Conglomerate Gully Walking Track

Conglomerate Gully is named after the fine exposures of conglomerate rocks which can be found along the valley of the creek. These rocks are at least 370 million years old and most probably much older and formed long before the Macedon Ranges.

What are Conglomerates?

Conglomerates are sedimentary rocks that are made up of rounded pebbles of other rocks which which have been cemented together in a fine grained sandy matrix.

In the Conglomerate Gully Flora Reserve the rocks are rounded, flattened, whitish pebbles which are cemented together in a reddish sand. The pebbles vary in size from 5cm up to 70cm with pebbles in the range 5-20cm most common.

How did they Form?

The conglomerates at Conglomerate Gully are part of a unit of rocks known as the Kerrie Conglomerate which is exposed in various parts of the Macedon Ranges (see geological map). The conglomerates were deposited in a system of broad, shallow, rapidly flowing streams. The water carried sand and rocks from nearby highlands, tumbling and shaping them into the smooth, rounded pebbles we find in the conglomerate.

The whole region was slowly subsiding, forming a depression which allowed a large thickness of conglomerate to accumulate. In the Black Range area the conglomerates are approximately 900m thick and about 840m thick at Mt Charlie. Substantial amounts of conglomerate have probably been removed by erosion so the original thickness would have been much greater. As the sediment accumulated the grains of sand and pebbles were cemented together under pressure of the overlying rock.

When were they Deposited?

Unfortunately the conglomerates do not contain any fossils, because the right conditions did not occur for these formations, which makes precise age determination difficult. By examining other features of the rocks it is possible to determine an approximate age.

The conglomerates overlie a series of sediments which were deposited in a deep ocean during the upper Ordovician period (approximately 440 million years old). These deep marine sediments were uplifted and folded and eroded before the conglomerates were deposited. The pebbles in the conglomerate represented material eroded out of these deep marine sediments. The conglomerate has also been heated up by the intrusion of granite nearby (see geological map), which is late Devonian in age (approximately 370 million years). This gives an upper and lower age for the conglomerates of 440 – 370 million years. Geologists favour an older age for these rocks, towards the 440+ million years.

Formation of the Macedon Ranges

After the deposition of the conglomerates of the Macedon Ranges region was affected by spectacular and catastrophic volcanism during the upper Devonian period (approximately 370 million years ago).

The volcanic complex which formed consists of an extremely thick sequence of silica-rich lavas (greater than 2km thick in places). These lavas were deposited in a 64km squared elliptical depression forced by the collapse of a large block of the earths crust. These volcanic rocks are very hard and much more resistant to weathering and erosion than the Ordovician deep marine sediments which they overlie. Hence the volcanics, which once filled a depression, now form the Macedon plateau rising some 400 meters above the surrounding lowlands. Associated with the period of volcanism was the intrusion of granite in the area (see geological map and diagram). To produce the range in this way the region has obviously undergone major erosion.

Approximately 6 million years ago (recent in geological terms) other volcanoes which occurred during the Devonian period but were part of volcanic activity which was widespread over Victoria up until the last few thousand years. Good examples of these volcanic rocks can be seen at Mt Eliza and Camels Hump on Mt Macedon Range. This wave of volcanism affected the drainage and stream patterns of the region. Lava flowed down stream valleys, filling them in. New streams have established along the edge of the lava flows, such as Riddells Creek.

The drainage system at Conglomerate Gully was not affected by this volcanism. This region would have been like an island surrounded by lava as the conglomerates at this time were high points on the landscape. The creek in Conglomerate Gully was probably established pre tertiary and may date back to the early Cretaceous (145 million years ago).

Conglomerate Gully Map and Rock Formation