This is a comprehensive
geological report about the origins and current structure "anomalies" of Murphys Haystacks compiled by Dr. Rowl Twidale [see below for full details]. For a much simplified
"laypersons" view of the Haystacks see
The Geology: Simply Speaking
This "web" version may
be a little difficult to read without it's included pictorial content and we
understand that. Linking in the multitude of thumbnails for easier "in
context" viewing is difficult for speed reasons. However, all of the
hyperlinks used for images and diagrams in the reading of the report open into
separate image windows to facilitate better viewing.
Murphys Haystacks consists
of two separate though clearly related groups of large granite pillars and
boulders standing near the crest of a broad domical hill, here called
Oakfront Hill, just to the west of the Streaky Bay - Port Kenny Road, some
30km northwest of Port Kenny [Figure 1:
The major landforms present
at Murphys Haystacks are pillars and boulders. Pillars are tall columns of
rock that are in physical continuity with the granite of the hill [Fig
3a]. Boulders are more-or-less spherical masses that are detached
from the underlying bedrock [Fig
3b]. Both pillars and boulders are thought to result from the
break down of thick slabs or sheets of rock into which the granite is
sub-divided and which have given Oakfront and other hills in the district
their characteristic domical or convex-upward shape.
Such sheet structure is
exposed in the flanks of nearby Freeman Hill, and even more clearly on
Ucontitchie Hill and Mt Wudinna where the granite is not masked by a cover
of limestone. The hills are also sub-divided by sets of fractures arranged
in a fan pattern when seen in section. They intersect the thick slabs,
resulting in the formation of essentially cubic or quadrangular blocks [Fig
The pillars and boulders at
Murphys Haystacks typically have side-walls that are smoothly concave:
they are said to be flared. Such flared slopes result from weathering, by
soil moisture, beneath the land surface, and especially along fractures [Fig
4]. Water penetrates along a fracture altering the rock with which
it comes into contact. The mica and feldspar in particular are readily
altered on contact with moisture. Fresh granite is difficult for water to
infiltrate, but the more the rock is altered the more permeable it
becomes, and the more readily can more water enter the mass. Thus a zone
of weathered rock is formed along a fracture. There is an abrupt
transition between the altered rock and the still fresh rock. This
boundary or junction is called the weathering front. It represents the
limit of significant weathering and at several sites on Eyre Peninsula it
can be seen to be concave-upward or outward where the slopes are steep [Plate
Because the near-surface
soil tends to dry out at times, weathering is slower there than deeper
down, where soil moisture is ever present. For this reason the weathered
zone in fractures is wider at depth than near the surface, or put another
way the weathering front becomes curved, and penetrates deeper into the
host block a few metres below the surface than just below it or at great
depths beyond the limit of soil moisture. Thus the weathering front is
curved, and concave with respect to the host block.
The main part of the block
remains fresh and cohesive and so massive that it cannot be moved by rain
and wash; but the weathered granite [known as grus, from the German word
for fine gravel] is washed away, so exposing the weathering front as a
smooth concavity or flare, one on each side of the fracture. This process
of subsurface weathering has been followed by the erosion or removal of
the weathered material. Thus flared slopes are due to two processes,
weathering and erosion. Where such weathering and erosion has taken place
along all six of the fractures that delimit each of the blocks, each
resultant pillar or boulder comes to be flared on all sides. Hence the
haystack forms that are so well and so abundantly developed near the crest
of Oakfront Hill. Penetration of water beyond the weathering front may
also be the cause of the flakes and shells [Plate
3] of rock that can be seen on some boulders and pillars.
In some instances the flared
slope has advanced toward the centre of the block from all sides to such
an extent that the platforms are wider than the remaining core [which has
also been lowered by weathering], so that the residual looks like a boss
standing above a shield [Plate 7].
Carried to its logical extension such soil weathering will presumably
cause the elimination of the boss leaving simply a flat or only broadly
domed surface a platform or a low whaleback [Plate 8].
At the base of some boulders at Murphy Haystacks a more or less narrow
platform is found. This is due to a recent lowering of the soil surface [Plate 6].
