Here’s one the squirrel missed.

The poplar twig-gall fly lays eggs in tender new growth on aspen trees, often near a leaf bud. Larvae live and feed in this twig, causing a gall to form. The adult emerges in late winter or early spring.

In our woods there is an oak with this large burl, or burr. The tree had some stress long ago, whether injury, disease, or fungus, which caused this deformed growth.

Because of the value of the unique markings and patterns inside, for wood workers or wood carvers, some trees in the U.S. have been poached by thieves for their burls.

I don’t remember seeing anything like this before. Maybe I never noticed. But once I noticed these strange patterns, mysterious and fascinating, I saw quite a few examples.

We usually call these poplar trees, but they are also known as trembling aspen, or quaking aspen, for the way their leaves tremble in even the slightest breeze.

My best guess, from what seems like hours of online searching, is Epidermal Bark-Mining Fly. I can find next to no information on this.

Or could it be a fungus?

The weather has been too mild, thawing some of the snow, and compacting it. One result is treacherous ice on walking surfaces. A few weeks ago though, before the thaw, the snow was deposited in fascinating forms on every branch and surface.