IT’S COLD and damp inside this long and narrow tunnel on a windswept island in the middle of Sydney Harbour. At 180m long, Dog Leg tunnel passes through the belly of Cockatoo Island. Built in 1915 to move workers and materials from one side of the island to the other, the tunnel is a geological wonder carved from Sydney Sandstone.
Sydney sandstone is the common name for Sydney Basin Hawkesbury Sandstone. The Hawkesbury Sandstone is a Triassic (about 200 million years ago)sedimentary medium to coarse grained quartz sandstone with a smaller proportion of feldspar, clay, and iron compounds.
Hawkesbury Sandstone was formed from the sands washed from the adjacent, at the time, continent of Antarctica. The sand was carried by a vast river system. Over millions of years these sands were consolidated into sandstone up to approximately 50 metres thick at this location. Layers within the formation show extensive variation in the environmental conditions building the sandstone.
Large scale crossbedding is a feature of Hawkesbury Sandstone. The crossbeds dip generally northeast, indicating the source rocks for these sands were to the south west of the Sydney Basin.
Multiple layers are evident in the tunnel. Specifically you are asked to inspect the condition of the sandstone on the outside curve of the tunnel bend, The Dog Leg. The lower layer shows typical high angle cross bedding. Above this cross bedded layer are two distinct homogenous layers and then at the top of the tunnel is another layer of cross bedding. From your observations please forward answers to the following questions.
-What is the apparent angle of the lower cross bedding? Does this angle change at the top?
-What is the colours of the middle layers? Why are they these colours?