What's going on?

Hello!

I was walking in the park with my friend, when I saw this. At first glance, it just looked like some dirt on the side walk, but when I stopped and looked more carefully, I realized they're ants. The interesting and weird thing was that they were not moving. Just few of them were going around slowly, but most of them were still. I haven't studied ant's behavior but I had never seen such a thing.
They were not dead, but they looked like a dead community, where everyone is still alive, biologically, but nothing is happening, people are sluggish and inactive, no interaction, no nothing...

Have you ever seen such a thing before?