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Mount
Rogers,
the
(very)
early
days
Back
in
the
Cambrian
and
the
Early
Ordovician
epochs,
close
to
five
hundred
million
years
ago,
the
landscape
of
the
ACT
and
Eastern
Australia
didn't
exist.
The
continental
landmass
of
Gondwana,
that
included
Australia,
was
bounded
by
marginal
sea‐basins
(back‐arcs),
with
volcanic
island
arcs
on
the
outward,
oceanic
side,
resembling
today’s
island
arcs
in
the
Caribbean,
Japan,
The
Philippines,
New
Guinea
and
the
Solomon
Islands,
Tonga
to
New
Zealand,
and
the
Indonesian
island
arc
in
the
Indian
Ocean.
Outside
them
the
planet's
crust
underneath
the
great
world‐
ocean
boiled
slowly,
like
porridge
in
a
pot,
moving
an
inch
or
two
every
year
toward
the
continent,
pushing
against
the
island‐arc
rampart
and
eventually
plunging
underneath
it.
The
hard
rocky
crust
of
the
islands
scraped
the
top
layer
of
oceanic
ooze
off
the
moving,
plunging
ocean
crust,
much
as
a
bulldozer
blade
scrapes
the
soil
off
the
rocky
subsurface.
The
ooze
contained
the
siliceous
skeletons
of
oceanic
radiolarians
‐
no
calcite
skeleton
could
survive
at
these
abyssal
depths
under
several
kilometers
of
ocean
water.
Eventually
the
pile
of
ooze
‐
the
Pittman
Formation
and
the
State
Circle
Shale
‐
built
up
and
was
pushed
on
to
the
lower
margins
of
the
island
arc
‐
called
the
MacQuarie
Arc
‐
in
gentle
folds.
This
deepwater
terrain
was
the
fore‐arc
basin.
As
the
Ordovician
ended
and
the
Silurian
epoch
began,
around
four
hundred
and
thirty
four
million
years
ago,
the
shale
was
covered
by
sandy
sediment
‐
the
Black
Mountain
Sandstone
‐
brought
to
the
coasts
of
the
MacQuarie
Arc
by
rivers
and
cascading
down
the
ramp
of
the
offshore
slope
to
fill
the
muddy
trough
at
the
bottom.
Thus
was
laid
the
foundations
of
eastern
Australia
and
New
Zealand.
The
pile
of
sediments
of
the
fore‐arc
basin,
eventually
extending
hundreds
of
kilometres
to
the
east
of
the
ACT
and
getting
ever
thicker,
was
pushed
up
the
slope,
and
the
coral
reefs
of
the
Canberra
Formation
began
to
grow
in
the
shallowing
water
around
the
island
arc.
The
area
that
would
become
the
ACT,
pushed
along
like
a
surfer
on
a
wave,
overlaid
the
plunging
oceanic
plate
which,
hundreds
of
kilometres
below,
had
melted
and
was
sending
plumes
of
volcanic
magma
up
toward
the
surface
as
the
planet
laboured
to
shed
the
heat
generated
by
the
Core.
A
dog
loses
the
heat
generated
in
its
active
muscular
body
by
panting.
The
Earth
does
the
same,
but
her
exhalations
each
last
a
million
or
more
years.
You
can
see
the
process
most
clearly
today
on
the
island
of
Tanna,
or
at
Rotorua.
A
dog
goes
through
cycles
of
panting
and
silence.
The
Earth’s
equivalent
is
called
an
orogenic
cycle
and
lasts
about
50
million
years
before
falling
quiet
for
about
10
million,
then
beginning
again
as
the
heat
builds
up
inside.
The
episode
described
above
is
named
the
Benambran
Orogenic
Cycle.
Eventually,
around
430
million
years
ago,
the
subduction
zone
at
the
ocean/island‐
arc
plate
boundary
became
clogged
with
sediments
and
volcanics
and
jammed
by
irregularities
and
entire
island
chains
forced
up
against
the
plate
boundary.
