Background to the Hartley Region

Geological Time Scale

During this field trip, you will be looking at rocks that formed and were metamorphosed at different geological times. Most of the rocks that you can find around the Hartley region come from the Late Devonian (blue shades), Carboniferous (red shades), and Permian (green shades) Periods, all within the Paleozoic Era.

Simplified Geological Map

Carboniferous Plutonic = pink & dark blue (Cm)

Late Devonian = purple (Dvi)

Permian Sedimentary = pale blue (Ps) & pale green (Rs)

The Survey geological map can be used to guide your thinking about the geology of the Hartley region. However, a simplified detailed geological map is provided for Hartley that has been modified to be useful for teaching purposes. A legend hasn't been provided here, as you will be required to determine the rock types yourself as you complete the nine stops of the virtual field trip.

Leaving Sydney

As we head out of Sydney we leave behind the sandstone topography of the Blue Mountains and Sydney Basin and enter into the Lachlan Orogen. During the bus trip to Hartley approaching from Mt. Victoria, you drive through the base of the Sydney Basin rocks with Triassic 220 Ma, Permian 260 Ma, and Devonian 380 Ma aged rocks. As you travel through these different aged rocks, you begin to see large differences in topography as you transition from environments dominated by sedimentary rocks to igneous rocks.

This figure shows a typical sandstone escarpment just to the east of the Hartley region.

Coming into Hartley

Some of the topography differences you may notice include the densely forested sandstone escarpments of the Blue Mountains giving way to the rolling hills of the Hartley Valley. These low rolling hills are seen to be cross-cut by rivers and streams, typical of granitic geology. Large granite formations often weather in a peculiar way, creating a landscape dominated by what are referred to as 'granite tors'.

Granite Tors

After entering into the igneous dominated Lachlan Orogen you will notice some peculiar dome shaped rocks outcropping abundantly in the surrounding fields. These are the classic granite formations known as 'tors'. Unlike other smooth rounded rocks, these do not form in a river/stream environment. Their shape comes by the processes of physical and chemical weathering.

As a large granite pluton is exposed at the surface, stress is relieved as the weight of the overlying rock is removed and the rocks can crack. The fractures form in regular patterns and are called joints. Over time, ground water may seep into the joints, inducing both chemical and physical weathering and allowing block like features to form. The edges of the blocks are weathered away the fastest, rounding these blocks until they begin to form the classic dome like shape. As weaker minerals are more easily removed, a granitic sand forms around the tor domes, allowing them to appear as if they are popping out of the ground.

Other outcrop patterns on maps and in the field

Granite tors are one distinctive type of rock outcrop pattern and these are common to massive homogeneous rock types. Outcrops can be highly variable. Some may comprise layered rocks such as sedimentary rocks. The layering may be deformed due to tectonic movements to form folded or faulted rocks. The different layers can have variable resistance to chemical and physical weathering such that some layers are more prominent and others are more recessive, creating distinctive layered outcrop patterns. You also should describe other features of rock outcrops such as how rounded or jagged the outcropping rocks are, how homogeneous or heterogeneous they are in terms of colour, texture, grain size, etc.

Geology maps depict the rock units at Earth's surface. At the large outcrop to map scale, the interaction of layered rocks with topography is also forms predictable outcrop or map patterns:

Flat-lying beds (horizontal strata): where sedimentary layers are nearly horizontal, the contact between different layers of sediment run parallel to contour lines. In the field, this is experienced when you walk along a contact between two sedimentary layers and you do not walk up or down hill but walk at the same elevation. Look at the green units on the geological map in the section on 'Geological Time Scale' above - these have map patterns very typical of flat-lying beds.
Folded rocks: where layered rocks have been folded, the outcrop patterns of rock units can double back onto themselves and must be traced carefully in the field. Sometimes measuring the orientation of bedding around folds is the best way to track the systematic changes in orientation around folds.
Faulted rocks: where rocks are faulted, abrupt changes in geology can occur on either side of the fault and the displacement sense of the fault can be determined when units are carefully matched either side of the fault. The outcrop patterns at the site of the faulted rocks are commonly recessive as faulted rocks are more easily weathered away.

If you are interested in investigating these outcrop and map patterns further, please visit this site from opengeology.org.

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