Wikipedia about wasps and learned a lot including that it is probably a wasps' nest as "social" wasps chew trees and other stuff to make a paper pulp to make their nests. Apparently there are lone wasps too, but they don't make big nests and live together. (Sounds like some people we all know!).
I expect this nest will get covered up as the tree leafs up but I'll keep an eye out in about a month and see if I can still see it.
The address for this wonder of nature is about 1406-1408 Q Street NW, just east of The Bike Rack shop nearby.