The Geilston Bay fossil site is a paleontological site of Late Oligocene (or possibly Early Miocene) age in south-eastern Tasmania, Australia. It lies within the suburb of Geilston Bay which, although it is a part of the municipality (city) of Clarence, is effectively a suburb of Hobart located on the eastern shore of the River Derwent. The Geilston Bay site is important as one of the few of its age to yield fossils of mammals from the Late Paleogene Period (earliest Tertiary); at the time of their redescription in 1975, the mammal (marsupial) fossils were considered the earliest then known from Australia, although that distinction has since been surrendered to fossils from the Murgon fossil site in south-east Queensland, whose fauna—known as the Tingamarra Fauna—is of Early Eocene age. In addition to the somewhat fragmentary mammal bones still in existence, a range of plant macrofossils have also been described from the site, which appear not to have survived. The limestone deposit (in a form known as travertine) in which the fossils occurred was extensively mined (possibly completely) for the production of lime in the nineteenth and early twentieth centuries, and its location, while still publicly accessible, now lies buried beneath landfill under the playing fields of the former Geilston Bay High School.
Geological setting The Geilston Travertine (also referred to as the Risdon Travertine in older literature, also spelled "Travertin" on occasion), is a freshwater limestone deposit that (according to Tedford and Kemp) "fill[s] the narrow end of a valley excavated along a fault displacing Permian sediments. More than 10 m of sediments, passing below sea level, are [or were] preserved in this Tertiary paleovalley. ... [Tertiary] Alkali basalts overlie fossiliferous sediments at Geilston Bay, Lindisfarne Bay, and at Selfs Point north Cornelian Bay. They pass below sea level beneath the Tasman Bridge and extend along the floor of the Derwent estuary. ... In the case of the Geilston Bay outcrops, travertine deposition continued after emplacement of the lava. ... The basalt overlying the Geilston Travertine (Tedford et al., 1975, see data) yielded 22.4 ± 0.5 Ma, which can be corrected following Dalrymple (1979) to 23.0 ± 0.5 Ma." The general scientific consensus is that the travertine was deposited as calcium-rich water emanating from a fossil spring which subsequently dried out and solidified, on occasion incorporating portions of the surrounding biota that fell into it, before the water could flow away. Since the corrected date (between 22.5 and 23.5 Ma) for the overlying basalt exactly straddles the Oligocene-Miocene boundary, accounts such as that of Tedford and Kemp refer to the deposit as [late] Oligocene, while other workers (particularly those who have published on the Riversleigh fauna, the earliest portion of which is also considered to be late Oligocene), refer to the Geilston Bay deposit as early Miocene. However, as Tedford and Kemp point out, the Oligocene-Miocene boundary date is for the overlying basalt, not the travertine, which therefore is most likely to be Late Oligocene in age. Johnston (1888, page 284) groups the Geilston Bay deposit with other Tertiary deposits in the Derwent Basin considered to be of similar age, notably deposits at Cornelian Bay, Sandy Bay, and One Tree Point (now Blinking Billy Point), and notes their similar flora and, possibly, fauna.
Contemporary descriptions Since the deposit is no longer extant, it is worth noting in some detail contemporary (plus some later) descriptions of it, ranging in date from the nineteenth to (approximately) mid twentieth centuries.
Some nineteenth century commentators believed that the deposit had been described by Charles Darwin during his visit to Hobart and surrounds in 1836 as "a solitary and superficial patch of yellowish limestone or travertine, which contains numerous impressions of leaves of trees, together with land-shells, not now existing", however subsequent, more detailed descriptions that he gave of the site, lead modern commentators to infer that Darwin was in fact describing a separate, hillside outcrop in Hobart itself (in Burnett Street), no longer in existence.
The earliest documented visit that is currently known to researchers is a note by Robert McCormick, surgeon on H.M.S. Erebus published in 1847, who visited the site during a visit of the Erebus (with its sister ship, "Terror") to Hobart in 1841 (along with a different travertine quarry, the one in Hobart Town) and noted the remains of "Dicotyledonous plants of an extinct flora" plus "a helix and bulimus" (land shells), the latter in the Geilston deposit only. Of the Geilston deposit he wrote: "The limestone is of the same yellowish colour [as that in the Hobart Town deposit], but more indurated in texture, and has been quarried to the depth of seventy or eighty feet. The shells are found in the upper layer, and both leaves and stems in the lower portion." We do not have a date that the quarry was started, but from this account it seems likely that it was already some years earlier; Hughes, 1957 gave a date of 1836 for the quarry opening, but no source is given, and his remark may simply be based on the subsequently discounted, supposed Darwin visit of that year; however there is an 1838 letter (quoted in Ward, 2021) which mentions a John Price (a tenant of the owner) "cultivating the land and burning lime for which he has an easy market in Hobart Town".
According to an account by von Ettingshausen (1883), McCormick collected plant material from both sites which was subsequently deposited (under the label "Erebus and Terror") in the British Museum in London and was later seen by von Ettingshausen there, refer next section for further details.
