We were on our way to Adels Grove, a 380km drive north west from Mount Isa. There's 110km of highway towards Camooweal then another 50km of bitumen on the Thorntonia-Yelvertoft Rd and a short blacktop section on the Gregory Downs-Camooweal Rd before the gravel kicks in. Just before the end of the bitumen, the clouds put on a show. It's not quite Burketown's Morning Glory but it is spectacular none the less.
The road is quiet with Burke Shire only just reopening after COVID-19 pandemic declared area status was lifted the week before we travelled. There were a few cars about but the only real traffic was this herd of cattle being mustered at or near Thorntonia station.
When the road turns to red dirt, it's a bumpy ride the rest of the journey with an average speed of 60 to 80kph depending on variable quality. We turn off the Gregory road onto the equally lumpy Riversleigh Road which will take us to our destination. Here we see the only building on the entire route: Riversleigh station. This is one of two stations run together as part of Lawn Hill and Riversleigh Pastoral Holding Company owned by the local traditional owners the Waanyi People.
Near Riversleigh station we cross the first of two river crossings, this one the O'Shannassy River. While I had a high clearance vehicle and it was the dry season, the current was deceptively strong and I felt a strong tug as I drove through wanting to drag me into the stream.
Though there has been almost no rain since March the O'Shannassy still has plenty of water. It feeds into the Gregory which in turn discharges into the Nicholson River (named by Leichhardt on his first expedition) which empties into the Gulf of Carpentaria near Burketown.
About 250km in we enter the Boodjamulla National Park. This magnificent national park is most known for Riversleigh Heritage Fossil site and Lawn Hill Gorge but stretches through a vast landscape for 2820 sq km towards the NT border. Around 260km into the journey and still a 100km from Adels Grove, we arrive at the Riversleigh Fossil World Heritage Area and it is too good to pass by. According to David Attenborough Riversleigh is one of the four great fossil sites in the world which keeps on giving when it comes to fossils, including a new marsupial lion identified only this year.
Riversleigh covers 35 sq km and there are 280 sites identified but the most famous is this one: D-Site easily seen from the road with this mound visible for kilometres in all directions. D-Site was one of the first major fossil deposits found at Riversleigh and is the only publicly accessible part of this World Heritage area. Its fossils are from the late Oligocene period, dating back 25 million years. Turtles, fish, snails, crocodiles, lizards, pythons, birds and many types of mammal and marsupial fossils have all been recovered here.
There is a small interpretive centre on the site and an 800m walk showing off some of the finds. Riversleigh is the richest known fossil mammal deposit in Australia and has been on the World Heritage list since 1994. The carvings are of a 25 million year old thunderbird which grew to 2.5m and a freshwater crocodile which killed using its blade-like teeth.
Today Riversleigh is dry and barren but 25 million years ago it was full of ancient lake deposits. It was a time when the ecosystem was evolving from rich rainforest filled with lakes and waterways to semiarid grassland community. The high concentration of calcium carbonate in the water has ensured that fossils have been extremely well preserved. When the skeletons of dead animals came into contact with water, the bones were quickly coated in limey mud. Later the bones were replaced with hard minerals from the limestone-rich water. Millions of years later, the fossilised bones have been exposed as a result of weathering by wind and water that dissolved and removed layers of surrounding soil and rock.
Fragments of limestone are scattered around the area and provide excellent examples of fossilisation. The fossil above belongs to a dromonthid, the big flightless bird of the diagram. They weighed up to 300kg and enjoyed swimming in the lakewaters of the era. The fossils are surrounded by small pebbles which the bird swallowed to help grind food before digestion.
These rocks are 530 million years old. Called "Cambrian pancakes" they are remnants of what was once a continuous bed of limestone. Ancient fossil trilobites - the earliest arthopods alive - are preserved in this marine limestone from a time when Australia was part of Gondwana.
Above is the view from the top of the rise looking north towards the interpretative centre, the carpark and the road. It is a vast and empty country. However you can get a feel for what's here in Mount Isa by visiting the revamped Riversleigh Fossil Centre.
The circular fossil above is the cross-section of a limb bone belonging to baru wickeni, one of Riversleigh's largest crocodiles. Baru was five metres long and an apex land predator. It killed its prey by shaking them with its jaws and slicing them with razor sharp teeth. The skull and jaw adaptations indicate it was specialised towards subsisting on large vertebrate prey of similar size to itself, ambushing victims close to water sources. Baru resembled a modern crocodile, but the deeper head and alligator-like overbite was more pronounced.
These fossils above are from a group of turtles called chelids which still live in Australian freshwaters such as nearby Gregory River. Other extinct turtles include the 200kg meiolaniid which had a horned skull and an armoured tail in a bony sheath.
On completion of the walk it was time to head back to the road north west. We were keen to see more of the national park and our bed for the night at Adels Grove Resort and it would take some time to get there on this beautiful but difficult dirt track.