Thanks to reader JS who sent two images of the windgap near Perth, Western Australia: a closeup of Walyunga NP, and a larger scale image of the Perth coast. They were obtained from Google Earth. JS said:

“The first is a Google Earth screenshot, in which the purple represents a filled-in contour of 80m asl [above sea level], and the blue of 120m asl. The blue area delineates the westward flowing paleochannel which would appear to have been cross-cut and captured by the SE flowing Swan R., whose channel is purple.”

Region around Perth, Western Australia, showing sea-level rise of 80, 180, and 240 m aslRegion around Perth, Western Australia, showing sea-level rise of 80, 180, and 240 m asl

“The second image is from a Google Earth mapplet called Sea Level Rise, which allows the user to set 3 different sea levels in red, yellow and orange, in order to simulate sea level rises. I use it to simulate emergence from inundation. On this image, red is set at 80m asl, orange at 180m asl and yellow at 240m asl.”

“What I found intriguing on this image is both the parallelism of the western Australia streams, and also a consistent jog in stream channels that appears as they approach the coastal plain. Stream parallelism is a prominent feature of the east coast of America as well, as are bends in the stream channels as they approach the coastal plain. Interestingly, 80m, 180m and 240m are also levels at which major groups of water gaps appear in the northern Appalachians of the US.”

Another feature this second image from JS shows is the flatness of the plateau east of the Darling escarpment. The Sheet flow stage of the retreating floodwaters explains the flatness of this plateau which has had kilometres of sediment removed in sheets from its surface….

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