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2005-11-28

Day in the (death) of a worm

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Just been digging around in the rock pile, and I found this rock which I'd recently described in a USENET post:

The cohesiveness of consolidated mud would 
provide a comparatively resistant surface over 
which coarser-grained sediments could be 
transported as a package. 
Actually, thinking about it, I've a nice specimen 
in my rock pile showing a similar event from 
100-odd million years ago. Mid-jurassic of the 
Isle of Wight, there was a muddy sea-bottom 
with a rich fauna of burrowing worms chomping 
away buried in their nice muddy burrows. Then 
along came a flood of silty sand which buried 
and killed off the worms leaving a series of 
sandy casts of the top ends of the burrows on 
the *under* surface of a circa-2cm thick sand 
bed.
One hundred million years later along comes a 
superannuated hippie of a geology student who 
extracts said thin sand bed from a 10-odd metre 
high cliff of similarly interbedded muds and 
sands, and carefully appreciates the day in 
the death of these worms before spending 
hours in the pub (and back in the honours 
year student labs) carefully picking out the 
bits of clay from between the casts of the 
burrows.

"AA" size cell and a ruler to give scale objects. The autofocus of the camera seems to be slipping, and there's no manual alternative. I'm thinking that I can almost justify getting a new camera.

2005-11-29
Carsten described these as "escape structures". I don't think so - I think they're burrows which the worms (lugworm or equivalent) died in after getting a belly full of sand. The underlying mudstone bed contains sub-vertically oriented sand tubes dispersed through the mud, representing the worms which swallowed a sand meal, stopped eating, swallowed (separating the swallowed sand from the continuous sheet), then died.
Please note that this photograph is of the UNDERSIDE of the slab as originally found, in a normally oriented sediment sequence.

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