Sedimentary

Geology of Bitou, Lailai and Beiguan, Taiwan

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Bitou – this small fishing village is about 70km north of Yilan City. Right next to it is the Bitou Geopark. Here you can take a clifftop walk above steep sandstone cliffs, or descend to the shore platform to see some strange mushroom like features at close quarters Here the shore platform is festooned with these strange mushroom shaped concretions. They really are unusual, and make this a famous geological location in Taiwan. As you can see it is also a popular spot for fishing. Due to storms and occasional freak waves there are many accidents all along this coast where people get swept into the sea. Our next stop was the well known shore platform at Lailai. Here the gently dipping sedimentary beds have been folded and faulted. with hard layers of sandstone being less easily eroded (and therefore sticking out more) than the more easily etched out softer mudstones. The shore platform is impressive, with the tilted sedimentary rocks folded into gentle curves, and a lot of faults cutting through the layers. It was a perfect area to use my drone to get these aerial images. A short distance away there is a dyke (an igneous intrusion that originally pushed into the sedimentary rocks as hot magma)  that can be soon cutting through the sedimentary layers of the shore platform. It stands out because it is made of harder rock than the surrounding sediments, and is therefore more resistant to being eroded. Here you can see the dyke is offset – sometimes by faults but also simply by the magma pushing up through slightly different pathways in the original country rock. You can see here how the dyke has baked the adjacent mudstone – giving it a darker colour for about 40cm  to either side of the once hot dyke. A closer view of the dyke standing up like a man-made wall on the shore platform. The baked sediments right next to it have also been hardened by the heating process, so they have also resisted erosion more than the softer surrounding rocks. This video shows a bit more detail of the rocks of Lailai ,which I think is an ideal place to run a geological field trip: Finally on our way back to Dongshan, we stopped in the small Beiguan Tidal Park where you can see these rocks with impressive joints forming a diamond checkerboard pattern. In the background is Turtle Island, another well known local feature.

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Geology on the Yilan Coast, Taiwan

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To find worthwhile locations that offer great learning opportunities in geology, you have to spend time exploring outcrops, trying to make sense of the geological features that are exposed and then think of ways that students can explore and make sense of them out of their own activity. This inquiry learning process can work well via guided questions that encourage careful exploration and observation and then the unfolding of ideas and understanding. however it doesn’t usually just happen by magic – it takes some working out to frame interesting learning activities at a given unique location. With a small group of teachers from CiXin School, we explored several locations along the coast north and south of Yilan. Heading South we went to a coastal fishing settlement called Feniaolin. Here there were some amphibolites (metamorphic rocks) that are part of a long outcrop extending further south. These are amongst the oldest rocks in Taiwan and have been exhumed from many kilometres deep in the earth’s crust. Just past the fishing wharf there is an area of sea stacks – classic coastal erosion features: We continued further south to the Nanao Valley where there is a mixture of rocks on the river bed including many huge boulders. Some of the boulders were granites (that were once molten magma deep in the earth). They had lumps of schist included in them – fragments of the crustal rocks (xenoliths) that must have been incorporated into the molten magma before it crystallised. – given them a very striking apprearence. All in all there is plenty here to discover – rocks and minerals that have been metamorphosed by intense pressure and heat a long way down in the earth’s crust. Here is a video I made about our trip:

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Nature’s Earthquake Recorders

