Dights Falls is an artificial weir built on a natural rock bar across the Yarra. The weir was built in the 1840s to provide water to the 'Ceres' flour mill, one of the first in Victoria. The falls were later to become known as Dights Falls after the owners of the mill.
This site and the adjacent Pumping Station Track cuttings, exhibit exposed bedding planes characterised by ripple marks, complex folding and faulting and fine grained sediments containing early marine fossils (eg. Graptolites - primitive invertebrate animals).
Looking at the Cliffs on the opposite side of the river you will see the rocky escarpment which is made up of uplifted silurian mudstone/sandstone sediments.
Geology:
The three main rock groups, Igneous, Sedimentary and Metamorphic are discussed below. You can think of these as families of rocks. Within each family there are many individuals.
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Igneous:
Igneous comes from the Latin ignis, or “fire”. Igneous rocks are new, born from within the earth. They comprise either fresh material from the mantle or reignited continental material that has been subducted and melted. Those igneous rocks produced at or above the Earth’s surface are extrusive (volcanic), those formed beneath the surface are intrusive (plutonic). They can also be a combination of the two.
More than 95% of the Earth’s outer 50 kilometres consists of igneous rocks. Igneous rocks are defined by their mineral composition and their texture. Examples of igneous rocks are granite, rhyolite, andesite, basalt, diorite, gabbro, peridotite and komatiite.
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Sedimentary:
When rocks break down due to the processes of weathering and erosion, sediment is created. Sedimentary rocks are formed from particles accumulated and consolidated on the earth’s surface after the rocks breakdown. Accumulation zones may be a lake, an ocean, floodplains, and quiet parts of a river or stream, desert dunes or even glacial fallout. Sedimentary rocks are named based on the size and relationship of the particles within them. Features of the rocks give clues as to the environment of deposition, such as different organisms present at the time, waves, seasonal aridity, wind and current directions, tidal flows, major climate change, Examples of Sedimentary rocks include sandstone, limestone, mudstone, siltstone, greywacke, conglomerate and breccia.
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Metamorphic:
Created when pre-existing sedimentary or igneous rocks are subjected to heat and/or pressure, metamorphic rocks will often retain many of characteristics of the original rock. They are generally more complicated than the other two rock types and can be quite spectacular. Metamorphic features can reveal things about rocks such as whether they have been buried, been in contact with volcanic products, or even been flushed with hot fluids.
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Silurian Sandstone
The Silurian sandstone is a Sedimentary Rock. These rocks consist of particles of variable size bonded together and hardened to form a solid rock. This rock unit was produced after a deep ocean accumulated fine sediments over a very long period. The layers alternate between sand and silt sized particle. Each layer represents a period or event of deposition.
There are no fossils known to exist within the Silurian rocks of Darebin Parklands. Within the same unit further to the north, huge populations of Brachipoda appear in intermittent layers. They have all died together most likely buried by a sudden large influx of sediment. Their size is small and the shells articulated.
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In order to understand geology, we need to think about the rocks below our feet in three dimensions. Those spatial relationships— what’s on top of what, how rocks are faulted or folded— give us all kinds of interesting information. While maps are useful in all kinds of ways, they’re a little lacking in the 3-D department. So when we’ve measured the orientation of a rock layer, we need a way to represent that on a map. Here’s a basic primer on how that’s done.
The geological lingo for this is “strike and dip”. The words may be confusing at first, but it’s really quite simple. Let’s start with some nice, horizontal sedimentary rocks. Pretend the top of this block is the surface of the Earth. Picture a kangaroo hopping across it, if that helps or entertains you.
![strikedip1]()
Let’s picture a single layer of that rock and tilt it downward to the right. The tilt of that layer is what we call “dip”. We describe this layer as dipping to the right, because that’s the direction it’s tilted downwards toward. We measure the dip as the angle between the layer and horizontal.
![strikedip3]()
Now, let’s try to imagine looking at that same tilted layer from directly above it. Again, it’s dipping downward to the right. Draw a horizontal line across that layer— that’s what we call the “strike”. (Don’t worry about why we use that word.) In this example, the strike is north-south. That means this layer is dipping to the east.
![strikedip4]()
The Fun Part:
At the published location you will be facing a sign about the geology of Dights falls, it is the sign that will help you answer the following
Q1: The sandstone and mudstone have been dated using marine fossils to be how old?
Q2: The course of the Yarra was pushed up against these older outcrops by lava flows which poured down the Merri and Darebin creek valleys from northern and western Victoria how many years ago?
Q3: What angle do you think the Dip is?
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We hope you enjoy the Earthcache and the area.
Regards
The Coffee’s