that this formation is known as a snow roller. A snow roller is a natural meteorological phenomenon in which large snowballs are formed as chunks of snow roll down hillsides or are blown by strong wind. They are rare because they depend on three conditions all of which existed when we were there : the snow was covered by a layer of ice to which snow will not stick; the layer of ice was covered by wet, loose snow with a temperature near the melting point of ice; and wind or gravity must push the snow into a roll. Other names for this formation are snow pipes, snow onions and snow logs.
Sometimes a snow roller becomes hollow since the inner layers, which are the first layers to form, are weak and thin compared to the outer layers and can easily be blown away, leaving what looks like a donut. This is called, you guessed it, a snow donut. I have not encountered any snow donuts in Boulder but a recent sighting in Seattle was covered on National Public Radio. Mandy and I will be on the lookout for winter trail donuts from now on.