Once out of Lochern Park, having crossed the Thomson River, the vegetation changes.

There is what I thought a nice geological description of laterite formation, the cap that preserves the jump up, and the erosion around the edges. Repeated for my continued education below.

One day I'll look up silcrete and calcrete, which also cap jump ups.

This pic is looking west across the Thomson River vallley.

Jump Ups are a feature of both the Mitchell Grass Downs and the Channel Country. They can form spectactular visual features on an otherwise monotonous horizon as the traveller traverses the broad featureless plains common to this area of Australia. Their elevation in a flat landscape provides wonderful vantage points, with the landscape rolling away for as far as the eye can see, exposing geographic features clearly.
These remnants of a much older landscape have not been wornn down by erosion to the extent of th elandscape below. They are capped by unusually hard rock formed when naturally occurring chemicals cement the surface creating a Lateritic formation, protecting the softer rock beneath.

Laterite, derived from the Latin word for brick or tile, is a surface rock rich in iron and aluminium, formed in hot and wet tropical areas. It is developed by the intensive and long term weathering of underlying rock which has been decomposed by the combination of high rainfall and elevated temperatures.

The rain water has caused the dissolution of the primary rock minerals and a decrease of easily soluble elements such as sodium, potassium, calcium, magnesium and silicon, giving rise to a residual concentration of the more insoluble elements, predominantly iron and aluminium. Laterites consist mainly of the minerals kaolinite, goethite, hematite and gibbsite which form in the course of weathering. Moreover many laterites contain quartz as relatively stable relic material from the parent rock. The iron oxides, goethite and hematite cause the red-brown colour of laterites.
 

These formations are the product of former geologic epochs. 220 million years ago, the Great Artesian Basin was created when enormous tectonic forces down warped the region which then began to fill with the sediments of ancient rivers and lakes. The environment was more akin to that of the Amazon Basin of today.
 

A series of shallow inland seas then flooded the continent. Over the next twenty million years, the seas retreated north, leaving behind deep layers of sand and mud which compressed into rock. It was these layers that underwent heavy weathering and erosion carving the landscape we see today. These processes also formed the world famous opals mined in the area.
 

With thin, rocky soils, these jump ups support flora and fauna ideally adapted for survival in harsh environment. Spinifex can fuel spectacular seasonal fires, either deliberately lit to clear dead plant material and promote new growth, or as a result of lightning strike from summer storms. Mulga is often twisted and distorted as trees scrabble in search of a foothold and nutrition. After rains, the jump ups soften. Hardy shrubs like cassia, turkey bush and senna burst into bloom. Water collects in deep rock holes.
 

Abroiginal tribes passing through this inhospitable country carved native wells to conserve precious water for dry times. The rock holes become a focal point for birds and animals while the water lasts. Domestic stock rarely venture up these formations, so they form natural barriers creating a refuge, particularly for a wide variety of reptiles, for wallaroos and for dingoes. However they also create a haven for thousands of feral goats.

Further on a native well. Carved out of the rock, with drainage to it for when it rains.

There is water about 600mm down. We don't know how deep.

Nice to see it well preserved.

We were looking for info on Hellhole, but info was closed (Saturday).

The sign saves me the effort of drawing the rivers, as I did for Kakadu and surrounds.

This version came from "Ashley Murphy - Evolutionary Histories and Futures of the Fishes of the Lake Eyre Basin".

Also saves me a bit more research on where the watershed is between Gulf Country (north flowing rivers) and the Eyre Basin (south flowing rivers). The northern boundary is, as we thought, around Camooweal, Mt Isa and Duchess.

Welford National Park is in the triangle where Thomson meets Barcoo to form Cooper Creek.

Last night at Lochern we were camped on the bank of the Thomson. Tonight on the Barcoo.

Red sand and spinifex.

A little concerned as the track is not on our maps and the markers are designed around clockwise rather than our anti-clockwise approach.

Widely spaced and short here, almost non-existent.

But very red and old looking.

Roughly to scale, the sand samples collected in the Simpson Desert in 2017, and far right today's sample.

Hopefully not just my imagination, its darker. More oxidation, more iron oxide on the sand grains.

The sand moved from south to north. Today's dune is older.

I've yet to catch a sample from the north of the Simpson.

And no dunes.

We camp just a little further south. Litttle Boomerang Water Hole on another branch of the very braided, like all the other rivers we've seen recently, Barcoo River.