We also went to a sure source of interesting rocks: the local graveyard.
Mix of rotting vegetation and rocks
Almost all rotting vegetation
Then we took guesses as to which one was the oldest. Our conclusion was that the all-rock one was oldest because the vegetation had already rotted away and left only the pebbles.
On our way home again at the end of the month (sniff, honk) we looked at all the layers in the cliffs that line the highway.