Friday, December 24, 2010
Paul Karrow
Department of Earth and Environmental Sciences, University of Waterloo
Marl is a soft, light-coloured (white to pale gray or) mud-like sediment, seen in shallow waters of some small lakes or ponds, or under swamp lands, where it typically is covered by black organic sediment (Figure 1).
Marl
forms
through
the
combined
action
of
algae
(e.g.
Chara)
and
other
aquatic
plants
and
animals
living
in
hard
water
streams
and
springs,
rich
in
dissolved
calcium
carbonate.
Such
conditions
are
associated
with
bedrock
such
as
limestone
or
dolostone,
or
Precambrian
marble,
and
with
calcareous
drift
derived
from
them
(Figure
2).
Large
areas
of
the
Precambrian
Shield
are
covered
with
calcareous
drift
derived
by
glacial
erosion
of
carbonate
bedrock
under
the
Hudson
Bay
Lowland
and
spread
outward
onto
the
Shield.
Such
widespread
occurrence
of
calcareous
drift,
even
on
large
areas
of
the
Precambrian
Shield,
has
led
to
problems
in
radiocarbon
dating
because
of
the
incorporation
of
old
Paleozoic
carbon
in
lake
sediments
and
associated
aquatic
plants.
This
has
resulted
in
radiocarbon
dates
which
may
be
too
old
by
several
thousand
years
(the
“old
carbon
problem”
or
“the
limestone
effect”).
To
avoid
this
problem,
careful
selection
of
terrestrial
plant
material
must
be
made
for
dating.
Unfortunately,
hundreds
of
erroneous
ages
have
been
published,
distorting
the
history
of
ice
retreat
and
vegetative
change
over
postglacial
time.
In
the
late
19thand
early
20th
centuries,
marl
was
a
valued
mineral
deposit
used
for
cement
manufactur
(Guillet
1969).
In
1907,
13
cement
plants
in
southern
Ontario
produced
portland
cement
from
marl,
and
interest
continued
long
after
its
use
was
superseded
by
limestone
quarrying.
Guillet
(1969)
described
a
variety
of
uses
for
marl
and
reported
on
107
Ontario
deposits.
Provincial,
state,
and
federal
organizations
provide
information
on
deposits
elsewhere,
and
marl
deposits
are
known
across
Canada.
Marl
deposits
have
been
subjected
to
various
kinds
of
scientific
study.
They
provide
good
study
sites
for
recovery
of
a
variety
of
remains
of
fossil
plants
and
animals
to
allow
analysis
of
changing
environments
in
postglacial
time.
One
of
the
most-studied
marl
sites
(now
covered
by
buildings)
is
the
Gage
Avenue
site
in
Kitchener,
Ontario,
where
contained
plants
(pollen
and
macrofossils)
and
insects
were
described
by
Schwert
et
al.
(1985)
and
molluscs
by
Yang
et
al.
(2001).
The
marl
there
was
about
4
m
thick
and
yielded
176
types
of
insect
fossils,
36
types
of
pollen
grains,
49
types
of
plant
macrofossils
(seeds,
needles,
etc.)
and
25
types
of
molluscs.
As
mentioned
above,
some
of
the
radiocarbon
dates
from
this
site
were
affected
by
the
“old
carbon
problem”
(e.g.
17,200
years
and
20,000
year
ages
were
determined
near
the
base
of
the
marl),
yielding
dates
that
are
thought
to
be
more
than
4000
years
too
old.
Probably
the
most
notable
kinds
of
fossils
found
in
marl
deposits
are
the
bones
of
large
extinct
mammals
such
as
mastodons
and
mammoths,
and
even
giant
beaver!
These
discoveries
are
accidental,
as
farmers
dig
ditches
to
drain
low
wet
areas,
or
excavate
to
create
stock
ponds.
The
bones
are
often
found
in
the
upper
part
of
marl
deposits,
which
would
be
soft,
mushy
ground
where
the
animals
may
have
wandered
in
and
got
into
trouble.
More
than
a
hundred
such
sites
have
been
reported
in
southern
Ontario
alone.
The
most
famous
of
these
is
the
Highgate
mastodon,
southwest
of
London,
Ontario.
See
the
history
of
this
site
on-line
through
Google
and
in
a
popular
book
from
the
Earth
Sciences
Museum,
University
of
Waterloo
(Russell
et
al.,
2010).
Acknowledgements
Thanks
to
the
Ontario
Geological
Survey
for
permission
to
use
the
photo
of
the
workings
in
Wilder
Lake
and
to
Tom
Arnott
for
information
on
the
Wilder
Lake
and
the
present
view
of
the
lake.
References
Guillet,
G.R.,
1969.
Marl
in
Ontario.
Ontario
Department
of
Mines,
Industrial
Mineral
Report
28,
137p.
Russell,
P.,
Hoganson,
J.,
Karrow,
P.,
and
Motz,
J.,
2010.
A
mastodon
in
a
biscuit
box.
Earth
Sciences
Museum,
University
of
Waterloo,
20p.
Schwert,
D.P.,
Anderson,
T.W.,
Morgan,
A.,
Morgan,
A.V.,
and
Karrow,
P.F.,
1985.
Changes
in
late
Quaternary
vegetation
and
insect
communities
in
southwestern
Ontario.
Quaternary
Research
23,
205.226.
Vreeken,
W.J.,
1981.
Distribution
and
chronology
of
freshwater
marls
between
Kingston
and
Belleville,
Ontario.
Canadian
Journal
of
Earth
Sciences
18,
1228.1239.
Yang,
J.,
Karrow,
P.F.,
and
Mackie,
G.L.,
2001.
Paleoecological
analysis
of
molluscan
assemblages
in
two
marl
deposits
in
the
Waterloo
region,
southwestern
Ontario,
Canada.
Journal
of
Paleoliminology
25,
313.328.