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The connection between citrus fruit and the American-Canadian border did not seem immediately obvious to us as we approached Eastport but our subsequent experience of crossing back into the States suggests some correlation. At the checkpoint itself questions centered round what we were bringing back from Canada and how much alcohol we were carrying, before antennae went up at the revelation that we had been out of the country for five weeks. Americans do not have five weeks holiday in a year, being grateful for two and lucky with four. We were obviously too young for retirement and so suspicion was aroused but quickly assuaged as Sterling pointed out the Internet satellite dish on the roof and assured the customs officers that it wasn't all fun and games. The final scene in this little drama revolved around food that we were bringing back and specifically a short inspection of the contents of our fridge and cupboards. The only offending item turned out to be a solitary orange, an Australian orange, not that its nationality was an issue, or so we were assured; the offence appeared to be more in it's intrinsic citrusy nature. The apples, bananas, onions, peppers, broccoli and other sundry fresh food was of no concern and as we drove off, we did wonder whether he just fancied it as a mid-afternoon snack.

The Forest Service campgrounds in the northwestern states were already preparing for winter, the water supplies turned off and fees abandoned. At Howard Lake in Kootenai National Forest, Montana, the summer campers had left the campground deserted. A huge fire-ring and a plentiful supply of wood for Sterling to split kept us warm and toasty for the first couple of nights there. We were less keen the third night following a warning from a day visitor that there was a deer carcass very close by with ample evidence of feasting and tell tale bear scat. It seemed wiser to stay indoors.

This brings to mind the dangers inherent in trying to communicate in a foreign language. While out walking in Canada we met and struck up a conversation with a German man who after a few cursory comments asked us if we'd seen any bear shit. The bluntness was a little surprising from a stranger and he was obviously sensitive to the issue as he went on to ask for a more polite term that he could use. Sterling kindly supplied him with droppings, linguistically not actually, and he left us happy with his new found and delicate term. We reflected afterwards that foreign language vocabulary taught in schools never includes anything remotely associated with bodily functions or the products there of. While Sterling's experience reflected the accumulation of an array of illicit words, I on the other hand have a sad lack of any juicy foreign vocabulary, but then I was educated in a convent. Strangely enough, even though neither of us has ever learnt German it is the only language in which we both knew the word for excretia.

Driving south through the bare high desert of Idaho the colours are muted, the landscape a washed out picture of gentle tones; the grasses are drained of chlorophyll, the sage is its usual hint of green, the rounded contours of the surrounding hills a faint pink hue. The scene is broken by a splash of bright and definite colour where the Salmon River reflects the deep blue sky, and the varying golds of the autumnal cottonwoods line the banks. It is an empty place; scattered farms and the occasional green patchwork of irrigated fields along with tiny towns strung out along the road are the only signs of human influence.

The openness and light were appealing and we stopped for a few days at Cottonwood BLM Recreation Area, where we camped on the riverbank, the water gurgling in the background, the light bark and golden leaves of the cottonwoods a perfect fit against the clear sky, with another huge fire ring and a plentiful supply of wood to split and burn. The nights were clear and the stars emerged into the growing darkness, filling the sky with the complicated pattern of the universe. Flocks of migrating geese flew high overhead their characteristic honks audible even before we had a sense of daylight down on the ground. In the early mornings a Bald Eagle glided along the valley following the river in search of breakfast and disturbing small groups of ducks in the process.

We followed the geese and continued south, our next stop heralded by an abrupt change in the colour and texture of the land. Craters of the Moon National Monument encompasses an area of volcanic landscape, its seventy-five square miles a frozen sea of lava, dotted with cinder cones, the result of eight separate eruptive periods over the last fifteen thousand years.

The Monument is part of a chain of volcanic activity stretching from the Idaho-Oregon border sixteen million years ago and culminating today in the geysers and bubbling mud of Yellowstone in Wyoming. A couple of interrelated factors are responsible; a hotspot in the earth's mantle is the first of these. Magma rises in a column towards the crust and when the conditions are right bursts out onto the surface in catastrophic, voluminous eruptions. The second factor is the steady but continuing movement of the North American tectonic plate in a southwesterly direction at a rate of about two inches per year. In consequence, a different point on the earths crust is directly over the hotspot at different times; today it is Yellowstone, 8 to 10 million years ago it was Craters of the Moon. The two factors have left a line of old calderas strewn in a southwest northeast line across the Eastern Snake River Plain in Idaho, marking the progress of the crust over the hotspot, Yellowstone being the northernmost end of the line.

