Traces of the Geologist
I haven’t really considered myself a geologist for a while now. However, once a geologist always a geologist and we just can’t help ourselves ‘geologising’!
One of the ways that this manifests itself is in the examination of depositional environments and wondering how they would appear in the geologic record if preserved in their current states. This is an occupational hazard here as we live at the mouth of an estuary on a sandy beach within a dune complex. All of which are diverse and very dynamic depositional environments. This means they change every time we walk through them. We can’t help wondering if they were to be preserved as rock in their current state would we recognise them as the environment in which they were deposited if we to come back and see them in millions of years time.
This is essentially the concept behind the study of sedimentary rocks. We look at what is happening in the present, assume that similar processes occurred in the past and then become depositional detectives in order to determine the type of environment and processes that caused a rock to form.
The fact that things are so varied and so dynamic here does make us wonder if we would get it right and just how accurate we could be. A simple walk along a mile of beach here would see us crossing sand dunes, pebbles banks, sandy beaches, shell-rich sand banks, muddy channels, rocky piles of slate and even preserved soils and fossil forest. A complete mishmash of sediments and all with their own distinctive features such as ripples, substrate alignment, particle size and shape and of course distinct fauna. It would be quite a task to decipher all of this in the rock record.
As well as the sedimentary signature of such environments, there would of course be fossils too. One of my favourite types of fossil is the ichnofossil. In lay-mans terms this is a trace fossil, the most obvious example of which would be a dinosaur footprint. The thing seen in the fossil record is a footprint which is a trace of the organism that created it rather than a fossilised remnant of the organism itself. I became interested in such ichnofossils after discovering and naming a new one whilst researching for my undergraduate dissertation. It was a large worm boring that I named Trypanites fosteryeomanii .
Whilst out on a walk the other day I came across what could in millions of years time become a very distinctive trace fossil. To be honest it is unlikely to do so as no doubt it has been washed away already, but these are the sorts of things that do get preserved from time to time and become topics of discussion and speculation by geologists of the future. Again, I realise that’s unlikely to happen as well as I very much doubt that there will be humans on Earth in 200 million years time, let alone geologists!
Identifying Trace Fossils
However, this is what the trail in the sand looked like.
It was quite a long trail through the sand. I’ve cropped it for now so as to hide what made it. Obviously if we were looking at this in the fossil record then the animal that made it would be long gone and we’d only have the trail to work from. What do you think made it?
It’s quite a wide trail about 30cm in width and it continued through the sand for at least 100m. It created a little ridge to either side of it as it moved through the sediment and ripples within the trail itself that were perpendicular to the direction of travel but angled just slightly. There were a number of them along the beach on the estuary. Any ideas?
Well, here’s your answer. I don’t think a geologist would have guessed it, not even in 200 million years! It was barrel jellyfish! As far as I can tell, it was a dead one as well.
Even if it was still alive I don’t think a jellyfish would actually have been crawling through the sand to produce such a trail. Instead, I can only guess that these trails were made by dead jellyfish that had washed up on the shore and had then been pushed through the sand by the incoming tide. Each time the tide surged the jellyfish would float a little and be moved up the beach only to settle in the sand and make an indentation. As the tide pushed the carcass of the jellyfish through the sediment it pushed the larger pieces of sediment to the side and created the longitudinal ridges. Each time the carcass settled into the sand the indentation made created the little ripples perpendicular to the direction travel. This happened over and over as the tide rose, creating what looks like a trail made by a living creature. Maybe not all ichnofossils are quite what geologists think.