What does “the Catskills have hardly sunk to them” mean?

Question

What does Henry David Thoreau mean here when he says, "the Catskills and Highlands have hardly sunk to them":

We get a dim notion of the flight of birds, especially of such as fly high in the air, by having ascended a mountain. We can now see what landmarks mountains are to their migrations; how the Catskills and Highlands have hardly sunk to them, when Wachusett and Monadnock open a passage to the northeast; how they are guided, too, in their course by the rivers and valleys; and who knows but by the stars, as well as the mountain ranges, and not by the petty landmarks which we use. The bird whose eye takes in the Green Mountains on the one side, and the ocean on the other, need not be at a loss to find its way.

http://www.thoreau-online.org/a-walk-to-wachusett-page8.html

Answer 1

It sounds like Thoreau’s saying that the Catskills (in NY State) and Highlands (in CT?) have dipped below the horizon as they fly towards the northeast (Wachusett is in MA and Monadnock in NH).

In other words, the mountains of one state have hardly disappeared below the horizon before a new landmark appears. (At least that’s the perspective if you’re a goose flying high in the air!)

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Note: I doubt very much Thoreau is talking about the erosion of mountains over time, for two reasons:

First, although he was quite a gifted naturalist and would have been familiar with much of scientific theory at the time, A Walk to Wachusett was published in 1843, and modern geology got its start later. (Wegener’s theory of Continental Drift is circa 1915.)

Secondly, Thoreau was very prone to allusions and metaphors and “purple prose”; the erosion of mountains over the millenia might have occurred to a man but would not have occurred to a goose. The idea of “mountains sinking” literally seems very unlikely for Thoreau to state while he is in the middle of imagery.

Answer 2

I think it's referring to the fact that the Catskill and Highlands mountain ranges have sunk over the eons, but they're still pretty dang high to these little birds, so they have "hardly sunk" as far as the bird (migrations) are concerned.

Answer 3

Thoreau is poetically waxing how high the birds can fly while demonstrating how the birds use the mountains as navigational landmarks. The Catskills and Highlands are mountain ranges in the eastern US, and it is commonly accepted that the old mountains were once much taller and have shrunk over time. Thoreau is suggesting that from a bird's commanding perspective, the erosion is not as noticeable or not relative, and that the mountains still play an important part in the birds' migration.