There a stratum of rock, softer than those above, had been hollowed out by the action of time until it was like a deep groove running along the sides of the canyon.
(There a stratum of rock, softer than those above; had been hollowed out; by the action of time; T:until it was like a deep groove) (it; was; like a deep groove) (a deep groove; running; L:along the sides of the canyon)
There a stratum of rock, softer than those above it a deep groove
had been hollowed out was running
by the action of time T:until it was like a deep groove
like a deep groove
L:along the sides of the canyon