Shaping the Landscape continued

Phase 5 Today's mountain catchments

The Dandenong Ranges Source - Museum Victoria Photographer - Rhys Jones

The present highlands of south-eastern Australia represent the remains of a deeply eroded mountain chain with a protracted and tortuous history starting about 500 million years ago. Erosion of the topography is ongoing and the present rivers will continue to erode for millions of years into the future until the highlands of south-eastern Australia are completely worn away and the landscape returned to a flat, undulating plain. Unless, of course, another mountain building event intervenes in the mean time!

Crystals in the mountains east of Melbourne

Quartz Source - MV; Mineralogy & Petrology Collections

Granite in the mountains started out as molten rock that cooled slowly deep beneath the Earth's surface. Virtually all the granite solidified to form a uniform mass of small interlocking crystals of quartz, orthoclase, plagioclase and mica. However, in places, cavities within the granite allowed larger crystals of these minerals, as well as several others such as topaz and tourmaline, to form. These beautiful specimens are difficult to find, but occasionally a granite quarry may unearth a crystal-filled cavity.

Topaz Source - MV; Mineralogy & Petrology Collections		Tourmaline Source - MV; Mineralogy & Petrology Collections

After the granite near the surface is softened by weathering, mountain streams can wear it away more easily. Small grains and crystals of quartz carried by water act like sandpaper wearing away at the underlying rocks. Crystal-bearing cavities are affected in the same way, and the mountain streams may concentrate larger and heavier crystals in their gravel beds. Gem fossickers soon learn how to find these crystal-bearing gravels.