Meaning of a sentence in Thoreau's Walden

Question

There is a passage where Thoreau writes about the pond and its bottom. At the end of the paragraph he seems to be using a metaphor or making a comparison between the objects related to the pond. I do not understand the grammar connection between the things mentioned (bar, plain shoal, valley, gorge deep water and channel). What is the meaning of the sentence?

The regularity of the bottom and its conformity to the shores and the range of the neighboring hills were so perfect that a distant promontory betrayed itself in the soundings quite across the pond, and its direction could be determined by observing the opposite shore. Cape becomes bar, and plain shoal, and valley and gorge deep water and channel.

Answer 1

The author is comparing geological features of the landscape (both seen and unseen) with geological features of the lake, and how they complement and align with each other. He is rather spare with his punctuation, so the sentence is hard to scan. Extra punctuation should help:

Cape becomes bar; and plain, shoal; and valley and gorge, deep water and channel.

He compares a cape (as in Cape Cod) to a sand bar, a plain to a shoal of gravel, a valley to a run of deep water, and a gorge to a navigable channel.

Answer 2

He's simply alluding to the continuous nature of the landscape as it goes below the water level. Like fjords, the slope of the hillside continues to the deepest part of the pond. He's using words that describe the same landforms above and below the water. I'd you're ever in the area, a walk around Walden makes this clear. I did that just last week...