Mass Migration on a Tiny Scale

I was wandering around with my coffee this morning, when I noticed what looked like a big smear on the driveway. It looked like a lot of spilled coffee grounds, so it warranted a closer look. 

Black ants en masse, crossing concrete. The overall effect is of coarsely ground coffee dropped in a thick line about 2 inches wide that follows a seam to a crack in the concrete. A lighter smear further on is also comprised of ants, more widely spaced.
Ants migrating across the driveway. Photographed on June 24, 2018.

It turned out to be the teeny, tiniest black ants that live in this yard moving across the driveway. We have other, somewhat larger red and black ants that move around regularly, but I haven’t seen these little ants migrate before. Maybe their former home got too wet—we’ve had a good amount of rain in the last couple of days. Which direction were they going in? I’m not really sure—when I looked closely, they seem to be milling around. I could not stay long enough to figure out which way they were going.

By the time I got back, they were gone, so I am sure they are safely underground setting up housekeeping in their new spot.