The
island
barrier
did
retreat
before
this
horizontal
pressure,
but
slowly
and
reluctantly,
compressing
the
back‐arc
basin
behind,
which
was
now
filled
up
with
sediments
carried
down
by
rivers
from
both
the
continental
landmass
itself
and
the
mountains
pushed
up
along
the
island
arc.
The
Canberra
area
had
risen
above
sea
level.
The
Pacific
Ocean
plate,
stalled,
blocked
and
unable
to
move,
retreated
oceanward
to
the
eastern
edge
of
the
mountainous
mass
of
sediments
and
volcanics
and
began
to
descend
there,
far
to
the
east
of
the
ACT.
The
crust
above
the
now
dislocated
plunging
plate
under
the
ACT,
relieved
of
the
compressive
pressure,
relaxed
and
stretched.
The
stretched
crust
fractured
and
the
Canberra
area
sank
down
into
a
rift,
a
valley
bounded
by
faults.
The
abandoned
oceanic
plate
below
it
continued
its
plunge
toward
the
Core,
melting
and
sending
plumes
of
molten
volcanic
magma
and
bosses
and
mushroom‐shaped
batholiths
of
molten
granite
upwards,
seeking
out
every
crack
and
cranny
in
the
crustal
rocks,
infiltrating
them
with
tendrils
of
superheated
hydrothermal
fluid,
hydraulically
fracturing
them
and
finally
forcing
them
aside
to
make
way
for
the
ascending
volcanic
lava
and
the
granite.
The
sinking
Canberra
Rift
with
its
marginal
extensional
faults
provided
a
ready
access
for
the
ascending
volcanic
plumes.
The
event
from
428
to
about
370
million
years
BP
is
called
the
Tabberabberan
Orogenic
Cycle.
At
the
beginning
of
this
cycle,
for
about
four
million
years
from
428
to
424
Ma
BP,
the
Canberra
area
was
subjected
to
recurrent
eruptions
of
the
Ainslie
Volcanics,
the
Walker
Volcanics,
the
Mount
Painter
Volcanics,
and
it
is
this
Middle
Silurian
suite
of
rocks,
collectively
called
the
Hawkins
Volcanic
Suite
(Smh
on
geological
maps,
or
just
Sv)
that
constitutes
Mount
Rogers
and
much
of
the
ACT.
The
Capital
Territory
was
part
of
the
‘ring
of
fire’
around
the
Pacific
rim,
which
is
still
going
strong,
now
more
than
three
thousand
kilometres
to
the
east
and
north‐east
of
us,
extending
in
our
vicinity
from
New
Guinea,
through
the
Solomon
Islands
to
Vanuatu
and
from
Tonga
to
New
Zealand.
While
the
volcanics
broke
out
on
to
the
surface,
above
sea
level,
sometimes
in
great
explosive
eruptions,
the
granitic
magma
rising
from
the
plutonic
depths
lodged
and
solidified
some
five
or
more
kilometers
below
it.
Like
a
giant
mushroom
it
pushed
up
the
country
rocks
above
it,
and
this
stretched
and
broken
cap‐stone
was
vulnerable
to
erosion.
There
was
little
or
no
vegetation
on
the
land
in
this
early
period
of
biological
history,
so
the
process
of
erosion
and
removal
by
rain
and
rivers
proceeded
faster
than
it
would
today.
Kilometres
of
surface
rock
was
quickly
removed,
and
the
now
cold
granite
was
eventually
exposed
on
the
surface
of
the
land.
One
of
the
biggest
granite
masses
became
the
Murrumbidgee
Batholith
whose
frost‐broken
top
we
admire
in
the
Namadgi
National
Park,
with
names
like
Legoland,
The
Spinnaker
and
Gorilla
Rock.
The
resulting
complex
of
plutonic,
volcanic
and
sedimentary
rocks
is
the
eastern
part
of
the
Lachlan
Fold
Belt,
one
of
a
number
of
structural
units
that
make
up
Eastern
Australia.
If
you
study
a
piece
of
the
rock
found
today
on
and
around
Mount
Rogers
‐
any
little
bit
will
do,
and
it
will
show
better
if
you
wet
it
‐
you
will
see
it
is
a
greenish‐grey
to
purplish
colour,
with
small
light‐coloured
fragments
of
irregular
shape
scattered
through
it.
