This Earthcache will take you to the Basalt Tessellated Pavement at the Darebin Parklands.

A lot of cachers avoid earthcaches due to their perception that they are difficult,  we have tried to make the earth science lesson's easy to help get more people interested.

NOTE - During times of heavy rainfall or flooding this earthcache may not be visible

There is no need to climb down to the Tessellated pavement, all observations are possible from the top of the cliff, If you have young children watch them in this location as they could slip down the steep edge.

Darebin Parklands

The unique nature of rock outcrops within Darebin Parklands makes it an excellent study site for amateur or professional geologists. The associations within the park are regionally common, but globally rare. Silurian rocks are not internationally prolific and Victoria hosts a significantly large unit beneath Melbourne’s eastern suburbs. Combined with this Silurian unit is an extensive unit of Tertiary basalt. The basalt’s eastern limit is the Darebin Creek and the Yarra River to the south. Briefly to the north, and moreover to the west, the Victorian flood plain basalt creates the gentle undulating topography of the plains of southwestern Victoria.

Darebin Parkland - Basalt Tessellated Pavement

This Tertiary basalt outcrop was formed around 800,000 years ago due to lava flows from several volcanoes near Beveridge 40 kilometres north of Melbourne. The lava flows moved along existing stream beds and filled old stream valleys, creating new paths for the Darebin Creek. Weathering and erosion over the years has exposed this basalt tessellation.

Tessellated Pavement

In geology and geomorphology, a tessellated pavement is a relatively flat rock surface that is subdivided into more or less regular rectangles, blocks approaching rectangles, or irregular or regular polygons by fractures, frequently systematic joimts, within the rock. This type of rock pavement bears this name because it is fractured into polygonal blocks that resemble tiles of a mosaic floor, or tessellations.

Branagan recognizes four types of tessellated pavements. They are tessellated pavements formed by jointing; tessellated pavements formed by cooling contraction; tessellations formed by mud cracking and lithification; and tessellated sandstone pavements of uncertain origin^.

The most common type of tessellated pavement consists of relatively flat rock surfaces, typically the tops of beds of sandstones and other sedimentary rock, that are subdivided into either more or less regular rectangles or blocks approaching rectangles by well-developed systematic orthogonal joint systems. The surface of individual beds, as exposed by erosion, are typically divided into either squares, rectangles, and less commonly triangles or other shapes, depending on the number and orientation of the joint sets that comprise the joint system. This relatively flat surface of individual beds of sedimentary rocks are frequently altered by weathering along joints as to cause the bedrock along the joints to be either raised or recessed as the result of differential erosion. This type of tessellated pavement is commonly observed along shorelines where wave action has created relatively flat and extensive wave-cut platforms that expose jointed bedrock and keeps the surfaces of these platforms relatively clear of debris^.

The second type of tessellated pavement consists of a bedrock surface that exhibits joints that form polygons that are typically regular in size, spacing, and junctions. Typically, these polygons represent the cross-sections of polygonal, typically hexangonal joints, called columar, that formed as the result of the cooling of basaltic lava. This type of surface can be seen at the Giant's Causeway in Northern Island.

The third type of tesselation recognized by Branagan associated with the shrinkage and cracking of fine-grained, either clayey or calvareous, sediments. They consists of polygonal cracking, often associated with individual 'plates' that tend to be concave upward, that characterizes the formation of mudcracks in fine-grained sediments. Often, the outlines of the polygons formed by this type of cracking are preserved and accentuated by the infilling of the cracks with material of a different composition from that of either the clayey or calcareous sediments in which the cracks form. The infilling of the cracks by sediments of a different character often preserved the polygonal pattern of the cracking where it can be exhumed by erosion as a patterned pavement after the sediment becomes lithified into a sedimentary rock.

The final type of tessellated pavement consists of relatively flat, sandstone surfaces that typically exhibit a complex pattern of five- or six-sided polygons. Typically, these polygons vary greatly in size from 0.5 to 2 m in width. These polygons are defined by well-developed fractures that sometimes have raised rims. The origin of this type of tessellated pavement remains uncertain. The size and shape of these polygons appears to be dependent to a large extent on the grain size, texture, and coherence of the rock. This polygonal tessellation is best developed in relatively fine-grained, uniform, and siliceous or silicified sandstones

Logging requirement:

To Log this EarthCache, please send an email with answers to the following questions to our Geocaching.com account.

1. Your caching name and the EarthCache name

2. What is the approximate size of each paver.

3. What type of tesselatted pavement do you think this is?

4. How many Boulders are there sitting on top of the tesselated pavement ? And why do you think they are here ?

5. If you like include a photo of yourself at GZ, but please don’t include a picture of any of the questions as we will need to delete the photo.

Sources:-

http://www.dcmc.org.au/parklands/geology.php

https://en.wikipedia.org/wiki/Tessellated_pavement