Final answer:
The author most likely intended to create an atmosphere of foreboding with the described imagery of the woodgrass.
Step-by-step explanation:
The author most likely intended to create an atmosphere of foreboding with the description 'behind them the line of the woodgrass like a dark open mouth.' This vivid description using imagery and personification creates a sense of unease and anticipation, suggesting that something ominous or threatening is about to happen.