Friday, December 24, 2010
Paul Karrow
Department
of
Earth
and
Environmental
Sciences,
University
of
Waterloo
A
previous
account
in
Wat
on
Earth
in
1995
described
kettle
holes
as
ice
block
depressions
formed
during
glacial
retreat,
where
stranded
blocks
of
glacial
ice
became
partly
or
completely
buried
by
meltwater
deposits
(outwash
sand
and
gravel).
When
the
ice
block
later
melted,
a
depression
was
left
in
the
ground
surface,
forming
a
kettle
hole.
A
second
kind
of
surface
depression
is
found
in
solid
bedrock,
and
appears
as
circular/cylindrical
erosional
forms
caused
by
rapidly
flowing
water
(Figures
1
and
2).
These
are
called
“potholes”.
Where
there
are
(or
were)
eddies
where
fast
water
vortices
entrained
cobbles
or
small
boulders
of
hard
rock,
such
as
fast-moving
streams
generated
from
melting
glaciers,
the
sediment
load
acts
as
a
large
abrasive
drill
and
can
bore
into
the
rock
surface.
Some
well
rounded
clasts
may
be
found
in
the
bottom
of
a
pothole,
possibly
left
behind
when
stream
flow
conditions
changed
and
the
pothole
was
abandoned.
The
wall
of
a
pothole
may
bear
grooves
or
flutes,
usually
spiraling
downward
and
formed
by
the
swirling
sediment-laden
water.
Also
sometimes
seen
is
a
low
central
hump
on
the
floor
of
the
pothole,
around
which
the
water
swirled.
Figure 1. Earth and Environmental Sciences professor Maurice Dusseault is seen on the “central island” remaining within a large pothole developed on banded gneisses of the Precambrian Grenville Province at Montmorency Falls, Quebec.
As
with
some
other
technical
terms,
“pothole”
may
be
applied
by
the
layperson
in
various
non-technical
and
unrelated
ways.
Thus
the
term
“pothole”
is
sometimes
used
by
farmers
in
referring
to
kettle
holes
in
their
fields.
City-dwellers
are
also
familiar
with
the
word
as
applied
to
splash-excavated
holes
in
pavement.
In
fact,
the
cities
of
Kitchener
and
Waterloo
have
specific
phone
numbers
to
call
to
report
road
pothole
problems!
Potholes
can
form
in
any
solid
rock
and
are
widely
distributed
in
Ontario
and
elsewhere.
They
may
occur
singly,
or
in
numbers
of
several
tens
or
even
hundreds.
Publications
describing
occurrences
in
Ontario
date
from
the
late
nineteenth
to
late
twentieth
centuries,
with
the
earliest
lacking
any
illustrations.
McKellar
(1890)
described
a
group
of
50
potholes
inland
of
Lake
Superior
in
“dark
green
hornblende
rock”
considered
to
be
Huronian
(Precambrian)
schist.
Some
20
of
the
potholes
are
individually
described,
with
diameters
ranging
from
over
1
m
to
18
m,
with
depths
commonly
unknown
because
of
partial
infill
by
sediment
and
vegetation,
but
exposed
to
at
least
6
m.
Stanley
(1934)
described
49
potholes
in
Huronian
quartzite
of
the
La
Cloche
Mountains,
north
of
Little
Current
on
Manitoulin
Island.
Several
maps
and
photographs
are
included.
These
potholes
range
from
0.5
m
to
4.5
m
in
diameter
with
visible
depths
up
to
6
m.
He
also
noted
several
potholes
in
similar
quartzite
at
Whitefish
Falls,
about
15
km
east
of
the
larger
group.
Kor
et
al.
(1991)
make
general
reference
to
potholes
as
part
of
a
suite
of
streamlined
erosional
forms
in
northeastern
Georgian
Bay,
centered
near
the
French
River.
Bedrock
of
the
area
spans
the
Grenville
Front
and
includes
highly
metamorphosed
gneissic
to
granitic
rock.
Kor
et
al.
(1991)
attribute
the
suite
of
erosional
forms
to
large
releases
of
sub-glacial
meltwater
from
the
north.
