It's brutal vandalism. I'm staring at what was, until very recently, the largest set of dinosaur footprints on earth – 581 metres of vicious slashes gouged by a carnivore's razor-sharp claws. But now, a third of the way into the run, as the predator salivated at the prospect of supper, the spoor suddenly vanishes, replaced by a slab of fresh, smooth rock.
It reappears 10 metres further on. The hunt has recommenced. But it's too late. A landslide has left a savage rip across the rock face, interrupting the trail laid down 65m years ago in Bolivia's central highlands. To palaeontologists it's almost iconoclasm, like a theologian finding a page torn from the Gutenberg Bible.
The lost footsteps of "Johnny Walker", as the dinosaur is known, are the latest tweak to the prehistoric landscape outside the elegant city of Sucre. Cal Orko is a constantly evolving record of life in the Cretaceous era – an epic canvas that due to erosion and local mining will always be a work in progress.
"It's just amazing," says chief guide Maria Teresa Gamón as we inspect the damage. "We see fresh footprints and fossils all the time. We lose some, we find some. It's always changing."
Whatever the latest tally, the vast wall of sedimentary rock still bears the largest, most diverse collection of dinosaur tracks on the planet. Across a limestone slab 1.2km long and 80m high, Cal Orko sports more than 5,000 footsteps, with 462 individual trails.
My own trail is slightly longer. Sedate Sucre breaks up the schlep from tropical Santa Cruz to the altiplano mining town of Potosí. With a student population served by great bars and cafes it's the perfect place to kick back for a few days, particularly in the dreamy Parador Santa María la Real, where colonial-era rooms surround light-filled courtyards.
But Spain's conquistadors are a pinprick in history compared with the dinosaurs. The Bolivian prints belong to eight main species, including the unpleasant carnotaurus with its terrible dentistry and laughably tiny upper limbs, and the herbivorous ankylosaurus – a sort of heavyweight armadillo. But they and their footsteps are dwarfed by those of the clumsy plant-guzzling titanosaur, weighing up to 100 tonnes.
Cal Orko lies three miles from downtown Sucre, where red, brown and ochre folds of earth smudge into the Andean foothills. I know it's inside a quarry – miners spotted many footprints after the first discoveries in 1985 – but it's still shocking to find heavy industry cheek by jowl with a fragile landscape of world significance. The extraction company's work covers everything – trees, buildings, men – in a fine fuzz of dust. Its vast lorries, titanosaurs of earth moving, are a surreal juxtaposition with the palaeontologists' delicate work.
The Parque Cretácico, opened in 2006, lies further up the hill, with a museum, vast models of dinosaurs and B-movie roars piped through loudspeakers. So far so Spielberg. It's only when I reach the viewing platform, 150 metres from the rock face, that it starts to become marvellously real – a widescreen view of prehistory. My eyes need to adjust. Cal Orko is a vast optical puzzle requiring time to decipher. Those dots, dashes and holes like super-sized horse hoofprints aren't random designs – they're rock-solid semaphore explaining Cretaceous life.
Visitors aren't normally allowed up to the wall, but with mining temporarily suspended, I'm granted rare access. The immense vertiginous rock face is slightly overwhelming, in the dust and searing heat. Maria, the palaeontologistss assistant, uses a mirror to transform the sun's rays into a spotlight, picking out specific tracks.
Her enthusiasm is infectious. "Look! Six footprints going up. Ankylosaurus. And over there! Those are about 80cm in diameter. It's a titanosaur coming down the wall for about 25m. See where they stop? That soft outer rock will soon crumble and you'll follow them right to the ground."
Maria uses the footprints to explain how meat-eaters walked with straight feet "while herbivores were pigeon-toed like Charlie Chaplin – apart from the long-necked ones, who had the same hips as carnivores".
The spoors reveal mundane details of daily Cretaceous life. It's CSI: Sucre. "That ankylosaur was running. It sank its four toes into the ground, rather than its heel." There's even a large carnivore that, like any true gentleman, preferred the female to stay on its left side.
Most fun of all is gauging the beasts' size from their tracks. For two-legged dinosaurs you multiply the length of the footprint four times to discover leg-to-hip length. Once you've got the legs, you know if it was Joe Average or Godzilla. A large titanosaur had 6m pins – without stilettos.
By the time we're placing our hands next to the tracks – a quick way to feel extremely insignificant – I've mutated into teacher's pet. I fear I'm raising my hand to ask questions.
"What's that?" demands Maria, pointing to a splatter of prints.
"Two round toes. A hoof. Waddling like Chaplin. A herbivore," I snap back. "Possibly an iguanodont."
"Very good, Ian" I feel stupidly proud.
But Cal Orko is about geology as well as palaeontology. Unless dinosaurs wore crampons, and size 72 ones at that, how the hell did they leave tracks up a vertical rock face? It's all very complicated, so pay attention at the back.
The Cretaceous era, starting 145m years ago and ending with mass extinction 80m years later, saw South America drift away from Africa and and join with North America, sparking wildlife migrations. Cal Orko, kissing a huge lake and boasting the continent's first flowers, attracted herbivores and subsequently carnivores.
But it was unique climate fluctuations that made the region a palaeontological honey pot. The creatures' feet sank into the soft shoreline in warm damp weather, leaving marks that were solidified by later periods of drought. Wet weather then returned, sealing the prints below mud and sediment. The wet-dry pattern was repeated seven times, preserving multiple layers of prints. The cherry on the cake was added when tectonic activity pushed the flat ground up to a brilliant viewing angle – as if nature was aware of its tourism potential.
Not surprisingly, Cal Orko has applied for Unesco listing, but the fate of Johnny Walker's steps underlines the vulnerability of this place.
Far from preserving the site, man is adding to the ravages of time. As I leave, a low explosion thumps across the quarry's fragile earth and reverberates through my chest. More tracks will disappear, more emerge – the endless dance of conservation and industrial progress.