It
resembles
the
deposits
found
around
explosive
volcanoes
of
the
type
that
destroyed
Pompeii
and
Mount
Saint
Helens
and
that
periodically
causes
havoc
on
Caribbean
and
other
island
chains.
This
dacite,
rhyolite,
andesite
and
‘burning
cloud’
ignimbrite
is
characteristic
of
volcanics
that
have
forced
their
way
through
the
upper
crust
and
sediment.
It
is
quite
unlike
the
black
basaltic
lava
that
flows
comparatively
quietly
out
of
volcanoes
on
oceanic
islands
like
Hawaii
or
Iceland.
That
is
dark,
basic
lava
straight
from
the
Earth's
mantle
and
it
flows
like
melted
glass;
this
is
lighter‐coloured,
acidic,
viscous
and
explosive.
It
comes
to
the
surface
full
of
gases
that
have
not
been
able
to
escape
through
the
thick,
viscous
lava,
and
that
now
do
so
with
violent
results.
This
marked
acidic
tendency
must
select
and
influence
the
plants
that
grow
on
the
mountain.
The
first
land
plants
began
at
this
time,
in
the
Silurian
Period,
including
a
famous
one
that
towered
above
its
understorey
to
a
height
of
at
least
an
inch,
25
to
30
millimeters,
a
veritable
Silurian
cedar.
It
is
known
around
the
world
under
the
name
of
Cooksonia
–
nothing
to
do
with
the
famous
sea‐captain,
its
discoverer
was
Elizabeth
Cookson,
an
Australian
paleobotanist.
It
is
briefly
described
(with
life‐
sized
sketch)
in
the
Geological
Guide
to
Canberra
(Finlayson
et
al,
the
principal
source
for
this
article,
see
Refs
below)
and
the
revolution
these
plants
triggered
in
the
Earth’s
ecology
is
canvassed
on
page
28.
The
next
time
you
see
a
lichen‐covered
rock
on
the
mountain,
have
a
closer
look.
The
variety
and
vitality
of
lichens
may
surprise
you.
Speculative
literature
sometimes
canvasses
the
possibility
of
‘terra‐
forming’
Mars,
making
it
more
like
Earth.
On
the
lichenous
rocks
of
Mount
Rogers
you
are
looking
at
the
descendants
of
the
pioneers
that
terraformed
the
Earth.
We
do
not
know
what
happened
on
Mount
Rogers
in
the
ages
that
followed
the
Silurian,
because
the
area
remained
above
sea
level
until
today,
and
the
solid
evidence
of
history
was
removed
by
erosion.
Some
20
million
years
later
and
40
km
to
the
north‐west
of
Mount
Rogers
a
shallow
sea
laid
down
exceptionally
fine‐
grained
limestone
that
entombed
and
replaced
the
bodies
of
fish
with
the
finest
details
preserved.
These
miraculously
preserved
specimens
from
the
Early
Devonian
(Emsian)
Taemas
Limestone
of
Wee
Jasper
and
on
Lake
Burrinjuck
are
world‐famous.
They
reveal
the
detail
of
development
of
early
eyes,
ears,
electromagnetic
senses
and
the
intimate
details
of
their
brain‐case
and
nervous
systems.
They
have
been
described
by
Gavin
Young
of
the
Research
School
of
Earth
Sciences
at
the
Australian
National
University.
These
four‐hundred
million
year‐old
fish,
the
placoderms,
are
the
ancestors
of
four‐legged
land
creatures.
The
armour
in
which
their
bodies,
and
even
their
eyes,
were
encased,
shows
what
a
competitive
and
dangerous
environment
they
lived
in
–
giving
them
a
sustained
and
strong
incentive
to
leave
the
sea
and
walk
upon
the
land.
One
of
their
predatory
competitors,
now
in
the
ANU’s
fossil
collection
and
not
normally
accessible
to
the
public,
is
formidable
enough
to
endanger
any
man‐size
creature
that
came
within
range.
Because
Australia
has
no
national
home
for
these
precious
fossils,
the
very
best
of
them
are
now
lodged
and
jealously
guarded
in
the
world’s
leading
natural
history
museums.