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In order to make sense of the sediment cores that can be retrieved from lakes near to the Alpine Fault such as Lake Christabel, it is worth having a think about what happens to the environment when the fault ruptures in a large earthquake. Under normal conditions, alpine lakes fill up very slowly with sediment that is fed into them by rivers. The particles settle onto the lake bed gradually, to create a sequence of finely layered mud. When an earthquake occurs, a number of consequences affect the landscape. The soft surface sediment on the bed of the lake gets deformed and folded, and the shallower slopes at the side of the lake collapse to create flowing avalanches (turbidites) that sweep down and across the lake floor. In the nearby mountains, large landslides occur that choke the river valleys with a chaotic mix of large and small rock fragments. In the months and years following the earthquake, the landslide debris is gradually washed into the lake, to form a recognisable layers on top of the turbidite deposit. Eventually, conditions return to normal, with the finely layered sediments gradually covering over all of the evidence of the earthquake and its aftermath. It may be hundreds of years before another earthquake sttikes that is near enough and strong enough to leave its mark in new layers of the lake sediment. Now lets have a look at the real thing – an example of a sediment core that has been retrieved from a New Zealand’s alpine lake. Back in the lab at the University of Otago in Dunedin, Jamie Howarth opens a core tube to reveal the layers of sand and mud from Lake Christabel. Here is a section of the core that shows the finely laminated lake sediments formed in normal conditions (on the right). In the centre you can see that the layers are slightly folded – this is the indication of an earthquake that has deformed these layers. They would have been at or just below the surface of the lake floor at the time. Here Jamie is indicating the remains of a leaf next to the blade of the knife. This is not far below the earthquake layer, and can be used to get a radiocarbon age which will help to date the earthquake event. This dark coarse layer is the next layer that was added to the sequence on top of the folded sediment. It is the base of an earthquake generated turbidite deposit. The material gets gradually finer to the left (‘upwards’) as the cloud of particles slowly settled onto the lake floor. The section shown here is the landscape recovery phase. Dating of the base and top of this layer in several cores has shown that it can take 50 years for the landscape to recover from an Alpine Fault earthquake. During that time, hillsides are destabilised, debris flows cover flat areas near to the mountains, and rivers are prone to changing course due to being overloaded with sediment. Finally we see the thinly layered sediment  indicating that normal conditions have returned to the lake environment. This map shows what can be done when this research is carried out at a number of lakes along the Alpine Fault. The coloured lines (purple, orange, green etc) show earthquake records that have been identified so far in some of the lakes along the length of the fault. You can see that the last earthquake rupture (in 1717 AD) was over 300 km long. The one prior to that around 1600 AD ruptured the northern end of the fault. Information about previous earthquakes is still incomplete, but the picture is starting to become clearer. With more research, Jamie and his colleagues will be able to show a more detailed history of the last 10 Alpine Fault earthquakes including the dates, lengths of rupture and magnitudes of the events.

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Lake Christabel

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Lake Christabel   J.Thomson@GNS Science This is Lake Christabel in New Zealand’s South Island. It is one of the many beautiful alpine lakes  to be found close to the Alpine Fault. Lake Christabel was formed when a huge landslide blocked the valley, thus damming the river that then backed up to form the lake.The present day outlet runs over the old landslide deposit of large chaotic boulders. Hidden beneath the waters of Lake Christabel are very distinctive sediment layers that tell the story of huge earthquakes that rocked the nearby mountains during ruptures of the Alpine Fault. Jamie Howarth from GNS Science, and Sean Fitzsimons from Otago University, have been investigating several such lakes to read the earthquake histories. I joined them on a recent expedition along with Delia Strong and Rob Langridge from GNS Science. The aim was to retrieve sediment cores from the lake to investigate the earthquake records. First of all a seismic survey was undertaken to find the best spots to sample on the lake bed. Sean is in the lead boat, towing a second dinghy that carries the equipment. The survey uses an acoustic source that sends pulses down into the water. The boat is towed along so that noise interference produced by a nearby motor is avoided. As the sound pulses are reflected back from the lake bed and its layers of underlying sediment, they are translated into a two dimensional vertical section image of the lake floor. A number of survey lines are made across the lake to give some idea of the 3 dimensional structure of the lake sediments. Once the best locations for sampling have been chosen from the survey results, the corer is prepared with a fresh 6 metre pipe that will be pushed into the lake floor to retrieve a sediment core. The corer is transported to the chosen point on the lake surface, and then dropped off the side of the boat once it is in position. After being connected with several airlines which are required to control the pressure coring process, the corer is lowered the 90 metres to the lake floor. The large barrel sits at the bottom, and is sucked into the mud to create a stable platform for coring. High pressure air is then applied to the piston which pushes the 6 metre coring pipe into the mud, releasing clouds of bubbles up to the surface. These bubbles allow Sean and Jamie to monitor what is going on with the corer at depth. Lake Christabel Corer Retrieval J.Thomson@GNS Science When the coring is complete, an airbag is attached to the line and filled up with air so that it  pulls the whole assembly out of the mud. The airbag bursts up to the surface from below in a spectacular fashion. Lake Christabel Corer Retrieval J.Thomson@GNS Science About a minute later, the corer assembly also emerges from the depths. It is not a good idea to be too close to this as it could easily sink a boat that was in the wrong place. The corer is then plugged and loaded into the boat to be brought back to shore, with its precious cargo of sediment. The PVC tube containing the core is then cut into 1.5 metre lengths for ease of transport. Each tube is carefully labelled to avoid any confusion  about where it was taken from and its relationship to the other samples. Lake Christabel Flight  J.Thomson@GNS Science Once all the sampling has been completed, the expedition is over. It takes several helicopter loads to transport the two boats, safety gear, corers, generators, samples and all our personal equipment back to the road end. The samples are then taken to Otago University for analysis. My next post will describe how alpine lakes like Lake Christabel have shown themselves to be very useful natural seismometers through this research approach.

