I'm guessing they were some sort of seeds from the deciduous trees/bushes in the area. Think of cottonwoods -- that's why they are named that. I think the willows along the trail may have cottony seeds that would release in September, too.
I'm getting one of these for sun protection, and it would help with dust, cottony seeds, etc, too:
https://www.amazon.com/Neck-Gaiter-Face-...T04RX8&th=1
