The Oxley Creek at ~70km long is Brisbane’s longest creek and a major tributary of the Brisbane River. It was originally named Canoe Creek, but was renamed after John Oxley who was the first explorer to survey it in December 1823. With its headwaters on the Northern slopes of Mount Perry (Beaudesert Shire), the catchment covers ~260 km2 and flows through 28 Brisbane suburbs before discharging into the Brisbane River at Tennyson. Land use within the catchment include rural, residential, industry, sand extraction, waste and sewage facilities and the Archerfield Airport. Being the only sand-based creek in Brisbane, sand was heavily mined between Willawong and Larapinta on both the Oxley and Blunder Creeks for the construction of buildings and roads. This mining converted the anabranch into the mainstream leaving a series of lagoons, and greatly affected water quality due to increased sediment. In this EarthCache you will learn about soil classification and understand what makes the sand in the Oxley and Blunder Creeks so valuable.
There are numerous soil classification systems, but they all classify soils by texture, plasticity, and organic matter. Texture is defined as the size, shape, and arrangement of grains. It can provide an insight to the history of the sediment and its lithification into rock. There are two general categories of texture: clastic (round) and crystalline. Plasticity is the ability of a soil to be moulded and retain its shape. High plasticity soils turn sticky when wet and resist shearing. Non-plastic soils, such as sand, cannot be rolled out and maintain their shape.
One of the first systems, the Wentworth Scale (1922) divides clastic texture by size into coarse-grained gravel, medium-grained sand, and fine-grained mud (silt and clay).
The Shepard’s Classification System (1954), builds the model by using a simple classification scheme for fine-grained sediments (soils containing <10% gravel) to produce a soil description by comparing the relationship between the sizes of grains.
The Unified Soil Classification System (USCS) is used in geology to describe the texture and grain size of a soil, with most unconsolidated material represented by a two-letter symbol, except for peat. Like the other models, classification begins with particle size, but then classifies the course-grained soils, gravels (G) and sands (S), further by determining cleanliness, particle uniformity and plasticity. Cleanliness relates to purity with dirty gravels/sands being mixed with either silt (non-plastic fines) or clay (plastic fines). The particle size of gravels and sands can be either poorly graded (P; uniform particle sizes) or well-graded (W, diversified particle sizes). The fine-grained soils, silt (M) and clay (C), are also classified by texture but then subdivided by plasticity: high (H) or Low (L).
Types of Soils:
Gravel (G): Unconsolidated rock formed from weathering and erosion of rock, which depending on size can be classified as either granular gravel (>2 – 4 mm) or pebble gravel (>4 – 64 mm). Gravel can also be produced by quarrying and crushing hard-weathering rocks, such as sandstone, limestone or basalt. However, in certain areas gravel can be compacted into a sedimentary rock called conglomerate.
Sand (S): Granular material composed of finely divided rock and mineral particles ranging from 1/16 - 2 mm (0.063 – 2 mm). The majority of sand is composed of either silicon dioxide (quartz) or calcium carbonate (aragonite or sandstone), but the bright white sand in tropical settings is from eroded limestone and may contain coral and shell fragments. Sand feels gritty when rubbed between your fingers whereas silt feels like flour.
Silt (M): A granular material sized 1/16th – 1/256th mm (0.002 – 0.063 mm; smaller than sand but larger than clay) composed mainly of quartz and feldspar. Sedimentary rock composed of silt is called siltstone. Silt is sometimes known as “rock flour” or “stone dust” as it is non-plastic with a floury feel when dry, and a slippery feel when wet. Silt is chemically distinct from clay, and unlike clay, grains of silts are dimensionally uniform. They can also be distinguished by plasticity: silt is non-plastic. Silt transported by air forms dust, and in water contributes to turbidity.
Clay (C): A fine-grained soil smaller than 1/256 mm (0.002 mm), which is a mixture of clay, metal oxides, and organic matter with variable amounts of water trapped in the mineral structure. Clay is formed from thin plate-shaped particles held together by electrostatic forces thus giving it plasticity. Clay is formed from the chemical and hydrothermal weathering of rock.
Organic Soils (O) and Peat (Pt): Unlike the soil types mentioned above, organic soils are composed of carbon from the decomposition of plant and animal material. Organic matter hold soil open allowing the infiltration of air and water, and may hold twice its weight in water. These soils are highly fertile and are readily identified by their colour, odour, and spongy feel. Peat, due to being composed solely of organic material is infertile.
To Log This Earthcache:
You will need to visit the listed co-ordinates, make detailed observations and answer the following questions. Prior to your visit print out the Unified Soil Classification System table at the following link (http://farm8.staticflickr.com/7359/14005182090_b327197780_o.jpg). You will need this to answer the questions.
- What evidence do you see of previous mining activity?
- From an area where you think the sand was mined take a representative soil sample (you only want the soil so exclude particles >75 mm, plant matter, insects, etc…).
- Using the Wentworth Scale what classification of soil was mined?
- Using the Unified Soil Classification System provide the two-letter symbol that best suits the mined soil?
- Based on your observations and soil classifications, what benefit would this sand type have for the construction of buildings and roads over coarse-grained soil (i.e. gravel) or fine-grained soil (i.e. silt and clay)? Keep in mind the following: moisture content/holding capacity, compatibility, and organic content.
Feel free to take a photo of yourself along Oxley Creek and post it with your log.
Please e-mail us the answers to the questions. Please do not post any of your answers in your log as it will ruin the educational experience for other geocachers. Any finders who do not e-mail through the answer and/or any logs with the answers will be deleted without warning. Once you have e-mailed us the answers feel free to log your find. If there are any problems we will be in contact.
Hope you enjoyed learning about sand mining in Oxley Creek.
Note: FTF was awarded at the Cleaning Up Lovely Logan (CITO Event). Also a big thank you to OrionJ for the background image.