They appear to be very dark, overly large worms - perhaps a handspan in length and half as thick as a finger. That and their matt, dull skin marks them as something other than a typical earthworm.
What do you mean, the little wriggly ones you throw at fish or the big rock ones that squish people?
Six months of hunting, six months of research and paying coin to scholars and sages, and now that we have the legendary wand of Geohadris the Stone King, it only summons stupid boulderkin? I am outraged beyond imagination!
Mapmaker's bane, a few of these can ruin a decade of cartography!
As the PCs travel the road, right after a bend they hear a sharp whistle and call: "Heeey, not so lazy, move your asses!" It is a large man that calls, and there are unwilling workers that listen. A small company, 10-15 men work on the road, push boulders aside, dig up roots from under the road, etc. The large man that shouted turns to you, smiles fast and mutters something under his breath, sounds like cursing some lazy worker. "Where does the road bring you from, travellers?" And does a little small-talk.
And what is really happening? A group of bandits is 'adapting' the road for shady purposes. The road will not be wider, but tighter, with enough cover around (and a few traps perhaps), and will become an ideal spot for ambushing travellers or entire caravans. The bandit leader wants them all to appear harmless. The 'lazy worker' he cursed was actually a guard that should give warning before any travellers come around (fallen asleep). Not surprisingly, the boss may decide for an ambush even now.