Over 280 million years Tasmania was part of Gondwana. Around 160 million years ago came the first signs of the break-up of the supercontinent Gondwana with the intrusion of vast volumes of molten rock into the upper layers of the Earth’s crust. The molten rock produced dolerite rock common on Tasmanian mountain tops such as Mt Barrow, Mt Wellington and the Great Western Tiers. Similar dolerite is found in India, Antarctica and South Africa, which were all parts of Gondwana at the time the dolerite formed.
Faulting caused both uplifting and lowering forming fault valleys called grabens. This process formed the present landscape, with the Western Tiers to the south-west of Launceston and the range of dolerite capped hills and mountains from Mt George to the Ben Lomond massif east of the Tamar Graben.
Various freshwater lakes developed and disappeared in the graben valleys. In these lakes considerable sand and mud was deposited. The sediments formed into rocks, which now occur under large areas of Launceston. Rivers and streams excavated channels into these sediments after the lakes drained away.
In Kings Meadows a major fault line exists west of the high school with dolerite outcrops west of this fault exposed along the Kings Meadows Connector. To the east is mainly lake sediments, right through to the North Esk River, with the exception of an area around the Punchbowl and Watchorn Street.
Punchbowl has an outcrop of dolerite “where there shouldn’t be any”. This dolerite outcrop has occurred because movement on the fault lines was not uniform. Erosion in the Punchbowl area will have originally been controlled by the nature of the lake sediments resting on top of the dolerite, but once the dolerite was uncovered, the direction of the fault lines in the dolerite have become the major control. Just upstream of the road bridge in the Punchbowl the course of the rivulet is very close to a fault zone, creating the unusual feature there.
Kings Meadows rivulet wanders across the surface of lake sediments until it meets the harder dolerite, where the power of erosion has created a deep, narrow channel along a fault line, until the rivulet passes eastwards off the dolerite and widens its valley on the lake sediments near Penquite Road.
As you head westward out of the Punchbowl dolerite-derived soil and small outcrops are visible almost to the exit, while a similar situation exists driving eastwards up and out from the bridge. The flat area of Norwood to the east rests on lake sediments, which also make up the steep hillsides west of the North Esk River.
Question 1 At WP1 S41 27.439 E147 10.071 how far away is the natural rock fissure.
Question 2 At WP2 S41 27.457 E147 10.063 near the bridge what colour is the rock formation straight ahead of you.
Question 3: Follow the track to the top. At WP3 S41 27.499 E 147 10.049 take a seat and describe what you see. You will need to take a photo of this area and send it to me.
Please send me your answers and i will reply to you if you have them right i will let you log a find. No email no find.
Ref: QVMAG Geology Section Douglas Ewington 28-10-2008