Geology And Geophysics

A Guide to Backyard Geology



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BACKYARD GEOLOGY, THE HARD WAY.

I've been lucky enough to have some of the weirdest geology imaginable in my various backyards. My original home, however, took the cake. I grew up on the side of a 1 in 3 hill, and was literally surrounded by geomorphic oddities. We had some of the best soils in the world, sitting on some of the lousiest, silica-riddled rubbish. We had huge clay deposits appearing out of the side of the hill, notably on a water easement that turned into an almost indescribable morass when it rained. Fragile, impacted grey soil that cut like cheese, and a fibrous, wonderful-smelling loam that I swear you could knit into a sweater.

Our house had been literally slotted into the side of the hill, my father and a friend working with a spirit level and a bulldozer. Between ourselves and the neighbors, a lot of the hill had been exposed, and it was a staggering mix of geo-peculiarities. It's good to be a kid in a situation like that. Mysteries, all over, and lots of bush to explore. We kids had the run of the neighborhood, as is the correct order of things.

At the bottom of the hill was a steep gully, with a really kid-prone creek running through it, exactly the right size for a primary school kid to leap over, even in flood. There were rocks everywhere on its banks, mainly sandstones, with a few escapees from other deposits, worn more or less smooth. Small rocks, tiny bits of things that had nothing to do with the sandstones, ornamented the silt.

I'm not kidding when I say that it was quite easy to spend hours just fossicking around in the bits and pieces the creek threw up for inspection. My dog, also a keen sleuth, was good at checking out anything new in the area, (terrible gossip) and by now I had another interest, paleontology, which had more or less subliminally explained to me that rocks were there for a reason. I knew, instinctively, that I was looking at history, and that the rocks were there as a result of the local geological record.

Not that any of it made a lot of sense, even now, thinking back on it. The bottom of the gully was a couple of hundred feet below our place, and a lot can happen in that amount of rock and soil. The all devouring clay deposit, when explored, went most of the way down the hill and disappeared. There must have been thousands of tons of clay, and it went from clay's answer to Godzilla on the gentler slopes to Little Miss Muffet at the steepest part. A cutting had exposed more of it under the topsoil, a bit more sandy, but equally uncompromising clay, and even it didn't argue with this weird arrangement, and didn't slide or make itself difficult. Just as well, too, because the houses above were effectively perched on its side. Rocks, there were, some sort of remnant deposits, not pervasive, which had been buried under the clay.

I do now understand why geologists get obsessive about their subject. There's a lot to learn, and more to figure out. It was as if all of that clay had been dumped there. The only real information about what was under all the clay and soil was in the gully. One of the beauties of the gully was an old bridge/tunnel built by convicts in the 1800s. It was made of granite, an arch bridge locked together in the old stonemasons' way, without mortar. I'd never seen so much granite before, and it was the first chance I'd ever had to look at it in detail. Granite, aged like that, is a truly beautiful rock, and among the ferns and creek trees, it was like a cathedral.

Better yet, it was passable, to the other side. As a little kid, I'd been wary of going through it, but a friend and I eventually persuaded each other to go all the way through. The dog was also in agreement, so we slithered along the green algae and came out into a new area, with the banks of the creek a bit higher, and some more sandstone hanging around. This side of the creek was closer to the sea, and in place of the tannin-soaked waters on our side, there was more sand, washed out of the sandstone, obviously, heading downstream.

The clay was still there, but the slopes were smaller, on one side, and steeper, on the other, so it was less conspicuous, just visible in places. One day my dog and I decided that if Magellan could do it, we could do it, and we picked our way along this new part of the creek, all the way to the sea. I promise you, you learn some respect for the local geology when it's your footing and your alternative to sliding into a creek several metres below.

Ever read one of those books where the explorers of the prehistoric world suddenly come into some valley or something where everything is completely different? The geology changed, when we got to the sea. Volcanic rock was everywhere, the odd, brown stuff, with the ancient bubbles. Very hard rock, and the demure creek sand had morphed into the more familiar sea sand.

The entire morphology was different. The gentler slopes were now cliffs, as if a cheese cutter had sliced them. The sandstone had vanished, entirely, overridden by the volcanic rock. This was all new to me at the time, but what we'd found, in total, equated to a history of really major upheavals. The huge amounts of clay, burying the ubiquitous sandstone, were one clue. The steep slope was another. A very deep, sharply angled, gully doesn't just happen. The pulverized pebbles, some marble, some other types of stone, weren't there as decoration, despite appearances. The glassy black fragments and little orange pebbles, quite unlike all the other rock in the area, had come from somewhere.

Every so often if you walk along any coast, you see rafts of rock which have turned at incredible angles. Landforms which look as if someone's been trying geo-origami and not quite getting it right are pretty normal, too. What stunned me was that this was my home turf. All this normality was what I was walking around on every day, and it was a real saga. There must have been several cubic miles of that clay, to start with, in one big irregular glop, sitting on that hill. I knew that on our land you could dig down for quite a long way and not find any clay, but it was on the surface, and deep, just metres away. I knew that the loam was really thick, but every so often you'd hit fragments of rock, sedimentary stuff, granite pieces, gravel of some indeterminate pedigree, just below the surface, and at other depths. The sandstone around the creek was actually quite fragmented, big chunks and little soothed by the water and the overgrowth, and their real significance easy to miss.

One of the more interesting facts was that there was hardly any impacted soil, even with the vast amounts of sandy silica that sandstone produces. Very large trees, including one giant pine, and a gum tree which must have weighed many tons, sat on this motley collection of geomorphologic bric a brac. Our house was rafted on it. Steep banks and retaining walls supposedly held up the cutting where the house stood, but even in earth tremors, there was little movement, as though some other forces were at work, a local version of specific gravity, holding everything in place. Maybe clay's a good shock absorber. It'd explain a bit. Whatever the reason, the ferocious drops in the area should have been good reasons for landslides, given that the sandy soil and clay weren't exactly water resistant.

I'm a horticulturalist, according to the certificate. Years later, and I now look at landforms and geology as a sort of site map. They tell me a lot. The presence of some rocks is a guide to the way the land is put together, how the drainage works, (or doesn't work) and why the trees and other plants are where they are. I can take a pretty fair guess about where the water table is, and am pretty much at home in any geological environment. I'm sure the dog understood all this a lot better than I did as a kid, but that's how I learned, with backyard geology as a teacher. It gave me so many questions, and so much information, that I couldn't avoid learning. It still is the strangest configuration of rocks, soil, landforms, and other features I've ever seen.

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