First image: The 80m level is the level at which the scarp west of Sydney pops out. It seems that 80m is globally significant in many places. Near Perth it also roughly marks the border scarp. On the east coast of the US, it’s a rough marker of the fall line, the demarcation between the Piedmont province and the coastal plain.
Others in the oil industry have noticed that 80m worldwide marks the boundaries of a lot of oil-rich provinces. Does that mean that there is oil around Sydney? Indeed, the lower Sydney formations do contain oil and gas (see Three Sisters: evidence for Noah’s Flood). Colours: yellow = 80m asl; red = 800m asl; Orange = 940m asl.
Second image: As we reduce the water level, at 940–920m asl, 3 major water gaps draining mostly to the south pop out near the northern edge of the divide between the Sydney drainage area and the drainage area to the north. In central Australia, the 940m level also shows up a lot of the Finke River water gaps and the Lake Eyre highland. (You can’t see the Fink River area on this image but you would if if you did your own Google experiments.) Colours: yellow = 80m asl; red = 800m asl; Orange = 940m asl.
Third image: With the sea level at red = 80m, orange = 320m and yellow = 620m, the drainage now flows to the east, with 3 major water gaps appearing and flowing eastward. It also looks like there’s a secondary terrace around 320m, west of the 80m scarp.
One striking feature of the Sydney area is how chewed-up and disected the area is. This is very different from the sorts of water gaps that cut through in linear mountain ranges, such as in the eastern USA. In this situation the water gaps are narrow and drainage is very structurally controlled. Around Sydney the situation is very different. Australian geologists are well aware that the drainage of the area is not structurally controlled, and comment on it in their reports. At first glance, there does not appear to be a lot of small-scale water gaps in this area, unlike the Finke River and Flinders Range areas, where small-scale gaps abound.
The ‘chewed-up’ appearance is similar to the patterns in the western side of the Grand Canyon area which Scheele explained as the effect of ponded water draining into a lower sea. He gives a modern, small-scale analogue of this process and shows how it carves the dendritic, fractal, branching valleys back into the escarpments.
One of my colleagues commented on the sharpness of the escarpments, gaps and gorges in these images. The fact that they are so crisp and sharp and that we can easily trace the drainage flows means that the landscape is not very old. If it was exposed to tens of millions of years of erosion the features would be smoothed and not sharp.