Friday, July 3, 2009
On the Fortescue Side
June 22nd – As described in an earlier post, last night’s swag out was along the old Wittenoom-Port Hedland Road. I’d driven north of the Hamersley Ranges, across the Fortescue River Valley. The work plan was to search for outcrops of one of the other Hamersley Basin impact layers, which should theoretically be exposed here. As I described, it wasn’t.
The rocks exposed on the north side are largely the Fortescue Group. They underlie and are older than the Hamersley Group strata on the south side. We’ve been looking at the latter on this trip. Most of the Hamersley Group comprises rocks deposited in the ocean. The dominant Hamersley lithologies are iron-formation, chert, limestone, and argillite (formerly mud). In contrast, most of the Fortescue is composed of terrestrially erupted basaltic lavas and surface sediments, such as sandstones deposited by streams.
The iron-formation and chert in the Hamersley resist erosion. As a result, the Hamersley Ranges are several hundred meters high and the gorges in Karijini National Park are deep and narrow. The Hamersley rocks wear a stain of orange-red from iron weathered out of the rocks.
The lavas of the Fortescue create a different landscape. They aren’t as resistant as the Hamersley; this makes for rolling hills rather than steep layered cliffs. The lavas weather to a rich brown-red; they look burnt. The rocks are shaped into lumpy, rounded to blocky outcrops. There’s a dolmen-like quality to them; they form vaguely linear, elongate bands, like teeth rising out of the ground. They look almost, but not quite arranged.
Beyond this surface appearance and what science tells me, the Fortescue Rocks feel older than the Hamersley. Their color and toothy appearance feels – deeper. If the Hamersley is the skin, this is the muscle underneath.
I actually meant to write about botany. Like much of Australia, the Fortescue area is very clearly an ecology controlled by fire. If you know me, you know that in the States I only look at rocks. In Australia, I diverge. I have at least four bird books, and three times as many on flowers and plants. I’ve resisted the urge to get a reptile book (maybe because all the snakes are deadly). The differences from North America intrigue me; I also simply love this area and want to know it better.
We camped in a broad valley several hundred meters wide, underlain by lava and argillite. It was punctuated by low mesas capped by the Marra Mamba Iron Formation – the bottom of the Hamersley Group. The swag out site was in a recently burned area. There was little spinifex remaining; all the other ground cover was gone. I slept on a coarse red-brown desert pavement of angular blocky rocks of chert, ferruginous chert, and iron formation. Pretty poky, I had to keep reaching under my swag at night and moving rocks away from tender places. I didn’t miss the spinifex. It was easier to walk around in the morning to take pictures and to look at outcrops.
Grey, black-charred trees were scattered across the desert pavement. Burned limbs littered the ground. Most of these were pretty big; all the smaller twigs and branches must have burned in the most recent fire. This made for easy firewood collection. This was our campground; bare rock and burned trees. It was windy.
I’d picked out an early morning photography site several hundred meters from camp, part way up one of the mesas. The flora here showed more signs of post-fire recovery. Mulla-mulla, a couple of cassias and a variety of peas were about ready to bloom. Wild tomatoes were fruiting (they are edible, according to my newest plant book: after you, Louis). I stepped on a number of baby spinifex in my haste to get to my site. A few surviving gum trees provided a bit of white and grey-green to the landscape. Many more plants I don’t know or have forgotten. The signs of regrowth weren’t surprising; I’ve seen the pattern many times further south. But on this cold morning, about halfway through the field work, it cheered me to see some color: evidence of resilience.
Later in the day, the spherule layer hunt took us through an unburned area. Very different. Dense brush: four meter high mulgas and corkwood, gum trees twice as big, mature spinifex knolls (too big to jump over), and lots of wattles. The ground flowers weren’t present. A few charred stumps were visible: evidence of previous fires. It felt more closed in to me. This is the turf I avoid when looking for rocks or tracks. I’m sure it’s better for everyone else birds, roos, and other fauna. I bet it excites biologists too.
Regeneration: I appreciated being reminded of this. It took a change of place for me to be able to feel it clearly.