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It's hard for me to tell the difference between lahar deposits and landslide deposits, given that they're both pretty much composed of the same materials. If it didn't come down the valley when the volcano erupted last, it came down when the landslide pushed it down later, maybe thousands of years later.
The trees in the debris layer might be 10-12,000 years old; old enough that there might be elephant bones in there if any had the bad luck to be in the valley when the event happened. I regularly scan the river bank for artifacts, bones or fossils exposed due to erosion, and I've found a few bones. One gave me pause - it looked a lot like a human fibula
I'll leave it to you to give your best guess.