Rock Elm Ulmus thomasii
Family: Ulmaceae (Elm)
Other names: Cork Elm
Rock Elm is best characterized by corky growths on its bark, though this feature is highly variable and may not always be obvious. The twigs are more slender, and slightly hairy, but not overly distinctive. It exhibits the least degree of the characteristic vase-like growth of Elms, but its twigs still grow in a horizontal plane off the main branch.
It's been brought to my attention that these pictures may not be of a Rock Elm (but rather White Elm), or at least not of a typical appearance for Rock Elm. For some other pictures see here and here.
End bud of Cork Elm. The lopsided appearance is characteristic of all Elms
Another view of a twig. (that is a spider web wrapped around it)
Young bark with the characteristic corkiness
Another view of the corky young bark
Bark just beginning to acquire corky ridges