One famous instance is the so-called boat-stealing episode in the opening book (lines 357-400; all quotations in this post are from the 1850, 14 book version):
One summer evening (led by her) I found
A little Boat tied to a Willow-tree
Within a rocky cave, its usual home.
Straight I unloosed her chain, and stepping in
Pushed from the shore. It was an act of stealth
And troubled pleasure, nor without the voice
Of mountain-echoes did my boat move on,
Leaving behind her still, on either side,
Small circles glittering idly in the moon,
Until they melted all into one track
Of sparkling light. But now, like one who rows,
(Proud of his skill) to reach a chosen point
With an unswerving line, I fixed my view
Upon the summit of a craggy ridge,
The horizon's utmost boundary; far above
Was nothing but the stars and the grey sky.
She was an elfin Pinnace; lustily
I dipped my oars into the silent lake,
And, as I rose upon the stroke, my boat
Went heaving through the Water like a swan;
When, from behind that craggy Steep till then
The horizon's bound, a huge peak, black and huge,
As if with voluntary power instinct,
Upreared its head.—I struck and struck again,
And growing still in stature the grim Shape
Towered up between me and the stars, and still,
For so it seemed, with purpose of its own
And measured motion like a living Thing,
Strode after me. With trembling oars I turned,
And through the silent water stole my way
Back to the Covert of the Willow-tree;
There in her mooring-place I left my Bark,—
And through the meadows homeward went, in grave
And serious mood; but after I had seen
That spectacle, for many days, my brain
Worked with a dim and undetermined sense
Of unknown modes of being; o'er my thoughts
There hung a darkness, call it solitude
Or blank desertion. No familiar Shapes
Remained, no pleasant images of trees,
Of sea or Sky, no colours of green fields;
But huge and mighty Forms, that do not live
Like living men, moved slowly through the mind
By day, and were a trouble to my dreams.
The central narrative here is simple enough. The young Wordsworth, led by Nature, finds a boat hidden in a lakeside covert. He steals the boat, and—vigorous and proud of his youthful skill—heads out onto the lake on a joy-ride. It's a beautiful scene, with the oars leaving a trail of rippling circles shining in the moonlight and the boy himself admiring the surrounding ridges and the stars and the sky beyond.
But then this peaceful image takes a scary turn. To understand what happens here, it is helpful to keep in mind the geometry of this lakeside landscape:
When rowing a boat, the oarsman actually faces backward so that, in order to steer a straight line, he needs to pick some spot on the horizon and keep that spot directly behind the boat. Accordingly, the boy Wordsworth fixes his view "Upon the summit of a craggy ridge, / The horizon's utmost boundary." At the beginning of the voyage, the boat is still close to the willow covert at position (A) on the diagram, and as the boy looks up he sees nothing but the "craggy ridge" and the stars beyond. But then as the headstrong boy warms to his task and starts "lustily" dipping his oars, he advances to position (B) and beyond. Now the sight lines have changed, and behind the first "craggy ridge" a "huge, peak, black and huge, / As if with voluntary power instinct, / Upreared its head." Imagine how this would look from the point of view of the boy: as he moves from (A) to (B), the "huge peak" will appear to rise up from behind the first "craggy Steep." The sight frightens the boy, but the harder he rows, the more the "huge peak" seems to come after him:
I struck and struck again,
And growing still in stature the grim Shape
Towered up between me and the stars, and still,
For so it seemed, with purpose of its own
And measured motion like a living Thing,
Strode after me.
Frightened and trembling, the boy—now humbled—turns around and returns the boat to its mooring and heads for home, "in grave / And serious mood."
Wordsworth, even as a boy, recognizes that this sense of a "huge peak, black and huge" rising up from the horizon and striding after him is really only an optical illusion—the peak "seemed" to have a "purpose of its own"; the mountain rises up "As if with voluntary power instinct." But this after-the-fact rationalization does not matter—one doesn't simply explain away the boy's experience, his sense of having been pursued by a "huge and mighty Form." Such is the point of the passage's closing lines. This has been a formative experience, a Wordsworthian "spot of time" that has shaped and continues to shape the poet's adult consciousness.
Taken as a whole, the boat-stealing passage offers an instructive glimpse into Wordsworth's childhood—the beauty of the Cumbrian lakes, the thrilling (if audacious) vigor of youth, the chastening power of nature, and so on. It's a useful episode to bear in mind while considering other celebrated poems. To offer just one example, in "Tintern Abbey," Wordsworth describes his boyish relationship to the natural world as one of raw enthusiasm,
when like a roe
I bounded o'er the mountains, by the sides
Of the deep rivers, and the lonely streams,
Wherever nature led. (67-70)
This energetic interaction with nature is then likened to a flight "from something that he dreads" (71), but eventually—after the recollections in the interceding years—it awakens in Wordsworth an awareness of a "presence that disturbs me with the joy / Of elevated thoughts" (94-95). The movement here seems analogous to the headstrong oarsman in the boat-stealing episode who subsequently is "chastened and subdued" (to adapt the phrase from "Tintern Abbey") and becomes dimly aware of "huge and mighty Forms, that do not live / Like living men."
