I was trying to figure out that final [punch]line...???

It would make comedy sense if the chalk was for marking a trail,
so he knows where he's already been.
But the rest of the final verse is at odds with that reading,
if that had been the writer's intention..

Though there's plenty enough old songs where the words and meaning
have become garbled, over time and careless copying,
into sheer perplexing nonsense...