How Were the Blue Mountains Formed?

The formation of the Blue Mountains landscape occurred over 300 million years ago and began with earth movements that created the quartzite base on which the mountains stand. From there, the geography was formed by rivers that regularly flooded the base and left sediment deposits. These deposits later formed layers of shales, siltstones and mudstones which turned into seams of coal. Roughly 500 meters of marine sediment came from this.

During the Triassic period that occurred 250 million years ago, rivers began depositing large amounts of sand on top of the shales and burying them. The layers eventually sank under the weight of the sand and formed a basin. As the sand continued to settle in the newly formed basin, the heat and pressure caused the layers underneath to turn into hard rock. These layers of hard rock became known as the Narrabeen and Hawkesbury sandstones. Approximately 80 million years later, sand stopped being deposited, and forces in the earth pushed the hard rock layers upward. The Blue Mountains is the name given to the resulting plateau that was formed. The reason that the plateau resembles a mountain range is due to the fact that the plateau has been dissected. Valleys and gorges resulting from volcanic activity are the main factors for the dissection and resulting appearance of the area.