Runs evenly — and true —
But let a Splinter swerve —
'Twere easier for You —
To put a Current back —
When Floods have slit the Hills —
And scooped a Turnpike for Themselves —
And trodden out the Mills —
F563 (1863) J556
The verbs in the last stanza dramatize the impossibility of a brain ever going back to its comfortable groove or, if you will, rut, once derailed. The first stanza begins calmly, both in language and image. The brain is running "evenly – and true" in its accustomed channel. It encounters some setback or glimpses some new truth or vision, but nothing cataclysmic. It is a "Splinter" not a logjam, not a stick of dynamite. Dickinson's insight is that sometimes a splinter is all it takes to set a life, or at least a mental life, careening out of control.
The second stanza looses the calm current of the millrace. The floodwaters "slit" the hills, an image managing to be surgical, violent, and yonic at the same time. A turnpike has been "scooped" out of the earth as if a giant trowel has been wielded against the landscape. The last image is that of the mills and factories "trodden" into oblivion. It is wild nature against the careful constructs of man. Such a flood can never again be tamed to its small channel.
It depicts perhaps part of the poet herself and helps account for the energy beating against the bounds of language in her poetry.