Midway Point is an interesting small peninsula that extends into the Coal River estuary part way between Hobart Airport and Sorell. As you will have noticed it is a convenient link that has enabled bridges to be used for road transport between Hobart and Sorell.
Midway Point sits between Orielton Lagoon to the east and Pittwater to the west and is comprised mainly of Triassic Sandstones and some sandy depositions similar to those found at the nearby Seven Mile Beach.
As with other parts of southern Tasmania this small peninsula was raised by activity that formed a rift (deep) valley for the Coalbrook River, and the intervening higher hill lines such as Mid-Way Point. The estuary of the Coal River (Pittwater) is a drowned river valley excavated during the last glacial period (approx. 20,000 years ago) and flooded by post-glacial sea-level rise. The estuary has been modified into a wide lagoon-like enclosed water way because of the development of the Seven Mile Beach Spit across the original wide and open river mouth. As with many of the areas of the South Coast of Tasmania rock features are exposed on the fringes of the river estuaries. These rocks are openly exposed to various types of erosion.
Rock face erosion occurs through various forms of physical and chemical weathering, and is influenced by agents like water (waves, rivers), wind, and ice. Key processes include hydraulic action, where the force of water and trapped air breaks down rock; abrasion, where sediment carried by water or wind grinds and scrapes the rock surface; attrition, where particles carried by water collide and wear each other down; and solution, where acidic water dissolves certain minerals. Mass movement, such as landslides and rockfalls, also plays a role.
The effect of the Seven Mile Beach spit is to alter the inward flow of tidal currents and also the outward flow of river currents (essentially at low tide). This is because the spit restricts clean outward flow of the river (and inflow of the sea) and alters the areas of highest flow (potential erosion) and lowest flow (potential deposition). Man-made modifications to the flow (such as the building of bridges and roads across the waterway – seen in this area) also modify current flows. In this area the river has been channelled into very small channels which take the water under the roads making significant change to all of the river development patterns that would be found if the river outflow was unrestricted.
The two diagrams below show the usual current flows at high and low tides.
To claim this cache, send your answers to the questions to the CO and then log the cache. Failure to send answers will result in deletion of logs.
Q1. From GZ describe the cliffs that you see tucked into to the SE side from GZ. You should mention colours, banding effects and any erosional features.
Q2. Referring to the tidal maps explain why these cliffs show a great deal of evidence of erosion.
How has the tidal flow has been influenced by the 7 Mile Beach Spit and by human interference.
Q3. Other than tidal erosion can you offer other ways in which the cliffs are subject to ongoing degradation.
Q.4 Please place a photo in your log of the wider Pittwater area avoiding showing the cliff area you have described in your answers.
If you wish to preserve the sequence of your logging you may log immediately, however, if appropriate answers are not received within 14 days logs will be deleted.