Franz Josef Glacier is like a giant white bulldozer shoving vast quantities of stone down the side of the Southern Alps to help build up the coastal strip of the West Coast.
Standing below its massive face, you can see on either side of the valley the great heaps of rock ripped out of the mountains and pushed ahead of the advancing millions of tonnes of ice.
That's because the glacier is advancing - one of the few that is - and has been for 25 years.
And, like the neighbouring Fox Glacier, it is also one of the very few glaciers to flow right down into the lush rainforest.
Of course, as Jo our guide from Kea Heritage Tours explains, glaciers are constantly advancing and retreating. When the explorer Julius Haast came here in 1864 the Franz Josef flowed much further down the valley before starting to retreat. But it reached its minimum around 1982 and since then has advanced about 1km back down its old path.
To get to the glacier face today involves a pleasant walk, first through some beautiful beech forest on the banks of the Waiho River, then across the floor of the broad valley carved out by glacier in its glory days, now covered with shingle ground out of the landscape by the ice bulldozer, and criss-crossed with riverlets fed by the melting glacial ice and the waterfalls flowing down from the steep bush on either side.
Because of the danger of being crushed by a falling chunk of ice from the advancing face you can't get too close these days, but the barriers are quite near enough for us to savour the extraordinary spectacle provided by what is, quite literally, a river of ice.
Some parts of the face are great chunks of frozen snow, glowing white and blue, compressed and pushed forward at a rate of several metres a day by the pressure of millions of tonnes of snow in a 20sq km field high in the mountains above.
But another big section of the face is grey and black, dirty looking, because it is covered in some of the debris the glacier has carried down the mountainside.
High above, on the top of the glacier, we can see lines of tiny figures, groups of visitors who have taken the opportunity to join a guided expedition on to the ice itself.
Meanwhile, back down at the observation area, entertainment arrives in the shape of a couple of kea, their red, green and brown feathers glowing in the occasional sunlight.
Jo, our guide, says there used to be a lot of kea around and they caused havoc, especially down at the carpark, stealing food and destroying rubber windscreen wipers.
"But since DoC introduced a policy of urging tourists not to feed them the numbers have gradually declined. These are the first I've seen here for some months."
There's entertainment of a different kind when we move on to the Fox Glacier, where the pathway up to the face is mostly over vast quantities of ice, left behind by the glacier's retreat and buried for half a century beneath the rocky debris.
Unfortunately it's now starting to melt, causing the seemingly solid gravel paths to collapse, in the process creating some breathtakingly lovely ponds of pale blue water.
There are warning signs around and Jo observes that the collapses seem to be increasingly frequent.
"The route up changes nearly every day. I half expect that one day I'll come here and there won't be a path to follow at all."
In the meantime there's plenty more to see in this spectacular part of the world, with the mighty mountains on one side, raging seas on the other and great braided rivers linking them.
It so happens we're here on the last day of the whitebait season so there's the added attraction of the banks of these torrents of water being lined by dozens of eager whitebaiters, all carefully placing their traps, in the hope of getting a final taste of the local delicacy.
The towns that dot this landscape may be small but they are all full of history and spilling over with great stories.
Ross, for instance, is where the largest gold nugget ever seen in New Zealand _ the Honourable Roddy Nugget, the size of a man's fist, weighing in at nearly 3kg _ was discovered in 1909. Jo tells us the land round here contains so much gold that most of it has been sieved a couple of times and as we drove through huge machines operated by local miner Evan Birchfield, were busy digging some bits up for a third time.
You may remember Birchfield as the guy who a few years ago got a 1953 Centurion tank for his 50th birthday and observed, "Every 50-year-old should have one of these." A classic Kiwi character.
Then there's Kumara, where the legendary King Dick Seddon, our longest serving prime minister, had his pub. These days only the foundations of the pub remain, but there is a tearooms on the corner to retain the tradition of hospitality, and a memorial marks the spot where he lived.
There's an impressive statue of Seddon in nearby Hokitika, which styles itself as the gold and greenstone capital, because of the number of greenstone carvers, goldsmiths and galleries that now occupy the lovely old buildings in the town centre. But I was particularly impressed by the Ruby Rock Gallery which makes jewellery from Goodletite, an amazing rock containing ruby, sapphire and tourmaline crystals, apparently found only in Hokitika.
You might be surprised to discover - well, I was - that Hokitika was the base for New Zealand's first scheduled air service, carrying mail and passengers to Haast and Okuru in the early 1930s. And just down the coast at Hari Hari, a plaque marks the spot where in 1931 Guy Menzies, the first man to fly the Tasman solo, made his historic landing upside down in the La Fontaine swamp.
The town of Whataroa boasts the smallest working courtroom in the country. But even more impressive is the Kotuku Gallery there, run by carver Kereama Armstrong, which has a sign outside saying it was voted the country's best Maori gallery. It's easy to believe because the carvings in wood, bone and jade on display here are superb. Some are carved in an old piece of whalebone discovered on the Chatham Islands which has been carbon-dated as 3500 years old.
With all the rain that falls on this side of the mountains there are also lots of beautiful lakes.
At possibly the most famous of them, Lake Matheson, the usual classic reflections of the surrounding mountains are obscured by the increasingly heavy cloud. The water here is incredibly dark, almost black, with leaf tannin, and even in what is by now a light drizzle gives perfect reflections of the lakeside trees. On a clear day it must be magnificent.
Driving past Lake Mapourika we spy two kotuku - whose only New Zealand breeding site is nearby - flying in to land. Jo quickly takes us down to the lake and we explore the shoreline, spotting all kinds of water birds, but no kotuku.
At Lake Mahinapua we are entertained by the sight of a bearded character who has slung his hammock inside the old steam ferry on display on the lake front and is lying inside reading a book.
I wander up to say hello. Pause. Grunt. Mind if I take your photo? Pause. Grunt. Okay. After taking a couple of shots I observe that it is a beautiful spot. Pause. Grunt.
Ah well, he might not have been inspired by the view, but I was.
Further information: You can find out about the Scenic Rail Pass, the cheapest way to see the country by rail, at tranzscenic.co.nz.
The Kingsgate Hotel is on the riverside in Greymouth.
For details of West Coast trips arranged by Kea Heritage Tours see keatours.co.nz.
For information about the Fox and Franz Josef Glaciers try glaciercountry.co.nz.
For tourist information about the West Coast see west-coast.co.nz.
Jim Eagles travelled New Zealand by rail with help from KiwiRail, Air New Zealand and the regional tourism organisations along the way.