There is a sweet spot between the harvest and the seeding of the new crop when a field is ploughed and fertilised. And, in an area like the Chiltern Hills, the plough will always lift lots of flint and chalk.
That's good news for me because it will also bring up the occasional fossil.
I say 'occasional' because it is a rare event.
I've walked these ploughed fields (with permission) for well over a decade now and only three have turned up so far - two sea urchins and an ammonite. But then again, I only get a few weeks every year when the ploughed field can be walked on. And there's a lot of ground to cover.
I reckon I'm pretty lucky to have found as many as I have.
Wherever there's chalk there were once oceans. Chalk is made from the decayed calcium in seashells and corals. And, sure enough, just 150 million years ago, towards the end of the Jurassic period, this area was once part of a warm, shallow sea teeming with life. The Earth has shifted on its axis since those days but, back then, what is now the Chilterns was about 35 degrees north of the Equator, (about where Tunisia is today).
I don't suppose I'll ever get over the thrill of personally finding a fossil. Firstly, there's the rarity value. The process is such an unlikely coming together of circumstances that only about one in a billion bones or shells get fossilised. To put that in perspective, there are 7 billion humans on the planet right now and we each have 206 bones. That's around 14.5 billion bones. So, if the entire human race dropped dead tomorrow, only 14 bones would survive for our remote descendants (or replacements) to dig up in the far future. However, considering how many humans came before us and are already in the ground, it's been suggested that the actual figure might actually be closer to 300 bones. But that's still only the equivalent of one and a half humans - not a great legacy for an entire species! But it does explain why the fossil record can only ever give us a tiny window into the worlds of the past - it represents just a small fraction of all of the species that have gone before. At present we have only discovered about 900 types of dinosaur. Compare that to the fact that there are over 6000 living species of mammal, 10,000 species of bird and an estimated 8,000,000 species overall. There are so many creatures we will simply never know about until someone invents time travel.
Secondly there's the fact that I am probably the first living creature to see these animals since they died.
I'm told that the sea urchins are maybe 65 million years old.
That's pretty humbling isn't it?
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