It is a very fine view. A century or so ago people compared it to the Rhine and even (a little ambitiously, I’m bound to say) the Alps. The artist George Innes came and made a famous painting called “Delaware Water Gap.” It shows the river rolling lazily between meadowy fields dotted with trees and farms, against a distant backdrop of sere hills, notched with aV where the river passes through. It looks like a piece of Yorkshire or Cumbria transplanted to the American continent. In the 1850s, a plush 250room hotel called Kittatinny House rose on the banks of the river and was such a success that others soon followed. For a generation after the Civil War, the Delaware Water Gap was the place to be in summer. Then, as is always the way with these things, the White Mountains came into fashion, then Niagara Falls, then the Catskills, then the Disneys. Now almost no one comes to the Water Gap to stay. People still pass through in large numbers, but they park in a turnout, have a brief appreciative gaze, then get back in their cars and drive off.

Today, alas, you have to squint, and pretty hard at that, to get any notion of the tranquil beauty that attracted Innes. The Water Gap is not only the nearest thing to spectacle in eastern Pennsylvania but also the only usable breach in the Appalachians in the area of the Poconos. In consequence, its narrow shelf of land is packed with state and local roads, a railway line, and an interstate highway with a long, unimaginative concrete bridge carrying streams of humming trucks and cars between Pennsylvania and New Jersey-the whole suggesting, as McPhee neatly put it in In Suspect Terrain, “a convergence of tubes leading to a patient in intensive care.”

Still, Kittatinny Mountain, towering above the river on the New Jersey side, is a compelling sight, and you can’t look at it (at least I couldn’t, at least not this day) without wanting to walk up it and see what is there. I parked at an information center at its base and set off into the welcoming green woods. It was a gorgeous morning-dewy and cool but with the kind of sunshine and sluggish air that promises a lot of heat later on-and I was early enough that I could get almost a full day’s walk in. I had to get the car home to New Hampshire by the following day, but I was determined to get at least one decent walk in, to salvage something from the catastrophe that was this trip, and luckily I seemed to have chosen well. I was in the midst of several thousand acres of exquisitely pretty woodlands shared jointly by Worthington State Forest and the Delaware Water Gap National Recreation Area. The path was well maintained and just steep enough to feel like healthful exercise rather than some kind of obsessive torture.

And here was a final, joyful bonus: I had excellent maps. I was now in the cartographically thoughtful hands of the New York-New Jersey Trail Conference, whose maps are richly printed in four colors, with green for woodland, blue for water, red for trails, and black for lettering. They are clearly and generously labeled and sensibly scaled (1:36,000), and they include in full all connecting roads and side trails. It is as if they want you to know where you are and to take pleasure in knowing it.

I can’t tell you what a satisfaction it is to be able to say, “Ah! Dunnfield Creek, I see,” and, “So that must be Shawnee Island down there.” If all the AT maps were anything as good as this, I would have enjoyed the experience appreciably more-say, 25 percent more. It occurred to me now that a great part of my mindless indifference to my surroundings earlier on was simply that I didn’t know where I was, couldn’t know where I was. Now at last I could take my bearings, perceive my future, feel as if I was somehow in touch with a changing and knowable landscape.

And so I walked five thoroughly agreeable miles up Kittatinny to Sunfish Pond, a very comely forty-one-acre pond surrounded by woods. Along the way, I encountered just two other people-both day hikers-and I thought again what a stretch it is to suggest that the Appalachian Trail is too crowded. Something like thirty million people live within two hours’ drive of the Water Gap-New York was just seventy miles to the east, Philadelphia a little bit more to the south-and it was a flawless summer’s day, yet the whole of this majestic woods belonged to just three of us.

For northbound hikers Sunfish Pond is something of a glorious novelty, since nowhere south of here will you find a body of water on a mountaintop. It is in fact the first glacial feature northbound hikers come across. During the last ice age, this was about as far as the ice sheets got. The farthest advance in New Jersey was about ten miles south of the Water Gap, though even here, where the climate would let it go no farther, it was still at least 2,000 feet thick.

Imagine it-a wall of ice nearly half a mile high, and beyond it for tens of thousands of square miles nothing but more ice, broken only by the peaks of a very few of the loftiest mountains. What a sight that must have been. And here is a thing that most of us fail to appreciate: we are still in an ice age, only now we experience it for just part of the year. Snow and ice and cold are not really typical features of earth. Taking the long view, Antarctica is actually a jungle. (It’s just having a chilly spell.) At the very peak of the last ice age 20,000 years ago, 30 percent of the earth was under ice. Today 10 percent still is. There have been at least a dozen ice ages in the last two million years, each lasting about 100,000 years. The most recent intrusion, called the Wisconsinian ice sheet, spread down from the polar regions over much of Europe and North America, growing to depths of up to two miles and advancing at a rate of up to 400 feet a year. As it soaked up the earth’s free water, sea levels fell by 450 feet. Then, about 10,000 years ago, not abruptly exactly but near enough, it began to melt back. No one knows why. What it left in its wake was a landscape utterly transformed. It dumped Long Island, Cape Cod, Nantucket, and most of Martha’s Vineyard where previously there had just been sea, and it gouged out the Great Lakes, Hudson Bay, and little Sunfish Pond, among much else. Every foot of the landscape from here on north would be scored and scarred with reminders of glaciation-scattered boulders called erratics, drumlins, eskers, high tarns, cirques. I was entering a new world.

No one knows much of anything about the earth’s many ice ages-why they came, why they stopped, when they may return. One interesting theory, given our present-day concerns with global warming, is that the ice ages were caused not by falling temperatures but by warming ones. Warm weather would increase precipitation, which would increase cloud cover, which would lead to less snow melt at higher elevations. You don’t need a great deal of bad weather to get an ice age. As Gwen Schultz notes in Ice Age Lost, “It is not necessarily the amount of snow that causes ice sheets, but the fact that snow, however little, lasts.” In terms of precipitation, she observes, Antarctica “is the driest large area on Earth, drier overall than any large desert.”

Here’s another interesting thought. If glaciers started reforming, they have a great deal more water now to draw on-Hudson Bay, the Great Lakes, the hundreds of thousands of lakes of Canada, none of which existed to fuel the last ice sheet-so they would grow very much quicker. And if they did start to advance again, what exactly would we do? Blast them with TNT or maybe nuclear warheads? Well, doubtless we would, but consider this. In 1964, the largest earthquake ever recorded in North America rocked Alaska with 200,000 megatons of concentrated might, the equivalent of 2,000 nuclear bombs. Almost 3,000 miles away in Texas, water sloshed out of swimming pools. A street in Anchorage fell twenty feet. The quake devastated 24,000 square miles of wilderness, much of it glaciated. And what effect did all this might have on Alaska’s glaciers? None.