Key Feature:
GOSSAN Outcrop: Gossans are formed by the oxidation of the sulphide minerals in a rich ore deposit. Outcrops contain highly oxidized, weathered and/or decomposed rock. At this location all three features are evident as are the high concentrations of iron compared with other sediments nearby. The only other established true gossan in the ACT is at the Paddys River mine near the Cotter Reserve. The word 'gossan' is a Cornish word meaning 'iron hat'.
Background:
We acknowledge the Ngunnawal people as the traditional custodians of this region. The Ngunnawal people held large gatherings here and used the coloured ochres as a ceremonial pigment. Local farmers Frederick Campbell and Samuel Shumack documented these gatherings in the 1850s and 1860s.
Geology:
Gossan Hill is one of complex geology with a number of different rock types present. While this hill is mainly composed of Ordovician Period sediments the most striking feature is the gossan outcrop that formed during the Silurian Period.
The reddish stones you see at GZ comes from the molten crust of the earth extruded 430-415 million years ago during the late Silurian Period. You can also see some of the different iron oxides that compose the gossan such as black magnetite, reddish brown hematite, and yellow limonite. Over the past 65 million years, the exposed area of the gossan has weathered, impregnating the surrounding soil with coloured ochres.
The gossan here cuts through folded sandstone and shale formed 450-445 million years ago during the Ordovician Period. The rocks visible at the road-cut on College Street was formed over 450 million years ago and is known as the 'Ordovician Pittman Formation' (Reference 1). It consists of folded, thin-bedded fine sandstone and shale and some chert. Volcanic activity in the area during the Silurian Period is also evident with weathered granite outcrops at the western end of the road cut.
Left side of photo, denser boulder with darker black/red iron oxides. Right side, boulder is less dense and contains more orange/yellow iron oxides. Photograph taken: June 2022.
Educational Value:
This site has significant scientific value as an educational resource for the geological features described above. It is used by many schools including the University of Canberra science students. There is no need to visit the 'Ordovician Pittman Formation' to complete this EarthCache. However, we have provided you with the coordinates, and a gallery photo taken in June 2022, for future reference and/or visits.
Access:
See waypoints provided. There are a number of ways that you can access GZ. However, we recommend that you take 'Walking Trail - Option 1: (Trailhead)' to walk on the deep red ochre rich soils on part of Middle FT(foot trail). The trail section of note is from Radford College up to the information board, see 'Information Board: Reference Point)'. This feature is the result of extensive gossan weathering.
Logging Requirements:
To log this cache you will need to address the following 4 points (questions and a mandatory photo).
Please send your answers via 'Message Center' to either CO as the preferred method. You can log this cache immediately attaching your photo/s. If there are issues with your answers or photo you will be notified.
Questions / Photo: Complete only during daylight hours.
Stage 1: Cleared area of woodland with exposed soils and gossan outcrop.
(Consider posting photos that would encourage others to visit this interesting location.)
1. Stand at the gossan outcrop at GZ and look around. What is so distinctive about this area?
Multiple choice:
a) There is evidence of highly oxidized iron rich rocks.
b) There is evidence of rich burnt-red/orange rocks/soils, and weathered rocks.
c) There is evidence of widespread iron rich decomposed rocks.
d) All of the above.
2. In what geologic period was this gossan outcrop formed?
(One word - use text in the description above to decide your answer.)
Stage 2: Cluster of spaced small gossan boulders, and a scattering of rocks:
3. Pick up a some of the different coloured rocks: Why do you think there is a difference in mass between the yellower rocks and the darker black/red rocks? In answering this question assume that you have rocks of the same size. (Please replace the rocks where you found then for the next budding scientist.)
4. Post a photo of yourself, or something that identifies you, on or near the larger gossan boulder.