Kor
and
Cowell
(1998)
made
similar
reference
to
potholes
in
the
Bruce
Peninsula,
developed
in
Silurian
dolostone,
with
comparable
origin
in
large
subglacial
floods.
A
few
potholes
are
shown
in
photographs
and
some
occur
on
the
cliff
of
the
Niagara
Escarpment.
The
earliest
description
of
potholes
was
that
of
Panton
(1888)
for
a
few
of
a
large
group
at
Rockwood
(east
of
Guelph)
in
Silurian
dolostone
associated
with
small
caves.
The
area
of
potholes
has
been
included
in
a
park
by
the
Grand
River
Conservation
Authority.
Here,
as
well
as
in
other
pothole
groups,
some
are
composite
or
overlapping,
suggesting
sequential
phases
of
pothole
development,
with
later
ones
superimposed
on
older
ones.
The
Rockwood
potholes
are
in
more
populated
southern
Ontario,
readily
accessible
to
the
public.
The
highest
in
elevation
and
largest
–
and
presumably
oldest
pothole
at
Rockwood
–
is
locally
known
as
“the
Devil’s
well”
and
had
its
“15
minutes
of
fame”
in
1983
when
it
was
cited
as
“the
world’s
largest
glacial
pothole”
by
the
Guiness
Book
of
World
Records,
but
mislocated
at
Bloomingdale
(north
of
Kitchener)
where
there
is
no
bedrock
exposure
(McKenzie
1994).
The
record
size
claim
is
dubious
(6.4
m
wide
and
13
m
deep)
as
McKellar
(1890)
describes
larger
ones,
and
a
world
record
needs
fuller
documentation.
This
pothole
is
additionally
interesting
because
excavation
at
its
bottom
has
revealed
an
apparent
till,
suggesting
it
was
formed
before
the
last
ice
advance.
A
more
detailed,
well
illustrated,
and
recent
description
of
the
Devils
Well
was
provided
by
A.V.
Morgan
in
Wat'on'Earth
in
2001.
Most
recently,
Kunert
and
Coniglio
(2002)
consider
alternate
origins
for
the
Rockwood
features
and
conclude
that
they
are
true
potholes,
formed
during
late.glacial
retreat
by
meltwaters.
Other
accessible
places
to
see
potholes
include
the
Niagara
Gorge
area,
at
Warsaw
caves
near
Peterborough,
and
at
Chippewa
Falls
north
of
Sault
Ste.
Marie.
References
Kor,
P.S.G.,
and
Cowell,
D.W.,
1998.
Evidence
for
catastrophic
subglacial
meltwater
sheetflood
events
on
the
Bruce
Peninsula,
Ontario.
Canadian
Journal
of
Earth
Sciences
35,
1180.1202.
Kor,
P.S.G.,
Shaw,
J.,
and
Sharpe,
D.R.,
1991.
Erosion
of
bedrock
by
subglacial
meltwater,
Georgian
Bay,
Ontario:
a
regional
view.
Canadian
Journal
of
Earth
Sciences
28,
623.642.
Kunert,
M.
and
Coniglio,
M.
2002.
Origin
of
vertical
shafts
in
bedrock
along
the
Eramosa
River
valley
near
Guelph,
southern
Ontario.
Canadian
Journal
of
Earth
Sciences,
v.
39,
p.
43.52.
McKellar,
P.,
1890.
On
pot.holes
north
of
Lake
Superior
unconnected
with
existing
streams.
Bulletin
Geological
Society
of
America
1,
568.570.
McKenzie,
D.I.,
1994.
Quaternary
geology
of
the
Waterloo
area,
Ontario.
Geological
Association
of
Canada
Field
Trip
A6
Guidebook,
46p.
Panton,
J.H.,
1888.
The
caves
and
potholes
at
Rockwood,
Ontario.
Proceedings
of
the
Canadian
Institute,
3rd
series,
6,
244.253.
Stanley,
G.M.,
1934.
Pleistocene
potholes
in
the
Cloche
Mountains
of
Ontario.
Papers,
Michigan
Academy
of
Science,
Arts,
and
Letters
19,
401.416.