Fossilized crayfish burrows on steroids...

from the Triassic Chinle Formation. This outcrop, easily located in a small canyon at milepost 13 on Utah 128, shows a dense profusion of burrow casts (click to enlarge). Note the DNAG photo scale card in the center of the image, at the mid-point of an individual burrow that's at least several meters long/deep. This morphology suggests a stable, but slowly aggrading substrate, at the time of its construction.

Here's a related post that describes my research interest in these trace fossils.