Australian
scientists
must
go
to
places
like
London
and
ask
permission
to
study
them
there
–
they
are
just
too
valuable
to
be
lent
to
Australia.
Darwin’s
famous
question
‘where
are
the
early,
transitional
models
of
the
modern
eye’
is
or
will
be
answered
in
part
by
these
priceless
indicators
of
our
continent’s
(and
our
planet’s)
biological
history.
In
the
east
the
plate
margin
again
and
again
stepped
back
to
one
side
or
the
other
of
the
compressed
mass
blocking
the
subduction
zone
at
the
ocean/island
arc
boundary.
The
diagram
on
p17
of
Finlayson
and
co‐authors
(reference
below)
shows
this
happening
during
one
short
period
of
this
complex
history.
After
one
of
these
plate
reorganisations
the
Tasman
Sea
began
opening
up.
New
Zealand
drifted
away
to
the
east.
The
eastern
Australian
coastal
rim
moved
slowly
upward,
forming
the
elevated
scarp
and
plateau
that
we
cross
to
get
to
the
coast.
Rivers
responded
by
cutting
downward
to
form
gorges
in
numerous
locations,
and
they
deposited
the
waste
‐
gravels
and
alluvium
‐
on
their
flood‐plains.
I
have
skated
over
the
long
and
intricate
history
of
the
intervening
ages,
when
Australia,
attached
to
Gondwana,
drifted
into
the
Antarctic
Circle
and
the
glaciers
advanced.
They
retreated
again
when
our
continent
left
Antarctica
behind
and
began
its
solo
journey
north.
Giant
rivers
the
size
of
the
Ganges
carried
billions
of
tons
of
clean
washed
river
sand
down
into
the
Sydney
Basin
to
form
the
Hawkesbury
Sandstone
and
create
its
spectacular
landscapes.
Dinosaurs
roamed
a
lush
well‐wooded
land
watered
by
abundant
rain.
Mount
Rogers
was
undoubtedly
part
of
the
landscape,
gradually
wearing
down
to
the
charming
little
hill
it
now
is.
Its
soils,
for
too
long
exposed
to
weathering,
oxygenation,
and
leaching
by
rain,
are
not
particularly
fertile.
Our
flora
have
responded
to
these
deprived
conditions
with
a
variety
of
ingenious
strategies,
extracting
the
maximum
nutrition
from
a
sparse
and
miserly
larder.
They
deserve
our
admiration
‐
and
our
solicitous
love
and
care.
They
are
unique,
and
if
our
climate
changes
alarmingly
fast
and
warms
as
predicted,
we
may
come
to
depend
on
this
clever
flora
to
keep
our
land
from
becoming
a
desert
like
most
of
the
long‐
cultivated
and
much
abused
Middle
East.
Refs:
A
Geological
Guide
to
Canberra
Region
and
Namadgi
National
Park,
Finlayson
D.M.
[author
and
compiler]
and
co‐authors.
Geological
Society
of
Australia
(ACT
Division)
Canberra
2008.
(140
pages
with
CD
of
geological
excursions
and
places
of
interest.
Bibliography
and
useful
websites)
Can
be
bought
at
the
shop
in
the
Botanical
Gardens
and
the
National
Library.
ISBN
978
0
646
48734‐2
Geology
of
Canberra,
Queanbeyan
and
environs,
notes
to
accompany
the
1980
1:50,000
geological
map,
G.A.M
Henderson,
(compiler)
Australian
Gov’t
Publishing
Service
Canberra
1981.
Available
from
the
Sales
Division
at
Geoscience
Australia
in
Symonston.
An
Internet
search
for
‘Finlayson
DM
Seismic
Imaging’
will
yield
a
plethora
of
research
papers
by
this
and
other
authors
on
the
complex
plate‐tectonic
history
and
structure
of
Eastern
Australia
and
the
South‐West
Pacific
margin.
A
good
ocean‐
floor
map
of
the
same
area
shows
its
intriguing
structure,
centering
today
on
Fiji.
Aidan
Moore
Melba,
ACT
2012