Pulpit Rock - Lower Derwent Valley

Pulpit Rock provides an excellent vantage point over New Norfolk and the Lower Derwent Valley.

You will be able to see a vista of the hills and mountains to the south from this site, with a view of the Derwent River below as it meanders around to the freshwater point (around 10km to the southeast, near the Bridgewater Bridge). Looking Southward, you will see a valley that leads towards Lachlan with rolling hills in the near and mountains over 1000m further away in the distance.

Earth Science Significance

There are several interesting and significant features to observe.

Landslip & Fault - East South East

Look downstream slightly towards Boyer and Sorell Creek, you will see opposite the ponds and the exit of teh Lachlan River where the cliffs have started to fall and material has been desposited.  The erosion action of the river in flood against this siltstone/mudstone has, over many years cut further into the land to the north, creating these cliffs, with deposits at the base (where the road is).  The landslides come from material washing down.  Several cracks exist, where a firm line can be seen with missing material.

Past the landslides, as the road disappears around the bend, opposite the Lachlan River the topography changes dramatically.  There is a fault in this rock travelling North-east and a transition can be seen - the hills are suddently much gentler and aren't cut into like they are closer to the lookout.  There is a mostly flat alluvial plane closer to where the Boyer newsprint mill is located. 

The terrain where you are standing is described as "Generally unfossiliferous glaciomarine interbedded non-fissile and fissile siltstone and silty sandstone, with common bioturbation and lonestones, rare pebbly beds and fossiliferous beds; top beds of laminated grey to brown siltstone with thin beds of" from the Permian period.

 After the point approximately 1km from where you are, this gives way to "Generally poorly fossiliferous interbedded glaciomarine fine- to medium-grained sandstone, fissile and non-fissile siltstone, lonestones and pebble-rich patches, productid bed at top, basal interval commonly with thick beds of coarse-grained sandstone" and "Colluvium derived predominantly from Lower Parmeener rocks".  The areas further down (which are most visible on the top side of the river) are "older alluvium of river terrace".

Lachlan River Mouth

South from your current location, you can see a sheet that is highest opposite where you are standing and in the vicinity of the old homestead (Turriff Lodge).  There are cliffs down to the river on this side, however the terrain slopes constantly down to the area past Millbrook Rise; albeit interrupted by the area around Tynwald Park and the Sewerage treatment plant, where the Lachlan River has eroded a path through the mudstone/siltstone.

If you look closely it is like a single continuous flat surface, except around the edges and the missing part in the middle.

Township and steep Mountains

The raised area of the township itself is perched on an outcrop of Basanite aged between 8.5 and 70 million years, with Peppermint Hill (the hill to the right with the 3 towers on it) made of the same rock that you are standing on, and the steep area between that hill and the township being siltstone/mudstone.  The nex layer out is triassic quartz sandstone followed by the distant mountains being primarily jurassic dolerite.

All of the peaks and steep slopes over 500m are Jurassic Dolerite (shown in an amber colour on the map below).  The lower, gentler hills are siltstone and quartz/sandstone.

From the left - you can see: Mt Faulkner just over the trees and the hill (and the secondary "south Mt Faulkner), then in the gap with the 1x large and 1x small diagonal powerline running over it that is Goat Hills (between Montrose and Collinsvale), in the near Big Rocky and behind that Mt Hull (just visible) in behind Big Rocky, and behind that Mt Arthur and Mt Connection.  The point is Collins Cap, and behind that is Collins Bonnet. and Trestle Mountain, Mt Marian, and Mt Charles.  The ridgeline continues to the right of the valley of the Lachlan River, with Jurassic Dolerite hilltops mostly above 500m all the way to the right field of view.

The red lines below show the field of view, and the "near", "middle" and "far" hills/mountains.

Many steep, north facing Jurassic dolerite hillslopes do not maintain adequate pasture cover and are subject to wind, sheet, rill and gully erosion which can be seen in places in the foreground.

References

References:

• WASSON, R. J. Last-glacial alluvial fan sedimentation in the Lower Derwent Valley, Tasmania, Sedimentology, Volume 24, Issue 6 https://doi.org/10.1111/j.1365-3091.1977.tb01915.x
• ANAND ALWAR, M. A. 1960 GEOLOGY AND STRUCTURE OF THE MIDDLE DERWENT VALLEY, Geological Survey of India and University of Tasmania https://eprints.utas.edu.au/14089/1/1960_Alwar_Geology_Middle_Derwent_Valley.pdf 
• ANAND ALWAR, M.A. 1958 Thesis https://eprints.utas.edu.au/18899/1/whole_AnandalwarNamdyamAnanthacher1958_thesis.pdf 
• MUSK, R.A. and DE ROSE, R.C., Department of Primary Industries, Water and Environment 2000 Derwent Report Land Capability Survey of Tasmania https://nre.tas.gov.au/Documents/Land_Cap_Report_Derwent.pdf
• Maps sourced from https://www.thelist.tas.gov.au/ 

To Log This Cache

You may log this cache immediately however you do need to send the CO your answers.  Logs may be deleted if answers are not sent or follow-up questions are not clarified

Questions

Q1. Do you think the river has eroded into the rocks where the cliffs are down to the left? Why?

Q2. From your vantage point, can you see any faults or changes in the composition?  Where and in which direction?

Q3. What is different about the shape of the higher/further mountains/hills (over 500m) in the distance compared to the close by hills? Why do you think that might be?

Q4. Why do you think the Derwent River bends around the raised area of the New Norfolk township but the Lachlan River cut a gap through the tilted siltstone sheet (around Tynwald Park, between Turriff Lodge and Millbrook Rise which is otherwise a steady descent)?

Q5. Can you describe the ground and the rocks that are visible in the immediate vicinity of the lookout (not the gravel but have a look off the track/sides a bit)?

Q6. How does this make you feel? (feel free to include this in your log).

Photo (Optional)

Please feel free to upload a photo of the vista and/or with yourself or a personal item.