[Aqualist] the (in)definitive silt boundary, and a further problem

[Paul Hesse]

Thanks to all who responded to my message yesterday.  I thought the
responses might be of general interest.

Paul



The Vote (who actually uses what)

62.5* microns	(4 phi; *reported as 62, 62.5, 63, 64)		17 people
32 microns	(?5 phi; 31.25 microns)				1 person
20 microns							2 people

Other variations mentioned (but not necessarily used by the reporter)
50 microns (World Soils Book, Bridges, 1978 - Uni Cambridge Press)
16 microns ('The bible': Bodem en bemesting - Commission for soils 1939)
Later revised to 50 microns  (i.e. Dutch usage.)
60 microns (in NZ soil science since 1991.  Previously the old
'international' boundary of 20 microns was used).
20 microns (Australian soil texture classification, based on Int. Soc. Soil
Sci system.  See Marshall, T.J., Aust. J. Soil Res., 2003. 41: 245-249, and
references therein.)

Justifications (theoretical)
62.5 microns.  Because if you also use the phi scale (where 0 phi = 1
mm)for descriptive purposes, 62.5 occurs at - 4.0 phi and is the very fine
sand/silt boundary, -3 phi the fine sand /very fine sand boundary etc.
None of the other values you indicated fit a whole phi unit. (Roger McLean)

The 63 micron boundary is part of a geometrical progression that makes some
sense numerically. (Stephen Gale)

63 microns: based on the Wentworth scale (Leanne Armand's techo)

20 microns: the International system based on Atterberg limits being the
log scale 2.0 mm, 0.2mm, .02mm and .002mm. Has the logic of the log scale
size variation. (Eric Colhoun)

What theory tells us
You can justify your choice, whatever it is.
There are adherents to a log base 2 system (the majority) and adherents to
a log base 10 system (the minority)

Justifications (practical)
Geoscience Australia have a very handy litle class card, with ruler, that
puts the cut off between silt and sand at < 62 microns (ie silt is classed
between 3.9 and 62 microns).
(Kate Harle)

1. So far as I'm aware, 20 and 50 micron sieve meshes aren't easily
available, though the 74.3 micron mesh is pretty close to the coggies' 75
micron boundary.
2. The 63 micron boundary is part of a geometrical progression that makes
some sense both numerically and in terms of proprietary sieve intervals.
(Stephen Gale)

I don't use 63 microns, wouldn't use 75 microns, haven't actually used 50
microns although my US/Canadian reference card puts 62 microns as bottom of
very fine sand and guess it follows that silts starts at the next
seive-step down - in quarter-phi steps this would be 52 microns. However
and inconsistently I habitually use 32 microns as the top of the silt
range!!! (John Chappell)  (this response has not been edited except to
remove the word 'bugger')

The practical application of this [the Udden & Wentworth classification] is
that in working in the Quaternary coastal environments of northern NSW and
southeast Queensland, particularly sand dune systems up to 98% of my
samples fall within the sand fraction as defined as > 63 micron. (Maria
Cotter)

More justifications (the Nuremberg defence)
(all identities have been concealed to protect professional reputations,
but the number of Xs is a hint)

I have always used 63 microns as the sand/silt boundary- thats the boundary
that has always been taught in quaternary science, coastal geomorphology
and sedimentology pracs at XXXXXX Uni

63 microns....from XXXXXXXXX Uni PSA that we were taught when we were kids,
wet sieving to separate sands from silts and clays!

63 micron - is what I use (but not sure why?) XXXXX

We have always used 63 microns.  XXXXXX varsity.

[63 microns] BUT I use this because that is what I was taught and never
really considered it an issue.  XXXXXXXXXX Uni

Disciplinary (National?) differences
although most respondents used the 62.5 micron cutoff, there are clearly
some disciplinary differences.  Both respondents who use the 20 micron
cutoff have been known to deal with pollen and microfossils but it's not
clear if that is a disciplinary bias as a confirmed palynologist does not.
20 microns, Australian soil science
50 microns, American and Dutch soil science
60 microns, NZ soil science (David Lowe) and British?
Volcanology 63 microns is boundary between medium and fine ash. (David Lowe)
tsunami and storm surge literature uses 63um (Adam Switzer)
63 µm for our sea sediment samples (Leanne Armand, Antarctic CRC)

According to the Glossary of Geology - Bates & Jackson (3rd Edition) there
are two definitions for silt  - one for sed and another for soil.
silt (sed) -  a rock fragment or detrital particle smaller than a very fine
sand grain  and larger than a coarse clay, having a diameter in the range
of 1/256 to 1/16 mm (4-62 microns, or 8 to 4 phi units)
silt (soil)  - a rock or mineral particle in the soil having a diameter in
the range of 0.002-0.05 mm. The diameter range recognised by the
International Society of Soil Science is 0.002 - 0.02 mm

Eric Colhoun, in support of the 20 micron (Atterberg) scale says; 'I am
aware of the others but I think this is the most widely used in Geomorph
and Geol circles'.

A New Controversy?
Kate Harle's response hints at another issue I had never considered: what
is the silt/clay boundary?
Stephen Gale notes 'An even more widespread gulf exists in the location of
the silt-clay boundary (there's a comment on this in Gale and Hoare's
'Quaternary Sediments' p. 60)'.

Apparently Geoscience Australia use the 3.9 micron (8 phi) boundary (as did
Folk, 1968).

I have always used 2 microns (9 phi) which I have always regarded as
'standard' (the Nuremberg defence dressed up).  Brent Alloway has
highlighted the difference in his response (above) with a '4 micron'
boundary for sediment and a 2 micron boundary for soil.  Personally, I
can't conceive how we can continue to think of sediment and soil as being
unrelated materials.

Final words of wisdom
I use and strongly suggest (ie insist) that my postgraduate research
students use 63 microns.
ROGER F MCLEAN

I guess you're covered if you define your cut-off.  In all soil or sediment
work I do I use the 62 um cutoff
Brent Alloway

The US has always been out of step with most things! (J. Goff)

References (not mentioned above)
Reineck & Singh 1980, Dep sed env., says nothing
Goudie, 1990. Geomorph techniques,  mentions the 63 micron sieve.
Gale and Hoare's 'Quaternary Sediments'
Folk's "Petrology of Sedimentary Rocks" puts the boundary at 62.5 micron.

Contributors
Eric A. Colhoun, Newcastle University
Jim Bowler, Melbourne University
Nikki Franklin, Sydney University
Scott Mooney, UNSW
Mike Sheperd, Massey University
Trish Fanning, Macquarie
Henk Heijnis, ANSTO
John Carter, VUW
David J. Lowe, Waikato University
J Chappell, ANU
Nerida Bleakley, UTas
James R. Goff, Geoenvironmental Consultants, NZ
Adam Switzer, Wollongong
Craig Sloss, Wollongong
Brent Alloway, GNS - NZ
Stephen Gale, Sydney University
Kate Harle, ANSTO (soon to be CSIRO)
Roger F Mclean, ADFA
Maria Cotter, UNE




     *****************************************
          Dr Paul Hesse

           Senior Lecturer in Physical Geography

           Department of Physical Geography
           Macquarie University, Sydney
           NSW 2109
           Australia
           Phone  (+61)  02-9850 8384  Fax. (+61) 02-9850 8420
           e-mail  phesse at laurel.ocs.mq.edu.au
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