Royal Park was originally reserved by Governor LaTrobe in 1854, and is now managed by the Melbourne City Council. The railway line through the park was completed as a single line in 1882, and widened to a double line in 1888. The cuttings on this line soon became a popular site for geological excursions, at least partially because of the tertiary shells that were formerly abundant there.
A quarry in Royal Park (now filled in) supplied one of Melbourne's earliest building stones - dark brown Tertiary ferruginous sandstone. This stone was used to build Melbourne's first stone church, St James Old Cathedral, completed in 1842 and still standing on the corner of King Street and Batman Street.
Railways, roads, playing fields, and buildings have so extensively altered Royal Park that almost no trace of the original vegetation remains, and the original geomorphology is largely obscured.
The Site
From the Royal Park Station walk along the northern side of the cutting to the given coordinates from where you will be able to view the site (watch out for muggle golfers).
Looking at the south face of the cutting you will see a true broad river deposit in red-brown sands and clays. These are unconformably overlying Silurian sandstones and mudstones.
Silurian sandstones and interbedded mudstones are the oldest rocks in the central Melbourne district; they form part of the Dargile Formation, which reaches a maximum thickness of 1700 metres. These sediments were probably deposited on an ocean floor in moderately deep water. The mudstones accumulated during periods when the water was still and very fine clay particles could settle out; the sandstones, because of their coarser grain size, were deposited during higher energy conditions, probably by turbidity currents.
The sediments overlying the Silurian rocks are mostly coarse-grained sandstone, showing considerable variation in grain size. These Tertiary sediments are composed of clear, angular quartz grains with occasional well-rounded pebbles of milky quartz. The latter were derived from quartz grains in the Silurian sediments, whereas the angular quartz grains were probably eroded from the granites that outcrop about 15 kilometres to the north.
The interface between the Silurian sediments (420 Ma) and the Tertiary (4-10 Ma) is called an unconformity because it is the erosion surface on which the Tertiary beds were deposited. It represents an erosion break from about 410 Ma (when the Silurian sediments were uplifted and folded), to about 10Ma (when the red sandstones were deposited). Unconformities usually indicate a long break in sedimentation.
That a large ancestral river deposit can be found at the highest summit level of Royal Park today is due to uplift. In the last 4 million years, the land has slowly been uplifted along a northwest-southeast fault known as the Melbourne Warp, which runs roughly parallel to the east coast of Hobson's Bay gently raising the land to the northeast. Since that time, the Moonee Ponds Creek and the Yarra and Maribyrnong rivers have cut terraced valleys in their old former valleys, leaving their original valley deposits capping the summit levels on the Silurian sediments. A monoclinal warp has dragged the ductile, plastic beds down over an escarpment rather than displacing them as in a normal fault (as illustrated below).
References:
Clark, I. & Cook, B. (Eds.), 1988, Victorian Geology Excursion Guide, Australian Academy of Science, Canberra
Nelson, S.A., 2003, Deformation of Rock, Tulane University, www.tulane.edu/~sanelson/geol111/deform.htm
Schleiger, N. (Ed.), 1995, Roadside Geology, Geological Society of Australia (Victorian Division) & The Field Naturalists Club of Victoria, Melbourne
To successfully log this earthcache you must email me the correct answer to the following question:
From the given coordinates what is the apparent dip direction of the Silurian sediments facing you in the cutting?
(A cardinal, primary or secondary intercardinal direction will suffice. There is no need to give an accurate bearing nor for close inspection or measurement of the rockface. Some reading/googling may be required if you don't know the difference between a dip and a strike)