What is being moved in these containers?

Horizontal wells near us (Loveland, CO) have recently been drilled. One has been frac’ed and presumably started flowback. A number of others are just starting being frac’ed. There has been a 24 hours stream of these trucks (about every 20 minutes) going to and from the wells. What are they carrying?

(Hopefully there is a photo above showing the trucks).

Looks like Frac sand

Frac sand from the frac’ing or from flowback?

I would guess either heading to location for frac job or back to the yard from the frac job.