Katherine Hudson, of Upper Hutt, asks :-
I saw a photo of dinosaur tracks taken in Golden Bay. They appear to have stepped across several sedimentary strata which must have taken millions of years to lay down but must have been softish when walked upon. Wouldn't rainfall have washed these away at the time?
Greg Browne, a sedimentologist with GNS science who discovered these tracks, responded.
The footprints do indeed cut across several layers of sandstone and mudstone. The layers were deposited by tidal currents of the then beach and probably represent a much more rapid time period than Katherine suggests.
Rather than millions of years, such layers probably form across several tidal cycles (i.e., days) if considering the few millimetres of layering shown in the photo. Geologists can't be certain of the exact time period involved because we have no independent method to quantify what we call the sedimentation rate. We can however infer from nearby structures called climbing ripples, which we know from experiments to typically form very quickly, effectively instantaneously in terms of geological time. And yes the footprints do cut across several layers, and the feet of the dinosaur cut through several centimetres of the then beach sands in the process of walking. The deepest of the dinosaur footprints is over 50 cm deep.
One of the amazing things about this site (and the reason the footprints are preserved after 70 million years) is that the tides of the time came in and covered the footprints with a few millimetres of mud, and later buried by a much greater thickness of sediment. The only reason why the footprints are there is because of this blanket of mud which prevented the footprints from, as you say, being washed away.
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