Over the years, I've spent many a sunny day crawling over a scree slope just outside Bannockburn. It's a town in Central Otago, most famous for its wine. In the summer, it regularly sits above 30°C and the air is drier than Weet-Bix. These are conditions most city folk couldn't live with for long. So what have I, City Boy Stevenson, been doing on those hot days outside Bannockburn?

Well, I’ve been hunting. You see, I don't hunt with a rifle or a crossbow. I go hunting with a big hammer and a magnifying glass, because I am a fossil hunter. That slope near Bannockburn is the old, worn face of a sheet of mudstone; literally mud turned to stone. It's packed full of fossils, the flattened remains of plants and animals that lived in the area a long time ago. If you're ever in Otago and you feel like expanding your collection, this is a place well worth seeing.

As you can probably guess, I was that kid in school who was obsessed with dinosaurs and other prehistoric life. I bamboozled my teachers by being able to pronounce names like Anomalocaris, Ogygiocarella and Pachycephalosaurus. I read books about the North American Bone Wars when I should have been learning my times tables.

DCIM100MEDIASheer fascination led me on to study Geology at the University of Otago. Last year I got the chance to speak with some very talented children about my experiences, with the help of a trilobite they named Steve.

Searching for fossils offers unique challenges and rewards that you don't quite get from other hobbies. I remember finding my first preserved leaf in the mudstone. Purely by chance, I cracked open the right slab of rock in the right way. There in my hands appeared the clear impression of a leaf, dark brown in the sunlight. No living thing had ever seen it before me. A far cry from a dinosaur bone, but no less special. The feeling of awe I experienced cannot be put into words.

Every fossil is special, because every fossil gives us a link to the deep past. Fossils are the closest we'll ever have to a time machine, unless somebody figures out the wormhole conundrum.

For instance, the Bannockburn leaves tell us that this region was once very different. The leaves came from trees that lived on the edge of a lake. Lakes are fantastic places to make mudstone. In a lake, the water is generally very still, whereas in the open ocean it can be turbulent. In still water, fine mud particles drift down and settle in layers, slowly burying anything that has fallen into the lake. Over time, the mud layers are squashed by more mud piling on top, perhaps heated a bit. That is how mudstone forms. All the fossils are things that fell into the lake and were buried.

This raises some questions. I know from experience that you don't need scuba gear to visit Bannockburn. There is no lake there today, and the surrounding hills host completely different plant species. So what happened to the ancient lake? How big was it? What swam in it, or drank from its shores?*

The answers to these questions are known, but they're too long and profound to fit into this article.

Instead, I'd just like to give you some idea of what it's like to be a fossil hunter. I admire the writings of professionals like Neil Shubin, whose book Your Inner Fish is well worth a read if you're interested. He recounts his expeditions to the Canadian Arctic, battling stormy weather and lack of Wi-Fi to recover fossils of extraordinary fishes. I've never been to the Arctic, but I have been south of Dunedin, and they're kind of comparable.

Down there is a beach, affectionately called Measly Beach, where the challenge is to find a piece of rock that doesn't have fossils in it. Instead of abundant leaves, you find abundant seashells encased in a rock that's harder than calculus. These shells are older than the mudstone at Bannockburn. In fact, they were laid to rest at a time just after the extinction of the dinosaurs.

When I first visited this beach, I didn't have the traditional hammer and chisels to use. I was twelve years old and couldn't be trusted with anything more dangerous than a Popsicle stick. So how I filled two 25-litre boxes with shell-stuffed concrete is a bit of a mystery. The shells themselves are cone-shaped, with the biggest being around the length of a toothpick. They wouldn't look out of place on a beach today, but their sheer number and close packing is fascinating to behold. It might be clichéd to say they're packed together like sardines, but that's exactly what they remind me of.

On other trips, I've explored limestone quarries in the north of Otago, looking for everything from ancient urchins to sharks' teeth. Last year, one of my classmates even found the skeleton of a prehistoric whale! Imagine her excitement when she climbed over a yellowish mound of stone, bent down to examine the ground and saw a great lump of bone sticking out. The whole skeleton was so badly chopped up by quarry machinery that it was scientifically worthless. Therefore we were allowed to keep bits of it, and somewhere in my collection is a piece of one of its enormous ribs.

How does a fossil hunter know where to look? It always takes some planning. In my case, I have geologically-minded pals who tell me where to go, then help me get there. But if you're thinking about going somewhere new and unexplored, there are three things to consider. First of all, not all rocks contain fossils. I have talked about mudstones and limestones, which were formed by gentle and gradual processes in water.

DCIM100MEDIAYou don't often find prehistoric remains in rocks of Auckland or the Southern Alps. They are a different variety, with different origins. The rock underneath much of the North Island, including Auckland, is volcanic. It was formed by things like lava flows and debris from past eruptions. Fossils tend to be destroyed by volcanoes, not preserved.

In the case of the Southern Alps, we can tell from their chemistry and structure that rocks have been buried deep within the Earth. Down there they were baked, squished, contorted and obfusticated. Any fossils inside would have been lost forever.

So it's only in rocks like mudstone where we can see fossils. These are collectively called sedimentary rocks. Next you should consider the age of the fossils you're interested in. If you're looking for whale bones, stay away from areas where the rocks are as old as the dinosaurs. No whales existed when the dinosaurs were stomping around. If you find rocks that are only a year old, they're probably concrete.

Finally, you need a hunting site that is accessible. The town of Oamaru is potentially a great place to find fossils of whales. It sits on top of sedimentary rock (limestone) of the right age. Unfortunately, there's a town on top of said rock. I'm not sure who the current Mayor of Oamaru is, but I imagine he or she would be very unhappy if you started hammering through the floor of the train station.

If you are interested in fossil hunting, good for you! In New Zealand we're lucky to have many organisations and clubs that cater for this exciting hobby. Check out some of these links to find one in your area.

Going on field trips with experienced hunters is an excellent way to gain experience and knowledge. It's also the best way to get an appreciation for the immensity of deep time... but I'll leave that for you to ponder.

*Luckier people than me have also found fossilized crayfish around Bannockburn, so there was definitely life in the ancient lake itself

Photos by Thomas Stevenson