Acacia Ridge Geology Walk
This ridge top walk follows a track along the side of a ridge flanking Devil’s Pass, the relatively
narrow valley which holds the access road to Arkaroola Village. The ridge is of Blue Mine
Conglomerate, much of which is coarse sandstone. There are numerous bands of conglomerate, with
sandstone. Some of the grains are blue-grey quartz, others are rounded small pebbles of feldspar.
Both of these clearly derived from the erosion of Mt Neill Granite in the older Mt Painter basement,
which would have been part of high land to the north. Other pebbles are white quartz derived from
quartz veins.
Within the sandstone and conglomerate are thin layers of black grains; these are heavy minerals,
mainly iron oxides which were concentrated on surface layers (often outlining cross bedding) when
the sands were being deposited by currents. Ripple marks, cross bedding, mud cracks and salt casts
all show that the Blue Mine Conglomerate was deposited in shallow water.
Interbedded with the coarse sandstones and conglomerates are continuous thin beds of green
siltstones identical to those of the overlying Opaminda Formation. Thus the Blue Mine Conglomerate
is a series of overlapping and stacked beds, where rapidly derived sands and gravels washed into
shallow waters already accumulating muddy sediments at a slower steady rate. The pulses of coarse
sediment probably were due to episodes of uplift in the adjacent granite basement exposing it to
erosion.
The Wortupa Quartzite which overlies Opaminda Formation on the other side of the road
represents a second episode of deposition dominated by coarse sandy sediments rapidly laid down in
shallow water.
Distance and time: 5.6 km along the ridge including spur to the summit allow 2.5 hrs.
Altitude Range: 270 m (Arkaroola Station Trailhead) to 566 m (summit of Acacia Ridge).
Access: It is best to arrange a lift either through Arkaroola management or a friend to be dropped at Arkaroola Station
Trailhead, so that you walk back to the village along Acacia Ridge. If you start in the village, the return walk back along the
road can be tedious.
Track: Generally a gradual grade; some rocky sections between 2.0 km and Mt Elva dam.
Distances marked in km decrease as you approach Arkaroola village
Notes read as for a start at Arkaroola Station trailhead.
5.2 Checkpoint A2 – Arkaroola Station Trailhead
Cross the small creek and walk upslope. The track is on the upper part of the Blue Mine
Conglomerate which is interbedded with blocky green laminated dolomitic siltstone of the
Opaminda Formation which forms the banded hills south of the road.
4.9 Here the Blue Mine Conglomerate is a coarse grained sandstone with rounded pebbles of white
vein quartz, black chert, quartzite and fine grained pink granite. Some grains of blue-grey quartz
and rounded feldspar can be recognized as derived from the Mt Neill Granite a basement rock
which is exposed further north (see Terrapinna Tors walk).
4.7 Here are numerous thin layers of green siltstone and sandstone, many of which are ripple
marked; these are ‘casts’ made of ripple marks on sandstone beds (ripple marks do not form
easily in salts and clays).
4.6 to 4.4 Between these points the Blue Mine Conglomerate shows good examples of cross
bedding and heavy mineral laminations. Look for examples along the way.
3.5 The track crosses a face of coarse sandstone, on a ledge eroded from a narrow siltstone band.
The face is cut by numerous thin quartz veins which carry black tourmaline (schorl), a boron
silicate mineral. Water runs off this surface into cracks, and plants unusual for the north
Flinders are found here. They are common further south where there is more moisture (as at
the Dutchmans Stern).
From here look south east over the range of Merinjina Tillite (high point Mt Jacob) to Lake Frome
and south to the banded Opaminda Formation on the hills across the road, with lower Burra
Group dolomites and siltstones behind. The track here is on Blue Mine Conglomerate, often
with deep purple iron stains.
2.8 Visible ahead are large steeply dipping faces, which are beds of conglomerate – the
“Devil’s Slide”. Earth movements have tipped the once horizontal beds into a near vertical
position.
2.7 Checkpoint A3 Along a short walk to the summit lookout (Checkpoint A4) the Blue Mine
Conglomerate is heavily iron stained, brown and purple.
From the lookout (look at the topogram for guidance) the main peaks in the core of the Mt Painter
basement area can be seen to the north, and to the southwest, the Rawnsley Quartzite peaks
of Mt McKinlay and Benbonaythe Hill are visible, essentially at the top of the Proterozoic
sediment pile. From this point towards Benbonaythe Hill, you are looking at remnants of a pile
of sediments originally 15 kilometres thick.
To the north and north east are steep rolling hills of Woodnamoka Phyllite which is beneath (older
than) the Blue Mine Conglomerate. If the sun is shining, the hills have a distinct sheen due to
the mica in the phyllite.
