At the given coordinates, you will find a sign which describes Charles Darwin’s visit to Hobart.
Charles Robert Darwin, (Born 12 February 1809, died 19 April 1882) was an English naturalist, geologist and biologist, best known for his contributions to the science of evolution. Darwin published his theory of evolution with compelling evidence in his 1859 book “On the Origin of the Species”. Darwin included evidence that he had gathered on the Beagle expedition in the 1830s and his subsequent findings from research, correspondence, and experimentation.
By the 1870s, the scientific community and much of the general public had accepted evolution as a fact. However, it was not until the emergence of the modern evolutionary ideas (from the 1930s to the 1950s) that a broad consensus developed in which natural selection was the basic mechanism of evolution.
What is less well known is that Darwin recognised himself as a geologist first and foremost. Darwin said of himself - "It was soon after I began collecting stones (when 9 or 10) that I distinctly recollect the desire I had of being able to know something about every pebble in front of the hall door--it was my earliest geological aspiration." At University, he was encouraged by his mentors (including Professor Adam Sedgwick , considered one of the founding fathers of Geology in England) to participate in Geological field trips. On his later voyage on “The Beagle” he gathered a great deal of geological evidence and in a letter to his sisters Darwin confessed that he "literally could not sleep for thinking over my [geology]".
In the five years that the voyage of the Beagle lasted, Darwin wrote 1,383 pages of notes about Geology - compared to 368 pages of notes on plants and animals. After returning to England (1836) Darwin presented his first scientific discourse of the geology of the Andes at the Royal Geological Society and published some preliminary results about volcanic phenomena observed in South America. His major contributions to volcanology are two later books: "The structure and distribution of coral reefs", published in 1842, and the "Geological Observations on the Volcanic Islands" published in 1844. The first book covers the distribution, structure and formation of coral-reefs around the slowly drowning volcanic islands in the Pacific, the second presents the descriptions of the visited volcanic islands, like Ascension, St. Helena, the Galapagos and a short notification about the geology of South Africa and Australia (principally Sydney and Hobart).
Therefore, it is hardly a surprise that on his 1836 visit to Hobart (as “The Beagle” made its way back to England) Darwin noted some of the significant geological features of the Hobart region. On his fourth day in Hobart, Monday the 8th of February, Darwin explored the Bellerive foreshore and noticed rapid changes in rock types. The first stone he identified was greenstone (dolerite). He was particularly interested in the near-by sedimentary rock with its fossils of marine shells, rock fragments and ironstone veins which reminded him of the sediment near Cape Horn where he had seen Andean glaciers descending to the sea and breaking off to float away as icebergs. Darwin encountered his first glaciers in January 1833 during the survey of the Beagle of Tierra del Fuego. He describes them of a beautiful "beryl blue" and noted that the ice falling from the snouts into the sea formed icebergs and that these icebergs often incorporated and transported rocks and debris. When the ice melts the debris is released and deposited on the bottom of the flooded valleys. In the area around the Bellerive Bluff he described a “white pottery-like” very fine grained mudstone and siltstone which he concluded (from the mixture of grains and materials) had formed from the debris droppings from a floating ice sheet on a shallow sea. Darwin was probably the first to recognise the effects of ancient glaciers in Tasmania. He also identified horizontally layered sandstones towards Howrah and differentiated these from the harder sandstones of the Waverley Hills area. On his fifth day, he found volcanic rocks near Blinking Billy Point on his walk from Battery Point to Sandy Bay. He concluded that the cliff nearby, which revealed two lava flows and much volcanic debris, was the eroded heart of an ancient volcano, a view that was not confirmed until many years later. Darwin was undoubtedly the first to recognise the remains of a volcano in Tasmania. Darwin also commented on the width of the Derwent Valley ("this Bay should rather be called a deep Estuary, which recieves at its head the waters of the Derwent)" and the background to Hobart Town – “the unremarkable Mount Wellington”
Hobart Town from Bellerive circa 1836
Formation of the River Derwent.
In simplified form rivers are generally characterised as having 3 sections - upper, middle and lower. In the stylised view of river development the lower section of the river appears as a wide, generally flat valley with the river meandering across the width of the valley in a sluggish manner. Good examples would be the Murray river in Australia and the Mississippi in the USA. In some rivers the lower sections are characterised by deep and wide rivers where deep water harbours are found. These are classified as drowned river valleys. A third common type are found in glaciated areas where the glaciers leave deeply incised deep water channels. These are usually called Fiords.
Two physiographic features dominate the Hobart district, the Mt Wellington-Mt Marian plateau and the Derwent lowlands. Relief is generally moderate to high and without exception, elevated country is dominated by dolerite bodies which give protection against erosion.
In detail, the topography is marked by fault-controlled features, such as escarpments and straight, narrow valleys. The Derwent lowlands are part of a graben some 32 km across. Step faulting within the graben has produced linear blocks at various elevations.
Formation of Graben Topography
The Mt Direction and South Arm Peninsula blocks represent a central zone elevated with respect to the eastern escarpment and downthrown compared with the western escarpment. The original River Derwent produced a deep V-notch valley from Elwick Bay to Cornelian Bay via Moonah and then south between Rosny Point and Macquarie Point. The valley is engraved in a thick dolerite sheet to a base level at least 200m below present sea level. Valley side slopes of 15° or more were common. By Early Tertiary times the valley and its tributaries were well formed. The position of the valley was controlled by first order graben faulting and the actual line of excavation by a series of nearly N-S trending faults in the New Town- Moonah area. Subsequent periods of deposition raised the base level of the valley.
Today the coastline is generally one of submergence. The Derwent estuary which extends for a distance of 52 km from New Norfolk at its northern end to the sea, at Iron Pot, was formed between 6,500 and 13,000 years ago when sea level rose around 60 m to near its current level. The upper estuary extends from New Norfolk to the Bridgewater causeway, and is characterised by a narrow channel 3-6 m deep, flanked by extensive wetlands. The middle part of the estuary - between the Bridgewater Causeway and Bowen Bridge - is 1-2 km wide, with a more convoluted shoreline with some rocky headlands and numerous small embayments. South of the Tasman Bridge the lower estuary widens and is characterised by relatively straight western and eastern shorelines, with a maximum depth of 44m observed immediately south of the Tasman Bridge.
Main sources of the above information were:
“Darwin in Hobart Town” Published by the Royal Society of Tasmania 2009.
“Geological Survey Explanatory Report – Hobart” D. E. Leaman BSc. PhD Published by The Tasmanian Department of Mines 1976.
"Darwin the Geologist" Scientific American 2012
Once you have completed the earth cache requirements you can post your find without delay, as per the guidelines. You will also need to verify your find by sending us an e-mail or geo-message providing answers to the questions.
To log this cache, go to the nominated co-ordinates: -
- Describe the rock features found on the foreshore near GZ (colour, texture, other obvious features).
- Which of the rock types described by Darwin are likely to be represented here? Give your reasons.
- Describe the obvious erosion hole on the rock platform, on the river side of GZ, and make suggestions as to why it may have been left like that.
- Which natural and man made features indicate that this rock may be easily eroded?
- How would the river valley (as exhibited by the Derwent River looking across to Hobart from GZ) be classified. Justify your answer.
- Photograph of yourself with the Derwent and Mount Wellington as a backdrop (optional).
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