Sieving the sands of time

Grain size refers to the diameter of individual grains of sediment in ‘clastic’ or sedimentary rocks.

As a rule of thumb, grain size reflects the energy of the environment a rock forms in. Because of this wonderful fact of physics, the proportion of different grain sizes in a fossil-baring rock can help palaeontologists piece together a picture of the environment their fossils formed in. Was it a quiet burial by a lakeside, or a tumultuous end in the pits of a mountainous river channel?

Generally speaking, higher energy water or wind currents are required to move larger grain sizes.

The degree of size-sorting a.k.a. how uniform the size of the grains is, can also indicate the energy of the depositional environment; well-sorted sediments tend to indicate higher energy depositional settings.

Sample e.g. with grain size graphic
A visual example of grain size analysis, with the number of grains (frequency, on the y-axis) in each size category graphed against grain size (x-axis). Peaks in the graph, as well as the splay of grains either side, hold the key for geologists using this technique.

Picture the kinds of environments you might see grains of these sizes laying around, and you’ll be on your way to thinking like a sedimentary geologist:

  • boulders, >256 mm
  • cobbles, 64-256 mm
  • pebbles, 4-64 mm
  • coarse sand, 2-4 mm
  • sand, 1/8-2 mm
  • silt, 1/256-1/8 mm
  • clay <1/256 mm

 

Patients

Rock conservation’ has a certain ring to it… a ring that registers amongst the highest calibre of beyond-boring, watching-grass-grow type activities. It’s the kind of stuff we hope our dreams are never made. Not sending emails, not ironing, not rock conservation. These activities, we suppose, seep in and infest our colourful humanity with stupefying fuddy-duddyness, changing us from people into some embodiment of dreary.

The term ‘rock conservation’ refers to the treatment and storage of geological material in order to prolong its natural life. It is a curatorial process essential to the maintenance of important scientific resources worldwide.

“…these patients will help form the foundations of palaeontological research that will enrich and expand our understanding of life on this sweet planet we call home.”

Largely, the process involves an assessment of the stability of the rock patient in question: whether it is rich in clay minerals and likely to delaminate and crumble with time, whether it has through-going cracks or fossil roots that will encourage it to fall apart when handled, etc.. This assessment, and the treatment to be administered in response, is carried out by one we might affectionately (and very colloquially) call the ‘Rock Doctor’.

As the resident Rock Doctor in my lab, I paint my patients with paraloid B67, which seeps into the pore spaces between grains and holds them together like liquid bandaid. I take carefully measured strips of cotton bandage and wrap the rocks, to act as a splint and reinforce the paraloid. I patch broken pieces back to their other halves with epoxy resin. And I make custom-fit straight jackets using polyurethane foam, to hold the bigger rock slabs tightly together while they are split to reveal as-yet unseen fossil layers.

Although the execution of these tasks is roughly three quarters fuddy-dreardom, the remaining quarter of a rock doctor’s role can only be described as a shiny, rather pleasant privilege.

The privilege has it’s genesis here; these patients will help form the foundations of palaeontological research that will enrich and expand our understanding of life on this sweet planet we call home. Whether as small puzzle pieces falling into shapely gaps, or as ripples that perpetuate outwards, crossing paths with other concepts and creating something novel, the information extracted from these fossils can be invaluable. And this is true whether we seek to gaze into the labyrinth of deep time, or to look into the possible futures that extend before us.

Furthermore, a minority of lucky rocks that pass through the hospice of rock doctors find a moment in the limelight, featuring in geology exhibits that seek to capture the hearts and minds of public visitors. Although my patients are unlikely to see any such fame (destined for storage in the GNAS collection outside Wellington, New Zealand, or offshore on the Chatham Islands from which they hark), it is the insightful journey they travel in the interim that makes my patients special.

For the potential insights they conceal, my patients are worthy of the utmost care, considered treatment and delicate love.

Arcing from mundane to wondrous, I find myself with the utmost patience for these rock doctor tasks.

 

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