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Global Catastophe in a thin rock layer

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K-Pg boundary layer – when the Earth changed forever The K-Pg Boundary (or Cretaceous Paleogene boundary, or K-T boundary as it is still sometimes called) is a layer in the Earth’s crust that marks a very dramatic moment in the history of life on earth about 65 million years ago. There is a huge change in the fossil communities of plants and animals across this boundary. Over half the species that are found in Cretaceous rocks are missing from the younger Paleogene rocks above them. Included amongst the creatures that vanished forever at this precise point in time are the ammonites, large marine reptiles (such as mosasaurs and plesiosaurs), large flying reptiles and of course the dinosaurs. New Zealand has a unique record of the K-Pg boundary. These eight localities in the northern South Island provide the only Southern Hemisphere record of how the catastrophe affected land plants (Moody Creek Mine) and marine life (Waipara River and six localities in near Blenheim).   An artists image of the impact by Don Davis of NASA The now well established explanation for this dramatic crisis in the history of life is that an asteroid, about 10 km across, struck the earth near Mexico, causing a huge tsunami and a global dust cloud that darkened the skies worldwide for months, thus killing plant and animal life. After a period of recovery that lasted several thousand years, the remaining plants and animals were able to diversify into the ecological niches made vacant by this mass extinction. Mammals were one of the groups that flourished and ultimately gave rise to humans.  The dark line of the K-Pg boundary at Chancet Rocks Recently I joined a group of scientists visiting several sites in Canterbury and Marlborough, where the K-Pg boundary is exposed. K-Pg boundary at Chancet Rocks centre left of photo At Chancet Rocks, just north of Ward Beach, the light coloured Cretaceous limestone contrasts with the darker grey Paleocene rocks on the right side of the photo. These rocks were laid down in several hundred metres of sea water, and the fossils found within them are mostly microscopic unicellular plants and animals. These have been studied in detail and are very different assemblages. This slab cut through a section of K-Pg boundary by John Simes and Chris Hollis was taken from the coast south of Chancet Rocks. If you click on the image to enlarge it you can see some of the features in more detail. You can see the thin layer of clay that precisely marks the boundary itself. This layer has been found at sites around the world, including drill cores from the ocean floor, and is remarkable for containing high levels of an element called iridium. Iridium is common in asteroids and its abundance at the boundary was a key part of the evidence that lead to the asteroid impact theory. We also visited Woodside Creek, the first K-Pg boundary locality in New Zealand that was found to be enriched in iridium. Here you can see that the river was quite high, making access a little bit difficult. This is the Woodside Creek section. It has been sampled a lot over the years so that quite a lot of the rock has been mined away. The drill holes you can see in the rock layers on either side of the boundary itself show where scientists took samples for the analyses that led to the discovery of iridium enrichment. The image at the top of this page was taken from here. This is a close up view of the very top surface of the Cretaceous at Woodside Creek, just beneath the iridium rich boundary clay. The masses of tiny pock marks in this surface are thought to have been caused by droplets of glassy impact ejecta raining down onto the sea floor from high in the atmosphere after the impact thousands of kilometres away. Chris Hollis at GNS Science has done very detailed studies of the Cretaceous and Paleogene rocks in New Zealand. He is a paleontologist who specialises in tiny microfossils called Radiolarians. Radiolarians are marine plankton that construct complex shells of glass (opaline silica); each species has a distinctly different shell. Radiolarians are one group of organism that didn’t go extinct at the K/Pg boundary. Instead, some species became very rare, while new species evolved and flourished.  These microfossil changes are clearly shown in rock samples from the K/Pg section at Flaxbourne River, where over the distance of a few millimetres one group of radiolarians (nassellarians) are almost completely replaced by another group (spumellarians). This change is thought be a consequence of rapid cooling of the ocean waters around New Zealand. In this video Chris tells us about the Woodside Creek K-Pg boundary section:

