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Around 280 million years ago this area lay in a shallow sea to the east of the primitive Australian landmass. The Earth at that time, know by geologists as the Permian, was gripped in an ice age and glaciers moved across the landmass, picking up rocks as they traveled, then sheared off at the coast and formed numerous icebergs. As these icebergs drifted over the sea they melted dropping their encased rocks into the mud on the sea floor. This rock platform was formed in the Permian to Triassic periods around 180 – 280 million years ago.
Norah Head is an important Geology location and is located at the northern side and end of the area known as Sydney Sandstone.
Large cracks in strait lines and at right angles can be seen across the rock platform. These are major Joints or fissures caused by earth movements or the release of pressure.
Based upon the angle at which joint sets of systematic joints intersect to form a joint system, systematic joints can be subdivided into conjugate and orthogonal joint sets. The angles at which joint sets within a joint system commonly intersect is called by structural geologists as the dihedral angles. When the dihedral angles are nearly 90° within a joint system, the joint sets are known as orthogonal joint sets. When the dihedral angles are from 30 to 60° within a joint system, the joint sets are known as conjugate joint sets.
About 60 to 30 million years ago volcanic activity resulted in extensive outpourings of lava. This can be seen in the visible dykes and volcanic necks which allowed the molten rock to reach the surface. At the end of the activity the molten rock solidified. It has the same chemestery as basalt (volcanic rock) but because it solidified below the surface it is dolerite (plutonic rock).
Along the edges of the dykes is some variation in the adjoining conglomerate. This was caused by the heat of the molten rock. Along the edges of the dolerite there are vertical cracks parallel to the edge of the dyke caused by the quicker cooling of the rock. Away from the edge of the dyke there are cracks running parallel and at right angles to the edge.
Parking is at: S33 16.947 E151 34.434
The start location 1: S33 16.897 E151 34.584
Called “A Grand Old Lady”, Norah Head Lighthouse has provided coastal shipping with a navigation aid since 1903.
Location 2: S33 16.882 E151 34.638
In the platform there is a volcanic intrusion, a channel of darker rock running from one side of the platform to the other. The intrusion was created by waves quarrying a softer basalt lava flow out of the surrounding granite. The lava came from an ancient volcano, probably Mount Warrawolong to the west.
Question 1: a) What is the type of material is the dark material?
b) Did it solidify above or below the surface?
Location 3: S33 16.841 E151 34.575
At this location, you are observing a fault line across the rock platform. The interesting aspect of this is the manner in which erosion is occurring.
Question 2: a) Use your GPS or another method to determine the direction it runs?
b) As a result of erosion, the surface of the rock within the Joint or fissure is lower than the surrounding rock platform. What depth would it be?
Location 4: S33 16.933 E151 34.671
At this point you are standing between Big Gutter and Little Gutter(Two very large Joints or Fissures formed by wave action).
Question 3: Use your GPS or another method to calculate the width of both gutters?
That ends this examination of the Rock Platform at Norah Head. I hope you have enjoyed your visit to this impressive Geological feature.
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Joint First To Find goes to kamfindit & ALL7.well done