ditch
From LSJ
εἰ γάρ κεν καὶ σμικρὸν ἐπὶ σμικρῷ καταθεῖο καὶ θαμὰ τοῦτ᾽ ἔρδοις, τάχα κεν μέγα καὶ τὸ γένοιτο → for if you add only a little to a little and do this often, soon that little will become great (Hesiod W&D, 361-362)
subs.
P. and V. τάφρος, ἡ, ὄρυγμα, τό, V. αὐλών, ὁ.