Merri Creek Tessellated Pavement EarthCache

Merri Creek Geological site :  This is a very clear example of a common feature of massive lava flows and is the best example of this type of jointing forming a tessellated pavement in the Metropolitan area. The site can be used to illustrate how the direction of movement of a lava flow can be determined. 

A small outcrop of basalt about 10 m x 10 m in area just above water level displays a roughly symmetrical jointing pattern forming a tessellated pavement.  The type of tessellated pavement found here, 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 hexagonal joints, called columnar jointing, that formed as the result of the cooling of basaltic lava.  The joints are contraction cracks formed as the basalt was cooling and their elongation indicates the flow to be still moving as the joints were forming. The direction of axial elongation of the polygons indicates that the basalt was initially moving as it was cooling (following the ancestral course of the Merri Creek), This trend may have reversed later as the flow backed up a tributary of the Merri Creek along the alignment of the present Edgars Creek. The upper surface of many of the polygons of the pavement has a marked concavity. This feature (known as "ball and socket" jointing) may result from original curvature of horizontal joints but on exposed surfaces it may be initiated or emphasised by chemical weathering as water collects in depressions on the pavement.

Now find the Polygon Pavement in this picture, it is in the northern most group :

Email or Message me the following answers:

A. Can you see evidence of pavement concavity at GZ, describe your observations ?

B. Pavement elongation - POLYGON 1 : In which direction  was the basalt moving in ? (Two cardinal points - eg.  NNE to SSW )

C. From the picture observe POLYGON 2 at the site.   How many sides does this have?

D. Optional - Weed removal - As the site is small it is at risk of being covered by vegetation growth and by flood-borne debris or sediment. - Bring a bag and remove some weeds growing in the joints, and other rubbish.

E. Optional - A picture of yourself at the site showing the pavement and/or removing weeds.

You may log this before submitting me your answers. K.