Some of the pillars and
boulders are not only flared but have developed in their sides huge
hollows known as tafoni [singular tafone], an Italian word meaning windows
or perforations, and apt in the sense first that these hollows interrupt
or breach the smooth cut lines of the forms [Plate 9],
and second that in extreme cases [as seen for instance on Granite Island
near Victor Harbour, and on Mt Wudinna] a tafone can develop to such an
extent that the hollow breaches the outer shell so that a window or
opening is formed - a site much favoured by photographers who record
companions with just their heads sticking out from the rock!
There is considerable
argument about how tafoni form; they were first described from Corsica
about a century ago and the argument has gone on unabated ever since. But
as they commonly form alongside flared slopes [and not only Murphys
Haystacks but at Ayers Rock and other places also] the two may have a
common subsurface origin: tafoni may develop where the moisture attack is
especially effective. Second, it is obvious that the outer skin of the
granite, that which forms the projecting lips or visors, is somehow more
resistant than the rest of the rock. It may be that a slight accumulation
of iron and silica, formed either at the weathering from beneath the soil
surface, or concentrated by the hyphae [roots] of lichens for instance
[see projecting ribs in the tafone on block [Fig 2] are covered by lichens
[Pl 4]. The most difficult problem connected with the tafoni however
concerns how and why the inside walls of the tafoni have been and still
are being worn back. The rock there is loose and fragmented, and can be
loosened at the merest touch at some sites [as e.g. at Ucontitchie Hill
where the rock is breaking down in layer upon layer of thin flakes]. Some
workers have suggested that the crystallization of halite [common salt]
and gypsum, The effectiveness of salt crystallisation in rupturing rocks -
even those as cohesive as granite - is now beyond dispute for it can be
replicated in the laboratory. The real problem is how saline solutions
come to be within the rock.
At least one other feature
seen at Murphys Haystacks is due to moisture attack at the weathering
front. It will be noticed that the lower few centimeters [about a foot] of
the sidewalls of many of the pillars have a rough surface which stands in
marked contrast with the smooth higher surfaces [Plate 10].
Close examination shows that this roughness is due to quartz crystals
[glassy or grey looking] standing out in relief, and this is because the
feldspar and mica that was originally between the quartz have been rotted
by water, and when the weathered rock was stripped away the tiny spaces
previously occupied by the feldspar and mica are left empty. This feature
is called pitting and it provides a measure of how much the plain surface
has been lowered recently [in this case, probably since the clearance of
trees from the hill by Europeans and the erosion of the topsoil, the same
erosion that caused the solid calcrete later to be exposed over wide areas
on the hill]. Similar soil erosion is implied by the steep basal slopes of
some pillars [Plate 6].
Other features worthy of
note are the vertical grooves [called Rille or Karren] which are due to
water trickling down steep sidewalls, and even overhanging faces, and
gradually weathering and eroding shallow grooves [at "G"
Figure 2b and "Y",
There are good examples in both the eastern and western groups at the
Haystacks. There is a complication which is well illustrated at
Yarwondutta Rock, near Minnipa. There on one of the northern flared slopes
there are grooves that now stand out as ribs and it is suggested that the
trickles initially erode a groove, but that this becomes a preferred site
for lichen growth. The lichen protects the surface [as at x in the western
group of Murphys see
and it is the adjacent areas that are next weathered and eroded leaving
the original grooves higher and looking like ribs [Plate 12].
The same sort of thing can be seen on the northern slope of the Turtle
Rock, near Mt Wudinna.
Some linear depressions,
grooves and gutters are due to weathering along fractures or other
weaknesses in the rock, and there are many examples. One on the crest of x
2b] in the western group is largely developed along a vein of fine
granite rock [Plate 13].
Also on this crest is a shallow depression known as a rock basin or
gnamma, again the result of water rotting the rock [Plate 14].
When did the Haystacks form?
We have to distinguish between the granite on which the various landforms
are developed and the period or periods of landform development.
The granite is the Hiltaba
Granite and is part of a large mass that underlies much of the
northwestern Eyre Peninsula, the southwestern Gawler Ranges and adjacent
areas: hence its name, from Hiltaba Station, in the southwestern Gawler
Ranges. The rock is pink and coarsely equigranular with crystals of grey
glassy quartz and pink feldspar prominent. The granite was emplaced deep
in the Earth’s crust, probably some 7-10 km below the then land surface.
