How the Blue Mountains were created

The Blue Mountains are the remains of a plateau that has been mostly washed away by rivers and creeks.

About 250 million years ago the rock that makes up the plateau was laid down as layers of sand, silt and the remains of plants in a vast coastal inlet, now called the Sydney Basin, which stretched from present-day Sydney to Rockhampton in Queensland.

Rivers flowing into the trough eventually filled it with sediment, creating deltas and swamps, in which they continued to deposit sand.

Between 35 and 90 million years ago, several geological upheavals raised the sediment-filled bay to create a plateau that sloped gently downward from west to east. Existing rivers and creeks continued to flow across the plateau, cutting down through the sedimentary layers, now compressed and turned mainly to sandstone, and creating the dramatic landscape we see today.

Not all the prominent features of the Blue Mountains are purely sedimentary. Some may have once stood as islands of older quartzite in the sedimentary basin. Others are capped by hard basalt that was spewed out as lava during volcanic eruptions about 15 million years ago.

The plateau, created when the sediment filled bay was lifted, up to 90 million years ago, has a gently undulating surface. Creeks and rivers flowing across it erode rock, creating sand and silt. This material gradually scours even deeper into the plateau, forming V-shaped valleys. The more erosions debris the water carries, the more it cuts trough the sandstone strata.

Beneath the sandstone lie layers of rock dominated by shale, deposited during the early to middle stages of the silting of the Sydney Basin. This shale soaks up water, softening and easily breaking away. When the rivers and creeks reach it, they cut down quickly. The softened shales break up, forming a notch or cavity, undermining the sandstone slopes of the valleys.

The sandstone beds are broken into blocks by natural fractures called joints. The undermining creates sandstone overhangs so that the individual blocks eventually become unstable, break and collapse along the natural fractures. The shale still forms a V-shaped lower valley but is covered by a heap of rubble, known as a talus. The fracture form steep U-shaped cliff-lined valleys and the shales provide soils for thick scrub cover. The rivers often form waterfalls where they flow over the hardier sandstone.