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Lake Ohau Sediments

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Ever since its creation by the retreat of a huge glacier at the end of the last ice age, Lake Ohau has been gradually filling up with sediments washed down from the nearby mountain ranges. This is the view looking north from the lake, up towards the Dobson Valley. The valley profile has the classic ‘U’ shape created by glacial erosion, and the flat valley floor is blanketed by sediments brought down by rivers, especially during floods when the water flows with high energy. Here you can see the lake inlet. You can see the delta created as the sediments fill the lake. Lake Ohau has been receiving a high level of scientific interest over the last few years, by scientists from GNS Science in collaboration with others from Victoria and Otago Universities. Gavin Dunbar of VUW preparing equipment They aim to understand the processes of sedimentation in the lake, and work out how these processes relate to weather patterns affecting the catchment. With that information, a study of the lake floor sediments will potentially give a detailed record of how the climate has changed in the area over the last 18 000 years, since the lake’s formation. A number of limnological (lake) measurements are being made to help understand the way water currents, water temperatures and water clarity vary seasonally in different parts of the lake. This is important because it allows for understanding of the factors that influence the deposition of mud on the lake floor. In this photo Heidi Roop (GNS Science PhD student) is helping pull a sediment trap out of the water at the end of the lake nearest to the outflow. At the bottom of the trap there is a bottle of sediments that have accumulated over the last 4 months. The 1 litre bottle is removed and replaced with an empty one. The bottle is quite full because it contains concentrated sediment that has fallen into the wide mouth of the trap. Careful recording is one of the most important parts of any scientific data collection. Marcus Vandergoes and Heidi Roop prepare to lower a gravity corer into the lake to sample a small core of the top layers of sediment. As the corer penetrates about 25cm into the lake floor, the mud enters the plastic tube. A cap then seals the top end of the tube so that the mud is held in place by a vacuum as the corer is pulled back up to the boat. Once at the surface, the lower end of the tube is sealed to prevent loss of the core which is then prepared for transport back to the lab for close study of the different layers, including thicknesses of the different layers, grain size and density. Heidi and Marcus pulled up a second core to show me what can be seen when it is sliced through to show a flat surface. Darker and lighter layers are visible, which have been shown to correlate with summer and winter deposition. The thickness of each layer is thought to be related to the number and size of storms and flood events. This core includes sediment accumulated over the last 25 to 30 years. Heidi has devised a way of comparing the sedimentation of particles from different depths in the lake water at each end of the lake. She has a line with several upside down cut plastic bottles that act as mini sediment traps attached at different levels in the water column. This shows whether the currents that deliver sediments to the lake are flowing at the surface, the bottom, or at intermediate depths. It turns out that this varies between summer and winter. In summer, the warm water entering the lake carries the sediment load at a high level, whilst in winter, the particles travel along with cold bottom currents. This is why the summer and winter layers of sediment have different physical characteristics. Clear as mud – a successful day’s sample collecting from Lake Ohau,