This age has been determined using radioactive decay methods (in
particular lead-uranium series dating). That the granites are now exposed
implies an enormous amount of erosion, for some 7-10 km of rock has been
stripped away. The overlying rocks have been worn away, transported and
deposited as sedimentary rocks on the continental shelf and inland basins.
The landforms we see at
Murphys Haystacks are much younger than the rocks of which they are
formed. The various pillars and boulders that constitute the Haystacks
were already in existence in essentially their present form some 34,000
years ago. They are almost certainly much older but how much older is not
known. At various times during the last 700,000 years (during the Middle
and Late Pleistocene) huge dunes of calcareous sand spread far inland from
the coast. Some of the granite hills of this coastal zone where buried by
the dunes. Freeman Hill is an example for calcareous rocks are preserved
on its crest. At Murphy Haystacks however, the evidence suggests that
though the calcareous sand spread on the lower slopes the two groups of
boulders and pillars granite hills stood above the dunes, like rocky
islands in a sea of sand. The reason for suggesting this is that the upper
zones of the calcareous dunes became cemented by lime, forming a solid
limestone called calcrete as a B-horizon in the soil. Calcrete now forms a
crust over the slopes of Oakfront Hill, and is also preserved in cracks,
crevices and hollows at the base of the rocks [as at H in
but no higher.
The calcrete at Murphys
Haystacks contains is fossiliferous with fragments of marine organisms
(forminifera) blown in with the dune sand. However such fossils would
provide a date for the sea floor sediments from which the dunes were
derived, whereas the calcrete provides a minimum (albeit a rather
unreliable date - an approximation) age for the landforms over and against
which the limestone has been plastered. The calcrete was dated using
radioactive decay, by the Carbon 14 method. The calcrete at Murphys
Haystacks is at least 34,000 years old (and some from the crest of Freeman
Hill is about 26,000 years old). As the dune sand on which the calcrete
was formed was blown into cracks and hollows in the boulders, the latter
clearly predate the calcrete so that we can say Murphys Haystacks are at
least 34,000 years old and, given the possible complications of 14C
dating, probably much older.
Review of development
1: Emplacement of mass of
granite at depth of 7-10 km about 1590 millions of years ago.
2: Deep erosion and removal
of rocks that formerly covered the granite, which had been stressed
resulting in the development of sheet structure as the mass came to be
close to the land surface.
3: More than 34,000 years
ago, the upper shell was broken down into cubic or quadrangular blocks,
and flared slopes were developed on the sides of the blocks. Because the crestal area was in tension, the blocks there were weathered quickly.
Likewise those on lower slopes were rotted rapidly and also eroded, so
that only the blocks on upper slopes persisted.
4: During glacial periods
of low Pleistocene sea level (the last 2 million years) huge coastal
dunes, consisting mainly of shell fragments made of calcium carbonate
developed and spread inland, covering many nearby hills and certainly
lapping around the bases of the Haystacks.
5: In recent times a soil
developed on the lime sand and since European settlement the topsoil has
been eroded [to a depth of about 25cm] exposing the solid limestone or
calcrete that we see on the hill crest and slopes, and above which the
The report in this
geological report was produced by Dr Rowl Twidale [D.Sc. (Bristol), Ph.D. (McGill) and D.Hon.,Causa (Madrid)]
and Dr Elizabeth M. Campbell [Ph.D. Adelaide].
The original report is
being used with permission and is strictly copyright material. Any
replication using any medium is strictly prohibited without the express
consent of Dr. Twidale. Please respect copyright law at best and common
professional courtesy at the very least.
He can be reached via
Department of Geology &
University of Adelaide
phone : Int + (618) STD (08) 8303 5392
fax : Int + (618) STD (08)
See Eyre would like to
offer a personal thank you to Dr. Twidale for his assistance while
creating this report in a "web format". He was most generous in
his efforts to ensure accuracy and for this we are most appreciative.
The time that was provided by Dr. Twidale must have been significant.