Coal was formed over hundreds of millions of years.
During the Carboniferous period (290 to 360 million years ago) the ancient continent of Gondwana was largely covered by a huge ice sheet, which blanketed small rift valleys in the area now known as Collie.
By the start of the Permian (240 to 290 million years ago) the ice had gone, although the climate was still cold and wet. Broad rivers which meandered through the landscape deposited sand and silt, which was interspersed with peat formed in damp forests and bogs nearby. Compaction of these layers and some further movement on fault lines caused the Permian sediments to be gently folded down into the older valleys.
Erosion during the later Triassic (200 to 240 million years ago) and Jurassic (135 to 200 million years ago) removed most of the Permian sediments, sparing only those protected within the deepest of the older valleys.
This erosion was brought about by formation of the Darling Scarp, a massive fault which runs parallel to the Western Australian coastline for 1000 kilometres. The Collie Basin (and two smaller but similar coal basins near Boyup Brook to the south east of Collie) are the only Permian remnants east of this fault.
The Cretaceous (65 to 135 million years ago), spelled the end for Gondwana as the continent broke up into four main pieces, three of which are now call South America, Africa and India. The fourth piece consisted of Australia and Antarctica, which stayed more or less intact throughout the period.
In the Collie area, flat lying sands and clay were deposited in the bed of a shallow lake, covering the upturned and eroded edges of the Permian sediments with a layer now know as the Nakina Formation.
The surface between the Permian and Cretaceous rocks is called an unconformity and represents over 100 million years of “lost” geological history. The Nakina Formation covers the Permian sediments, which generally do not outcrop. The coal exposure found in 1883 is still the only one known and today is difficult to see.
The Collie Basin is about 26 kilometres long and about 13 kilometres wide. It contains three distinct groups of seams that are from the top termed the Muja, Premier and Ewington Coal Measures.
- Muja seams are mined by Griffin Coal in the Muja Mine.
- Premier seams are mined by Griffin Coal in the Ewington Mine.
- Ewington seams are found around the edges of the basin and were formerly mined in many of Griffin Coal’s early open cut and underground workings.