I was a long way away from another human, picking my way hapily between mesquite bushes, agarve spikes and cacti. Lizards scuttled away from under my boots. A hawk circled overhead, and a wren scolded me from a tangle of twigs. The sky was a child's paintbox blue. I began to feel the cleansing that isolation brings.

Then I rounded a cluster of rocks and, in the middle of a clearing, on a boulder, stood a rusty beer can, and, in the sand, there was a glitter of bottle shards. I felt almost a blow of disgust, and then rage, that someone should come all this way to spoil the world.

On my journey I have been astonished by the millions of glass bottles we've passed, tossed on both sides and on the central reservation of the Interstates, and beside almost any major route. Glass shards glittering for hundreds of miles. I'm alarmed, anyway, that so much drinking is going on while people are driving. But beyond this, there seems to be an endemic disregard for the often-handsome country they are driving through as they swill whatever cat's-pee (sorry, USA, but you can't make good beer) the bottles contain. And the wilderness appears to be regarded merely as a place into which to retreat with a few cases of beer, make loud whooping noises and a mess.

Of course, this is a country where clearing garbage from the highway, and presumably from the wilderness too, has been relegated to a voluntary activity, as the monotonous "Adopt A Highway" signs remind us constantly.

The refuse disposal site that covers three quarters of the earth's surface

Oh, and then there's that other source of anger, the ocean. The other day, when we walked along a mile of Oregon beach, we picked up a heap of garbage almost too big to carry.

Though most of it was plastic, much of it was glass, including huge liquor bottles. We found at least two containers of identifiable poison (bleach and paint thinner) and a piece of timber to which a label had been attached warning that it had been treated with a carcinogen. There were skeins of that trapper of birds, fishing line.

Our haul, which wasn't 100% of the garbage we saw, was the result of probably two tides. Multiply this by the length of coastline and you're probably getting the equivalent of a city's-worth of noxious crap being washed ashore each tide.

I'm not being parochial about this — it is an international disaster — but it is sad to witness such a treasure as the US environment being abused like this.

We arrive at camp sites to find pads strewn with cigarette butts, glass bottles smashed in the fire pits.

And we peer over view points looking on to the Grand Canyon, and there below us is the leering sparkle of glass.

This makes me so mad I could go out, get drunk and smash some bottles.