The light-coloured sedimentary rock downstream of the Organ Pipes were formed by the accumulation of rock fragments, sand, clay and mud under the sea into successive layers or sediments. These layers were eventually compressed into rock.
Fossils of sea snails, sea worms and extinct floating animals called graptolites are found in the rock, indicating that it was laid down some 400 million years ago. Tremendous subterranean forces over millions of years caused gradual upheaval and sinking of the land. A fall in sea level then left the rock almost 80 metres above present sea level.
Watch these videos to see how sedimentary rocks are formed:
How Are Sedimentary Rocks Formed? - STEAMspirations (shorter video than above)
Most rocks in the park are dark grey or brown. The Organ Pipes themselves are formed of the hard, dark rock called basalt, a volcanic rock derived from lava. Much of the basalt is pocketed with small air bubbles. The air holes are a result of steam trapped in the lava; as the steam escaped the air pockets remained.
For about 20 million years, volcanic activity was widespread in southwestern Victoria. The lava covering the Organ Pipes National Park area is a recent flow, only about a million years old. The source of this lava was probably the group of low volcanic hills which may be seen about 6 km to the north of the park. These volcanoes are now extinct, or at least dormant.
Although each individual lava flow was quite thin, the plain was built up by successive flows from many volcanoes over a wide area. The lava plain extends from the foot of Mount Macedon to Williamstown and Laverton and is part of the third largest lava plains in the world: the Western Victorian Volcanic Plains.
The quartz and quartzite gravel found halfway down the track from the car park to the Organ Pipes is part of a deep lead – an ancient stream bed buried by a lava flow, and later revealed by the downcutting of Jacksons Creek. Deep leads were mined for gold in Ballarat during the Gold Rushes.
Watch these videos to see how igneous rocks are formed:
geological features of organ pipes national park
The Organ Pipes
The Organ Pipes are a spectacular example of basaltic columns. Rising to 20 metres in height, the Pipes are up to one metre across and are hexagonal in cross section.
Very few of the columns are straight or vertical; a number of the smaller columns around the Pipes are very much tilted, some almost horizontal. The Organ Pipes were so named because of their resemblance to a pipe organ.
The car park and picnic area at the Organ Pipes National Park is located on the remains of a very weathered scoria cone. Approximately 800,000 to one million years ago – around the same time as the larger volcanoes to the north were producing lava – this cone ejected molten rock in a series of explosions, producing scoria. Scoria is brownish in colour and is filled with air pockets.
Five hundred metres upstream of the Organ Pipes, overhanging the northern bank of the stream, is a large outcrop of basalt with a radial array of columns resembling the spokes of a giant wheel. It was formed by the radial cooling of a pocket of lava, probably in a spherical cave formed from an earlier lava flow.
On the valley floor, about 250m upstream of Rosette Rock, is a basalt outcrop that has a tiled or mosaic-like appearance. It is another area of columnar basalt, but instead of the vertical faces being visible (as for the Organ Pipes formation), the horizontal faces are visible. You can walk and climb over them. The columns tend to be hexagonal, but many have sides of unequal length and there may be from four to eight sides on each column. four to eight sides on each column.