Lines 13-17 Summary

Get out the microscope, because we’re going through this poem line-by-line.

And there's a story in a book about it:

That road we're on? Well, there's a story in a book about it.
What kind of story, we're not sure. Maybe we're dealing in the realm of fairy tales. Or maybe this is more of a journey-quest type situation. In any case, it's clear that this road isn't just a figment of the speaker's memory or imagination. It has existed for other people, too. Otherwise, why write about it, right?

Besides the wear of iron wagon wheels
The ledges show lines ruled southeast-northwest,

These lines give us a bit more detail about the road. It's got wagon wheel grooves, for one thing. Picture parallel grooves cut into the stone or dirt of the road by iron wagon wheels from days gone by. Once again, we've got some imagery that connotes the past—and rural life, too.
But there are other grooves, too—"lines ruled southeast-northwest." To find out what those grooves are from, we'll have to stay tuned.
In any case, these lines hammer home a theme we've become quite familiar with: the passage of time. Wherever this road is, and wherever it leads, one thing is clear: it's been there a long time.

The chisel work of an enormous Glacier
That braced his feet against the Arctic Pole.

Our patience pays off when we learn that this second set of lines and groves—the ones that run southeast-northwest—came from a glacier of all things.
We've got two timelines now—human time (represented by the wagon wheels), and geological time (represented by the grooves the glacier has left behind).
Lover of nature that he is, the speaker personifies the glacier, giving it feet (just like the town had knees), which he braced against the Arctic Pole, so he could get a bit of traction to do his earth-carving.
If you haven't noticed, dear Shmoopers, we've amped up the time scale a bit. Now, not only are we remembering the past of the speaker's life, we're also remembering the past of the earth's life. And what a long past it us.