There was snow on the mountains nearby, and in the dark gorge the winter water was icy. We sloshed up the meandering stream bed, examining the rocky walls as we went. Parallel layers of whitish rock containing darker interior bands plunged down to meet the stream ahead of us. The formation continued for hundreds of metres up the steep cliffs above us, up into patches of sunlight. But down where we were, there was no hint of sun.

Joy Lines, my fossicking companion, and I were on a journey back to the end of an epoch. Sixty-five million years ago, life on Earth faced an apocalypse. Half of all organisms were rendered extinct, including the dinosaurs. Most ecosystems suffered massive destruction, and it took a million years or more for life to return to a semblance of order, though it was never the same as it had been. All that is left of that cataclysmic change is a scar of clay-like rock exposed in a few dozen locations around the world.

I peered at the sheaf of papers in my hand, looking for congruence between the detail that appeared in the grainy pictures and the rocks beside us. Judging from the rock formations, we were certainly in the right area, but finding the exact spot—a layer of rock just centimetres wide laid down 65 million years ago—could prove tricky.

When I’d spoken with Chris Hollis, a geologist from GNS Science, on the phone a few days earlier, I could hear doubt in his voice that we’d be able to find the spot. After all, we were well off the beaten track.

We’d travelled with Hamish Murray (one of the family who owns 40,000 ha Bluff Station at Kekerengu, midway between Blenheim and Kaikoura) for some 30 km along a farm road in his ute, climbing over a low saddle near the eastern end of the Seaward Kaikouras and winding west towards the lower ramparts of the Inland Kaikouras. Murray pointed us in the direction of the south branch of Mead Stream and wished us luck.

Geologists come from all over the globe to see the rocks in Mead Stream. Sixty-five million years ago, these formations were sludge on the seafloor off the coast of a very different New Zealand. Dinosaurs probably roamed the land and erosion was steadily nibbling away at the hills. Rivers bore the debris back to the seafloor in the process of relentless levelling that nature undertakes, gradually building up a geological record in the sediment that accumulated close to the coast.

But then occurred an event that changed life here and everywhere else on Earth. It wiped out pretty much every animal on land larger than a rat, and also put paid to much that lived in the sea as well.

For instance, the ammonites—a group of cephalopod molluscs resembling the chambered nautilus still found in the tropical Pacific—completely disappeared despite dominating the oceans for more than 200 million years. Most large marine reptiles vanished. Half of all organisms became extinct.

So major was the abrupt change recorded in fossils at this time that it split geological time, ending the 180 million-year-long Mesozoic era and beginning the Tertiary, also known as the Cenozoic era. This has been commonly abbreviated to the K–T boundary—K is from the German name for the Cretaceous period (Kreidezeit meaning chalk age), the last of the three periods of the Mesozoic. These days, however, the use of T for Tertiary is being discouraged by the International Commission on Stratigraphy, and “Paleogene” is replacing it—so it is now more accurately termed the K–Pg boundary.

What actually caused this sudden extinction is the subject of much debate and suggestions have been many and varied—including terminal hay fever brought on by the evolution of flowering plants!

But the most persuasive notion has been a theory posited in 1980 by Nobel Prize-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and two chemists. In New Zealand and elsewhere, the K-Pg boundary is marked by a layer of clay-like sediment, a couple of centimetres thick, sandwiched between layers of harder rocks. Alvarez and his team discovered that this layer contains high levels of iridium—an element that is extremely rare in the Earth’s crust, but abundant in most asteroids and comets. These extraterrestrial bodies also possess oddities in the abundance of other heavy metals, such as chromium and nickel, that Alvarez also found in the boundary rocks. The layer also contained tektite glass spherules and tiny quartz grains bearing stress lines. All this is evidence, the Alvarez team claimed, of a massive asteroid impact on the Earth 65 million years ago. The asteroid would have exploded on impact, spreading iridium-laden dust around the globe.