Fossil

I think, although I have no idea what of.

Walking along Seton sands I walked over couple of hundred metres (or more) of what must have been sedimentary rock tilted up at about 45 degrees and then eroded by the sea. You can see layers in it, and if they're 2mm thick and each is a year (which is just a guess but it seems plausible to me that you'd get some characteristic annual layering) you're walking across well over 100,000 years of history.

The whole thing probably goes on for several km, so that's millions of years.

No sign of alien incursion was found.