The tone or mood of Frost's poem is ominous and threatening, creating a mood of unease or anxiety about the future. Nature is portrayed as planning, with deliberate, evil intent, to harm the world.

Details that help create that mood of foreboding are as follows:

The water is described as "shattered" and making a "din" or noise. It is then personified and given malevolent human characteristics. The waves are described as looking and thinking of "doing something" that "water never did to land before." The vagueness of this threat adds to the sense of anxiety, especially as we know that waves are powerful forces. The imagery of the clouds adds as well to the unease: they are described as if they were animals or barbaric humans: "low and hairy." The "gleam of eyes" they seem to exhibit is also ominous. These clouds seems to eagerly anticipate doing something evil.

We are then told that it is "lucky" the shore is backed up by a cliff and a continent, suggesting that the sea is planning a destructive assault. The speaker follows that image by saying it looks as if "a night of dark intent" is coming, and moreover, an entire "age" of "rage." The final threat adding to the tone of foreboding appears in the ending couplet of this sonnet:

In the poem "Once By The Pacific" by Robert Frost, Frost shows his darker side.  If we define tone as the feeling the author is trying to create while defining mood as the feeling perceived by the reader, many details in the poem create both.  The first line uses the word shattered to describe water which is a negative connotation while the personification Frost uses when the waves are thinking about doing things not usually done to land also creates a dark feel.  The phrase "night of dark intent" shows Frost's dark tone and even more so the idea that someone must "be prepared for rage".  All of these phrases and more create a dark tone and a dark mood for the reader when the poem ends with an even darker thought that God would speak his last, "Put out the Light." Be sure that you look carefully at which words Frost chooses to describe something; for example, if water was described as flowing instead of shattered, the feeling created would be far different.