“Now, don't touch the rock here, it erodes very easily,” said Marcelo, our guide, as he ran his hand over the face of a particularly imposing orange rock by the side of the route 68, 7 kilometres outside of Cafayate. Pebble-sized chunks fell off in his hand's wake and he dusted it off on his trousers.
“Does he do that every time he makes the tour?” said Annika, a tall, blonde girl on the tour with us, half joking, half deeply concerned in her uniquely German way.
Welcome to Argentina, I thought.
We followed Marcelo from the road where he'd parked the minivan that had brought us, along with Annika, a trio of Canadians, a guy from Singapore and a French girl from Cafayate. We followed him along side the rock face, which hung over us while trees and bushes closed us in on the other side, giving the impression we were passing through a tunnel. Soon we emerged in to a valley, which I thought looked like it may have once been a river bed.
Marcelo proceeded to explain that it was at one time the ocean floor, as evidenced by the many marine fossils that have been found in the area. This made the name of the place suddenly made sense: Quebrada de las Conchas translates to Shell Gorge. As a national park, it has only been recognised since 1995, but is of course many millions of years older than that.

After explaining some more about the geology, fauna and flora (which gets most of its water from condensation caused by the extreme difference in day and nighttime temperatures), we wandered around the landscape for a while, admiring the strange formations, taking photos and talking to the others on the tour.
Back in the minivan, Marcelo drove us a few kilometres further down route 68, letting us out to see a few of the more obviously recognisable rock formations in the quebrada, such as los Castillos (the Castles), las Ventanas (the Windows) and el Obelisco (the Obelisk).



Then, another short stop at a llama farm and pottery school, where we tasted some very sweet white wine and bought some water (though there was no pressure or expectation that we buy anything), and of course, took pictures of the llamas.

From there, we walked to la Yesera (the Gypsum Quarry), a valley of multicoloured hills rich in various minerals that was once mined for its natural resources. However, in the 1970s the Argentine government decided to stop the operation in order to preserve its natural beauty and give the river a chance to recover from the contamination caused to it.

We hiked through the valley, and stopped in the shade before ascending a to a lookout point while Marcelo explained all the colours we were seeing. The light red rock was iron, the dark red, zinc. Blue was cobalt and the light green, copper.

The final two stops on our tour before returning to town were el Anfiteatro (the Amphitheatre) and la Garganta del Diablo (the Devil's Throat). The former, we were told, had 80% of the acoustic quality of the Teatro Colón in Buenos Aires, and is used as the location of a local musical festival every winter.
We egged each other on to sing inside the Amphitheatre to test its acoustics out, but no one was game, Zab and me perhaps the least so.

The Devil's Throat required a bit more of a climb to access than the Amphitheatre and only half of us in the group attempted it. The effect of climbing up through the rock formation with the rocks layered at a 45 degree angle from the plane of the ground was rather disorienting, and I felt like I might fall over just from looking up, as though my body were trying to compensate for the optical illusion by putting me on my back.

We returned to Cafayate as the sun was setting somewhere out of sight, giving the valley an eerie marine blue, shadowy glow. If I squinted, ignored the minivan and road and imagined I were some kind of prehistoric fish, it almost felt like I was swimming through the valley, somewhere at the bottom of the ocean several hundred million years ago.
Almost.

Practicalities
La Quebrada de las Conchas is easy to visit by car from Cafayate or Salta as it lies along route 68 that connects the two. If you don't have your own transport and don't fancy cycling or walking 15 to 40km in each direction from Cafayate to see all the points mentioned in this post, taking a tour is the best option.
In Cafayate town there are several tour companies offering identical itineraries for the same price (AR$100 in May 2013), mostly located around the main square.
If you're looking for a place to stay in Cafayate, we can recommend La Morada Hostal, which was simple but clean and comfortable with decent breakfast and wifi.
Also consider booking this and other tours in Cafayate in advance to save yourself time!