J.B. Jukes (inferred as the author, see note) referred in an 1843 newspaper article to "Mr. Price's limestone quarry in James's Bay" (James's Bay being an old name for Geilston Bay), lying "horizontally in a small valley". Jukes mentions the quarry again in an 1847 paper, stating that the travertinous limestone at "James's Bay" "rests ... nearly horizontally, and is but little elevated above the level of the sea."
M. Allport gives an account of the fauna and flora of the quarry in 1866, which he reports as having visited "for twelve years past" (in other words, since 1854 or thereabouts); the (newly opened) bed in which the fossil mammals occurred is stated as being at "a depth of thirty feet from the surface soil". Allport's account is reproduced in more detail in the section below.
"S.H.W." (identifiable via a later 1884 account as S.H. Wintle) gives an account of the quarry in 1868, reporting that it had a maximum depth of almost 100 feet. He wrote:
In 1884, F.W. Krausè included the Geilston deposit three times in his maps and cross sections of the "geology of the area around New Town", the latter now a northern suburb of Hobart. He depicts the Geilston deposit as forming a low rise above sea level, and included a cross section drawn from inside the quarry, (both drawings reproduced here) from which it may be seen that the cross section shown would be between about 19 and 23 feet, with the bottom of the excavation (including perhaps a further 2 feet of "stiff brown clay") given as about six feet below sea level.
There also exists an unattributed 1902 newspaper article entitled "Tasmanian Lime Industry" from 1902, in which the quarry is most likely the one wherein "Messrs Wilson Bros ... turn out a brown lime on the other side of the river at Risdon. This brown limestone runs beneath the river level, and in years past it has been found necessary to erect pumps to reduce the water in the quarries."
Nye, 1924, wrote that by that time, "several quarries have been opened up" on the site, and noted that "The limestone occurs as irregular beds up to 5 feet in thickness, and is very dense and homogeneous. Remains of numerous fossil plants, fruits, and wood, as well as fresh-water shells, have been obtained from these quarries."
Later Hughes, 1957 noted that "In the quarry, being worked in 1924, were several beds of limestone, varying in thickness up to six feet, inter-bedded with sandstone and mudstone and overlain by 10 feet of basalt. At that time the stone was used by the Electrolytic Zinc Company and earlier the lime had been burnt in a kiln on the spot. Today, all that can be seen is a large pit with water in it and the remains of the old kiln.
Possibly the last account of the former quarry plus its associated deposit was given by W.R. Moore in 1965 who wrote: "The only definite Tertiary sedimentary deposit that outcrops in the Risdon Vale area is the limestone at the head of Geilston Bay. This fossiliferous limestone has been worked since before 1836 ... The deposit is practically worked out and the pits have been flooded so that it is difficult to gain an idea of its original extent and appearance or the stratigraphic relationship between the limestone and the associated basalt."
From the above, all that can definitely be said is that the quarry had most likely been in existence for some years prior to the first documented visit by McCormick in 1841, and was still in operation, by now as a supply point for limestone for the Electrolytic Zinc Company across the river, in 1924, as well as being definitely defunct by 1946, as evidenced by the aerial photograph included here. The height of the working presents a problem, being stated indirectly or directly as something approaching 23 feet (Krausè account), at least 30 feet (Allport), "seventy or eighty feet" (McCormick), and "almost 100 feet" (Wintle), as well as [the deposit] passing perhaps 6 feet below sea level (or perhaps more?). From this present distance in time, it is hard to envisage an entire hill of up to 100 feet being quarried away without trace, but possibly this is indeed what happened, allowing that some of the deposit may also have extended underground.
The 1946 aerial photograph detail reproduced here seems to be the only photographic evidence extant of the area occupied by the former quarry workings, and appears to show 2 areas of former quarrying activity, the northerly one filled up with water as later stated in the 1965 account by Moore. Since that time, in preparation for the construction of the adjacent Geilston Bay High School in around 1971, the area was levelled for the school playing field and no trace of the palaeontological site is visible at the surface today; whether or not any of the original travertine and/or related strata still exist in a condition untouched by the former quarrying operations is unknown.
Plant and animal fossils The earliest "formal" published description of the deposit and its fossils, by M. Allport in 1866 under the title "Notice of some fossils recently discovered near Risdon, Tasmania" was brief (just 3 pages) but full of interest. Selected paragraphs from his initial description are as follows: Allport goes on to say that he is about to send the animal bones to Professor Owen in London for his opinion on their affinities (refer additional discussion below).
A slightly later account was given by R.M. Johnston ("Johnson" in that publication only) in 1879; Johnston goes on to detail a large number of specimens of fruit, leaves and cones from the Geilston Bay limestone, which he has "referred to Baron von Mueller" for identification and of which the specimens do not seem to have survived, plus several land snails, 2 of which he describes as new (one is named Helix Geilstonensis after the locality).