I made a visit to my local beach the other day, and look what I came across – a chunk of sandstone packed full of trace fossils! They are the fossilised burrows of creatures that lived on the seafloor – most likely marine worms or teeny little shrimps – around 20 million years ago, when Auckland was pretty much all below sea level. The cliffs along Browns Bay Beach (and elsewhere) represent deep-water sediments deposited in a huge marine basin during that time, and incorporate interbedded mudstones and sandstones; as well as some volcanic material. The burrows themselves were infilled with very fine mud that was then cemented by a mineral called limonite, which is harder and far more erosion-resistant than the surrounding sandstone. Given that I came across that particular chunk of rock pretty much by accident, I am definitely going to go rock-hounding again to see if I can find some more examples!