2.4 The track now follows a ledge on soft green laminated fissile siltstone, with purple iron stained
conglomerate to the left.
2.2 Here, there is a notch in the conglomerate bed, which is the one that forms the face of the
Devil’s Slide visible from the road.
2.0 Wave-formed ripple marks and casts of salt crystals in sandy and silty beds testify to deposition
in very shallow water. Para 3 of the introduction details the environments in which this
happened.
1.0 The spine of the ridge is mainly sandstone with a thin, pebbly conglomerate band.
On the slope down to the dam at 0.8 km are exposures of dark green brecciated basalt stained with
green copper carbonate.
0.2 just before Wywhyana Creek a very good exposure of Opaminda Formation can be seen in the
road cutting opposite; this displays the green talcy siltstones, laminated dolomites and ripple
marked sandstones which stand out as the regular banding seen from Acacia Ridge. Opposite is
Griselda Hill which is Wortupa Quartzite.
Return to the village along the road passing Arkhenge which displays some of the rock types found
on Arkaroola.
0.0 Checkpoint A1 – Arkaroola Village Trailhead
Text prepared from field notes supplied by Nick Langsford
GLOSSARY OF GEOLOGICAL TERMS
For descriptions of individual rock types and Groups, see Stratigraphic Column.
Banding Parallel stripes of different colours, generally, but not always, representing bedding in sedimentary
rocks.
basalt Dark green igneous or volcanic rock, made up mainly of feldspar and ferromagnesian mineralsalumino-silicate minerals with notable amounts of iron and magnesium.
boron a non-metallic element, greenish brown powder or crystalline; derived from the ore, borax
(Na2B4O7.10H2O)
breccia Broken up rock; can form by a wide range of processes.
casts of salt crystals The mineral halite, which is common salt, forms cubic crystals when sea water is
concentrated by evaporation. These eventually sink onto the shallow sea or lake bed and are buried by
silt. When the salt redissolves an impression remains on the sea floor. This is filled by further sediment
to produce the casts.
chert Very fine grained quartz.
conglomerate Sedimentary rock with a major component of coarse usually rounded rock fragments.
copper carbonate Brightly coloured, hence easily recognized secondary copper minerals formed by the
weathering of primary copper sulphide minerals. Most common are malachite (green) and azurite (blue)
cross bedding Mini-deltas (centimetre to metre scale) where the bedding traces, often outlined by dark heavy
minerals, occur at a considerable angle to the originally horizontal bedding.
dolomite Rock that is largely composed of the dolomite mineral, CaMg(CO3)2
feldspar A complex set of alumino-silicate minerals, with vary amounts of sodium, potassium and calcium. In
important constituent of many rocks, especially igneous rocks e.g. granite.
fissile Easily split, usually along bedding planes.
Formation A sedimentary or volcanic rock unit which can be physically mapped and defined by features
observed in the field.
granite An igneous rock, formed at considerable depth, made up of millimetre to centimetre sized grains of
quartz, feldspar and mica.
Group A unit which includes two or more Formations of usually sedimentary rocks.
interbedding Layers of sediment lying in or parallel to layers of other material.
iron A metallic element, Fe. An important industrial commodity.
mica A series of alumino-silicate minerals which have the property of splitting into thin sheets – hence ‘books’
of mica. The most common are white muscovite and black biotite.
mud cracks The wedge shaped cracks bounding polygonal plates formed when thin layers of mud or silt are
dried by the sun. The cracks are filled by overlying sand layers and are preserved when the mud flakes
are weathered away.
phyllite A somewhat metamorphosed slate, with abundant mica on splitting planes giving a distinct ‘sheen’.
Proterozoic From 2500Ma to 543Ma. The youngest part of the Proterozoic Era is the Neoproterozoic which
includes the Ediacaran Period, covering 1000Ma to 543Ma.
quartz The most common crustal mineral – silicon dioxide (SiO2).
quartzite A rock which was originally quartz sandstone which has been heated or compressed sufficiently to
fuse the grains together, producing a tough resistant rock.
ripple marks Sedimentary structures formed on sandy surfaces by wave or current action. Very common in
sandstones and on present day beaches. Has the appearance of corrugated iron.
salt casts see casts of salt crystals.
sandstone A rock made of sand – generally quartz sand.
schorl Black tourmaline – a complex boro-silicate common in pegmatites.
siltstone A rock composed of fine, silt-sized (less than 0.1 mm) mineral grains – usually mainly quartz.
tillite Rock formed by glacial activity, containing a very wide range of grain sizes, usually including some large
fragments obviously not locally-derived, known as erratics.
tourmaline A complex boro-silicate; black schorl is most common; gem varieties can be green, red or blue.