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Opunake

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Another great geological venue on the South Taranaki coast is at the Opunake boat ramp, where we took our Geocamp participants recently. On two sides of the car park there are cliffs showing a spectacular sequence of strata. information for you to visit this spot is on our GeoTrips website here: www.geotrips.org.nz/trip.html?id=56 It’s a perfect spot to practice drawing a geological section and making detailed observations. Drawing is very valuable as it forces you to be careful and attentive to details. The observations lead to the next question. What can these rocks tell us about the processes that put them in place? These colourful orange, grey and yellow bands contain numerous volcanic clasts (pebbles and boulders) suggesting that they have originated from the Taranaki / Egmont Volcano, an obvious source about 25 kms north-east of Opunake. Generally when you see stratification in a rock it suggests that it has been laid down in moving water (or from the air in some cases such as volcanic ash layers or sand dunes).  This layer shows well developed graded bedding – the larger particles were laid down first, followed by finer and finer material. The very coarse unit above indicates another very high energy phase of deposition. In some places you can find very large boulders that have been deposited and left in scoured out hollows that have then been infilled with finer material. In  this image you can see a channel on the right of centre that has cross cut the horizontal layers and been infilled with gravel. This also shows that the sedimentation process was occurring in a high energy environment. So a fair interpretion of the Opunake sequences is that of volcanic material that has been eroded off Mount Taranaki and deposited in a fluvial (river) environment, possibly as reworked lahars or debris flows that have been mobilised by floods. If you take a look at the landforms on the slopes of Mount Taranaki, you can see numerous gullies and larger valleys where the rocks have been eroded away, either by rivers, lahars or rock slides. Occasionally there have also been the massive debris avalanches such as the one that covered the forest at Airedale Reef)   Over time the mountain has produced the material that blankets hundreds of square kilometres of the surrounding plain. Here is the geological Qmap for Taranaki. The red rocks are volcanic lavas and related rocks centred on Taranaki / Egmont Volcano, whilst the pink rocks are pyroclastic and debris flow deposits. Opunake is on the coast  half way up the map on the left. You can see the profound effect of the volcano on the landscape, as it is at the centre of a radial arrangement of volcaniclastic deposits.  The volcanic rocks have been  spread across the landscape for large distances by the power of gravity and water.

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Waihi Beach Taranaki

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The South Coast of Taranaki near Hawera has extensive rocky beaches lined by high crumbling cliffs. It is a great place for geology, but you should be wary of the potential for cliff falls, especially after rain. This is the view east from the Ohawe beach access point. A first look at the cliff from a safe distance shows that it is made of two main rock types; massive grey muddy sandstone at the bottom, with darker brown soft stratified layers above. The boundary between the two layers is very distinct and can be seen for many kilometres along the coastline. If you take a closer look at some of the muddy sandstone boulders lying on the beach, you can find some nice fossils such as this scallop. In places these rocks are very bioturbated. In other words they have been churned up by organisms that burrowed through them when they were part of the sea floor. For a close up look at the boundary between the two layers that form these cliffs, a good place to go is the beach access track 4.5 kilometres east at Waihi Beach (end of Denby Road see www.geotrips.org.nz/trip.html?id=55 for location and geological info). There, right before you reach the beach, is an easily accessible outcrop where you approach the boundary safely.   Here is a slightly closer view – you can see the change from the lower grey unit containing oysters and scallops with the shell rich layers above. The fossils in the lower unit indicate an environment of deposition about 20 to 50 metres deep. This layer is approximately 3.5 million years old. Here is another image where can see the incredibly abrupt change from the lower muddy sandstone to a much looser sandstone packed with shells. Just below the boundary there are some vertically positioned shells in a line. These have burrowed down into the sediment from above and have been preserved in life position. Although they are found within the 3.5 million year old sandstone, they are actually only as old as the overlying shelly layer, which is about 125 000 years old . This means that the 3.5 million year sea floor sediment has been uplifted, eroded down to sea level, and then covered with shelly beach or estuarine deposits of much younger age. Nearly three and a half million years are missing from the sequence. Interestingly the same unconformity is widespread across Taranaki. Here you can see it at Wai-iti Beach on the north coast. Here the time gap is even greater, as the underlying grey sediments are about 8 million years old and represent deposition at about 500 metres water depth. This shows that there has been greater uplift and erosion in the north compared to the south Taranaki coast.Here is a video of Kyle Bland explaining the Waihi outcrop and the story revealed by fossils:

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Airedale Reef

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Earlier this week I was up in Taranaki, exploring the geology of the area  with two GNS Science researchers Kyle Bland and Richard Levy. One of the sites we visited was Airedale Reef, a short walk east along the coast from the mouth of the Waitara River. There are spectacular remnants of an ancient forest on the shore platform at low tide, with tree stumps in growth position and large logs sitting in a black layer of peaty soil. The forest layer reappears at the base of the nearby sea cliffs, with the roots and tree stumps gradually being eroded out. Just below the dark layer is an olive green bed of dune sands. The carbon rich forest layer is thickest in the depressions between the dunes. This  is one of the tree stumps emerging  from the cliff. But how long ago was this forest still living? How did it die and why was it preserved like this? The answers are in the layer just above it. This overlying layer is made of an unsorted mixture of different boulders and less coarse particles of rock. You can also find chunks of carbonaceous material scattered within the layer that must have been ripped up into it as it was emplaced. This 4 metre  thick layer of material has been mapped  over a minimum area of 255 km2 around north Taranaki, and has a total volume of at least 3.6 km3. It is believed that Mount Egmont (Taranaki) volcano is the source of this layer. Like some of our other andesitic volcanoes, Mount Egmont is made up of layers of unconsolidated volcanic deposits interbedded with more massive lava flows. Because the slope angle of the volcano is very steep, the cone is inherently unstable, resulting in occasional enormous avalanches of debris launching down the mountainside, spreading across the surrounding countryside and out into the sea for distances of up to 40 km from the source. For this reason Egmont is a significant geological hazard that is monitored by GeoNet. On our visit up the  mountain the following day, amidst the lava flows and ash layers we could see deposits such as these – not too different from the bouldery layer at Airedale Reef, although likely to be much younger. Back at Airedale Reef this photo shows a good view of  the layer that buried and destroyed the fossil forest. It is known as the Okawa debris avalanche deposit and has been dated at about 100 000 years old. This means the forest was growing during the last interglacial period. Pollen analyses shows a dense podocarp forest, but lacking specifically coastal plants. It seems that when the forest was alive, the coast was further out than its present position. Rimu Pollen  (Dacrydium cupressum) 43 microns across There is a lot of pollen preserved in the Airedale Reef cliff section. Scientists found over 10,000 pollen grains per cm3 in places.They were analysed to study the plant communities from the period of time represented by these layers. Cyathea treefern spore, diameter 30 microns  This allows research into climate variations through time, as different species appear and disappear up through the cliff section from the base to the top. The Rimu and tree fern species in these two images indicate a lush podocarp forest that grew in warm, wet conditions. In the next layer above, the species found represent a sub-alpine shrubland community that grew in a cooler climate. In this photo you can see two pale coloured tephra (volcanic ash) layers near the top of the carbon rich layer, showing periodic eruptive activity from the volcano. In the last image you can see that another carbon rich layer formed in a depression at the top of the Okawa Formation (centre left). Above that the rest of the section is made up of orange and pale brown soils.

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Fossil Whale Hunting

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Last weekend I returned  to our fossil whale locality in Palliser Bay with John Simes, the paleontology collections manager at GNS Science. This is where I had found three large jaw bone fragments of a fossil baleen whale last November. We decided to have a good look through some of the loose debris in the area where I had already made some finds.After some time. John spotted another large piece of mandible, very similar to the ones that we had from the previous visit. I decided to try the direct route up the cliff, to get closer to the source of the bones. The ice axe proved to be quite useful for making progress up   the very crumbly mudstone. This got me about half way up the gully, to a point that I had reached last time and where I had found one of the three original bones. On this first re-inspection I didn’t come up with any more fossils. The next plan was to abseil down into the gully from the top, in order to have a very close look at the steep headwall which seems to be the actual source of the fossil whale. I had to take care not to dislodge any large rocks with the rope. Unfortunately this inspection of the cliff didn’t reveal anything even with careful scrutiny. Back in the bed of the gully, I dug around with my ice axe some more and did at last come up with three smaller pieces of bone. Here you can see one of them – we think it is the end of a jaw bone, although it is much thinner than the other pieces. Back in the macropaleontology lab at GNS Science, the thin layer of mudstone coating the bones was quite easily cleaned away with the help of a pneumatic air scribe. The 30 cm long piece shown here turns out to fit perfectly with the previously found  segments of the mandible, giving us a combined total length of 1.5 metres for it. The missing link puts it all together. The latest piece in the puzzle is second from right. The rest were found on the previous trip. Here are the smaller pieces after a bit of cleaning. It is tantalising to think that there must be a lot more of them waiting to be discovered in the mudstone of Palliser Bay.

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