Download Around 300 million years ago huge earth movements caused the

Around 300 million years ago huge earth movements caused the earth's crust in the region to subside and the
quartzite landscape on which the Blue Mountains stands was flooded by a shallow sea from the east. Streams
flowing into this sea carried huge amounts of sediment, which were deposited in horizontal layers. Later, these
layers formed rock beds of shales, siltstones and mudstones.
The soft sandstone of the Blue Mountains is easily eroded over time by wind, rain and rivers, and the cliffs
surrounding the Jamison Valley are being slowly broken up.
The sandstone has strong vertical jointing in a grid like pattern and over time erosion widened these joints to
leave the vertical turrets of sandstone which form the Three Sisters.
Formations like the Three Sisters are created when water seeps into small cracks in the rock, gradually enlarging
them over time to form large indentations.
The section of sandstone that makes up The
Three Sisters also contains many horizontal
layers of erosion resistant ironstone that have
probably helped the Sisters to resist erosion
and stand proud.
In the future, joint widening between the
Sisters will endanger them as they become
more and more narrow.
They will eventually tumble into the valley below.
http://www.geomaps.com.au/scripts/threesisters.php