Monday, June 15, 2009
On a second day of looking at gorges, gaps, and rocks...
The road eventually turned south, just in time for the sun to continue shining on me from the west. I guess I’ll return to North America with the usual asymmetric drivers tan.
A lookout turnoff appeared. At the base of a larger telecommunications mast I had my first look at the Gosse (or Gosse’s) Bluff impact structure. It was formed when something extraterrestrial – the guidebooks say a comet, although there is no positive evidence for this that I am aware of – hit this part of Australia about 143 million years ago. The hypervelocity impact was enough to make a crater that was 20+ kilometers in diameter. That structure has long since been eroded. What I was seeing from the lookout was its roots; the rocks that were deformed by the impact an estimated 2 kilometers below the surface. Today, these remnants form a range of hills more than 5 kilometers wide. From the lookout, it was clear why the early white explorer Giles called these bluffs. They form a series of sharply eroded, overlapping flat-topped hills that rise from Missionary Plain. A distinct contrast from the rolling spinifex hills on which we were standing.
We made a decent camp several hundred meters off an access road. Accessing what I don’t know, maybe better not to. A second night of frost and wondering why the hell swags are still made in Alice Springs. This camp was notable for its wildlife. I saw brumby tracks all throughout our campsite. Louis went walkabout and eventually spotted a pair of wild horses which looked about the right size to have made the tracks. They came to visit throughout the night. Paul suggested the fire either attracted or agitated them. It just might have been how we smelled too.
After the morning gear drying event, on to Gosse Bluff. I’d discovered we could visit this spot using Google Earth. Driving down the bumpy corrugated dirt road, I imagined viewing the truck on the access road from space. A different way of knowing where I was. I’m used to visualizing locations on maps, but placing myself on a satellite image so precisely felt strange. Almost like knowing too much information in advance.
Gosse Bluff is a circular ring of hills comprised of vertically dipping sedimentary strata. In English, this means the beds trend straight up and down. These rocks were flat-lying at the time of the impact. The massive overpressure of the event deformed them downwards, and they then sprang back, freezing vertically. Think of how a stretched rubber band moves; now imagine several kilometers of rock doing the same thing in the space of a few seconds. I sure can’t; but it’s the best scientific explanation for what’s happened.
The bluff is pierced by a stream on its east side; the access road enters here, stopping at a day use area in the center of the structure. Odd; impact features this size often have some kind of a central uplift. Not here; maybe it was eroded.
We parked and hiked around. I wanted to find shatter cones, which are a hand-scale feature formed in rocks by extraterrestrial impacts; they’re diagnostic of such. This was easy. A sandstone layer in the first bed we walked up to was full of them. They were defined by slightly radiating lines – microfractures – which cut across the layering in the sandstone. I’ll stick a picture in here somewhere. We did not see any cone apexes, but this shape was obvious.
We climbed up the side of on of the bluffs. I took the obvious wide angle and telephoto images. Paul shot video. Louis took pictures, climbed around and kept finding more shatter cones. He has a good eye.
Eventually I just sat. I was struck by how uncanny the bluffs looked. We’d seen plenty of tilted sedimentary rocks in the MacDonnell Ranges, folded by mountain building during the Alice Springs Orogeny. They were of course fractured and eroded, but these modifications occurred in a rational context. Tilting, folding, and erosion are comprehensible. Besides being vertical the Gosse Bluff rocks look – smashed. Fracturing is common in rocks, but here pervasive crushing had broken every layer into shards. The strata remained defined, but on close examination look like a splintered and reassembled sheet of glass. Maybe I was preconceiving this, but it was easy to infer that some catastrophic event had formed these features. However, confronted with the evidence, my mind could not comprehend what happened. I’ve read and studied impacts for a long time, but seeing an actual crater, I could not imagine what happened. The forces and time scale are too far out of my experience.
How would this have appeared to the Aboriginals who lived throughout this area? They would have astutely observed the difference in the rocks that I’ve written about. There’s no other structure like Gosse Bluff in Australia. In the local Western Arrente language Gosse Bluff is named Tnorala. Its mythic origin is in fact both cosmic and cataclysmic. According the interpretation of the NT Parks and Wildlife Service, “Tnorala was formed in the creation time, when a group of women danced across the sky in the Milky Way. During this dance, a mother put her baby aside, resting in its wooden baby carrier. The carrier toppled over the edge of the dancing area and crashed to the Earth, where it was transformed into the circular rock walls of Tnorala”.
I didn’t connect with this story when I first read it in California. Here, looking at the far wall of the bluff, it made more sense than trying to force my mind around an actual impact. This place is Tnorala in my mind from now onwards.