What was the sea floor really like?What was the sea floor really like?

So Brown End = Sudden End...in the dark! Whatever’s next?So Brown End = Sudden End...in the dark! Whatever’s next?

Mysterious animals eating mud?The muddy sea floor wasn't lifeless! There are many tracks made by creatures. We do not knowwhat made them, but we think most were probably soft bodied, like worms, with no hard partsto become fossils.

A genuine bottom-dweller!We do at least know one animal that crawled on the sea floor. This was a trilobite, whichlooked a bit like an overgrown wood-louse. But there are also many tiny fossils and lots of‘faecal pellets’ (poo). Geologists think that the water was about 300 metres deep. Bits andpieces of dead creatures, and poo, would have rained down on the sea floor.

Some of the tiny fossils are really tiny - here are four on a pin-head! But thesehave razor-sharp blades and lethal stiletto points, and were once in the mouthof an eel-like fish a few centimetres long. Imagine being a squidgy wormycreature and getting something like those digging and slicing into you!But bigger fish - maybe the sharks? - would have eaten these littleeel-like fish.

More killers - and in the dark!At 300m no light would have reached the sea floor. From time to time, a bigstorm or earthquake brought dense currents full of mud, bits of rock and bits ofcreatures hurtling across the sea floor like an underwater avalanche. Nothingwould survive in their path. Then all the sediment would settle out and life wouldreturn. Until the next time...

A shallow, sun-lit sea lily garden, as in the picture?You can see how the picture is based on fossils found in thisquarry, and in rocks of the same age from quarries nearby. Butmost of the rock in this quarry is very fine grained: it was oncemud. Today mud settles out in quiet water, so using “the presentis the key to the past” idea, we think the rocks in Brown Endwere laid down in deep, quiet water. Not in a wave-washed sealily garden. Maybe storms swept the creatures from their shallowwater home - the sea lily garden - down into deep, quiet water.

Geologists use the tiny fossils for telling theages of the rocks, but all this churning up ofthe sea-floor makes problems as the fossilsare jumbled up. Ones which were long deadand buried get mixed up with much youngerones. At least geologists can describe the typeof rock. Very often they produce a summaryof their findings as a ‘log’: there’s one forthe whole quarry below.

... or something else entirely?

Answer:There are atleast 3 types of tracks.

the brick pattern meanslimestone (to a geologist,at least!)

the width of this columntells us how large the bitsin the rock are

these are the fossils

this is the thick layer youcan see in front of you

‘resedimented’ meanspicked up and carried bythe sea floor current,then deposited

‘intraclasts’ are bits ofnew rock torn up anddeposited

‘peloids’ - pellets

Q: How manytypes and sizesof tracks canyou see?

We do not know what the sea floor wasreally like - but this looks like the end for some trilobites.

1 2 3 4 5

1

25

4

3

Original diagramcourtesy ofthe JNCC

‘calci’ means the rock is alimestone - made of calcite