This is the nest of a social wasp of some kind, most likely in the genus Vespula, which normally nest underground, in abandoned rodent burrows and other natural cavities. The scalloped pattern is owed to individual applications of paper (the wasps chew wood scrapings into a pulp and apply it in an arc-shaped pattern). So, each little line represents one load of pulp. Yellowjackets that nest above ground generally have a less dense-looking nest, but it could still be a product of the aerial yellowjacket, Dolichovespula arenaria.

I had one under my outdoor umbrella, same pattern, but was dark and light brown swirl. Maybe just fresher mud. There were numerous Cuckoo Wasps on it. If I am correct, the Cuckoo Wasps are parasitic on other wasps so that may have been the case. I didn't keep track of it and it eventually fell.