During an eruptive cycle, lava poured forth in phenomenal volumes and each consecutive event added to its overall depth. In places on the Eastern Snake River Plain the basalt is six thousand feet deep and the area is slowly but surely sinking under the sheer weight of the additional material spewed up onto the earths crust.

Today, Craters of the Moon shows no sign of current volcanic activity but appearances can be deceptive. The last event here was over two thousand years ago and it is estimated that the interval between eruptive periods has averaged two thousand years. In the meanwhile the lava from previous eruptions tells the story of different processes in the movement and cooling of the magma and it was to get a better idea of these that we booked to go on a day long guided geology hike at the Monument.

From a distance there is an appearance of uniformity, both in darkness of the rock and the apparent lack of vegetation but on closer inspection and with the illumination provided by our geologist guide, an intricate and varied world opens up. The hike took us off the trails, out across lava fields not normally open to the public and scrambling around and up the faces of cinder cones. It was a very immediate experience of the place, the movement of the molten lava seemingly captured in the cold dark rock. Lavas of different density cooled in different formations, the very dense creating a surface of angular blocks like those we had previously seen at Devils Postpile in California. Others cooled into the distinct patterns of smooth raised ridges, known as pahoehoe, taken from the Hawaiian word meaning "ropy", while another type formed a rough jagged surface and is known as 'a'a after the Hawaiian for "hard on the feet". Some flows have cooled with a distinct colour, the most obvious here being the Blue Dragon flow, covered in a metallic blue sheen not dissimilar to a certain sort of pottery glaze.

There are formations that tell a particular story of the cooling process; the distinct ripples of billows, formed where the front edge of the flow came up against an impassable obstacle, being concertinaed into rounded ridges as the lava continued to try and move forward; lava tube caves created where the outer crust of a flow had already cooled but the inner molten rock flowed out leaving a hollow space beneath the surface, some several miles long.

Evidence of the more explosive eruptions caused by gas-rich magma reaching the surface can be seen in the cinder cones. As the molten material was sprayed high into the air, individual pieces cooled in flight raining down as small pieces of cinder that gradually built up to create cones. Other projectiles, known as bombs were much larger and denser, taking on different appearances depending on conditions; resembling spindles, cow pats and rounder pieces with a crust like that of a loaf of Tiger bread and known strangely enough as Breadcrust bombs.

A few days later we set out on our own along a couple of trails and were immediately struck by the silence. In a group there is always someone talking and the silent context is lost. With just the two of us, the wonderful quiet is every bit a part of the landscape as the rock itself. There are few birds; the odd raven, a couple of chickadees and a Hairy Woodpecker intent on its percussive search for food. The chipmunks and squirrels go about their business, the odd rustle or flash of movement giving their presence away. A strong smell of sage wafts on the breeze supplemented occasionally by the scent from one of the scattered Limber Pines.

Mistletoe grows as a natural parasite on these trees. Its presence stimulates the production of extra nutrients resulting in areas of thick bushy growth on the Pines rather resembling a witches broom and known by that name. Some decades ago Park staff decided that these growths deformed the trees making them look ugly and definitely not conforming to Park's notions of natural beauty, and so they cut down and poisoned six thousand of the trees in an attempt to tidy up the Monument. This seems stunning within the context of today's attitudes but presumably back then it appeared to make sense in spite of the fact that both species were native to the area and the parasitic relationship was a natural part of the ecosystem.

Trees do not survive well in molten lava and yet there are tell tale signs of their presence to be found along the Tree Mold Trail. A tree standing in the path of a molten magma flow faced one of two fates. Some remained vertical, encased by the surrounding lava and as the tree burnt away in the intense heat a mold of the trunk was left in the cooling rock. Elsewhere, trees were knocked over by the advancing lava and imprints of the smouldering trunks have been left in the rock, reflecting the characteristic charred surface of burned wood.

Whilst visiting these sites of previous volcanic activity we often speculate about the prospect of seeing a lava flow in action rather than frozen in a snapshot of hardened rock. Not that we have an urge to be endangered but the thought of seeing red hot magma flow into the sea on the Big Island in Hawaii is certainly sufficient incentive for a trip at some point.

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