I believe this ... no ... really ... I do.
And your explanation would be...? Or maybe you're suggesting these simply don't exist?
(BTW, I would not assume that this was hot ash (although it might have been). The ash is amazingly light, and easily compacted, so would take an impression well. Mt. Saint Helens Ash is very, very light and I assume this was likely the same kind of thing. I bet it would only take a light rain to turn it into something like concrete.)