These images from the surface of Mars show the unmistakable pattern of sinuous channels. Sinuous channels are seen all over the surface of the Earth in numerous environments -- from the many kilometers-wide Amazon River to much smaller channels that form on tidal flats. These examples from Mars aren't active channels, of course, but remnants left over from the planet's past.
These images are from a new paper out in the open access journal Mars, The International Journal of Mars Science and Exploration by Ryan Anderson and James Bell. Ryan (@marschronicler) also writes the popular blog The Martian Chronicles and has a great summary about this paper on his blog. (I welcome Ryan's comments here since he is the expert.)
If you look carefully you'll notice that channels are visible because they are topographically higher than the surrounding area (this is especially evident in image 'a'). These channels are superb examples of inverted topography. In this case, the likely explanation is that when the channels were active they carried sand and some of that sand ended up being deposited along the channel. The areas outside of the channels were probably dominated by mud and silt. As the area began to be slowly eroded (following a long period of burial and lithification) the channels were more resistant to erosion and ended up forming mesas.
It's called inverted topography because what was once the lowest elevation -- rivers/streams follow valleys after all -- is now the highest topography.
A great example of inverted topography on Earth is Stanislaus Table Mountain in the foothills of the Sierra Nevada (not far from Modesto). The images below, about 11 km (7 mi) across, show the feature nicely. I've drawn the edges of the ridge in yellow on the right image to highlight it.
In the case of Stanislaus Table Mountain the more resistant rock is lava. Approximately 9 million years ago a lava flow made its way down a river valley filling it in. Subsequent erosion wore down the the surrounding rock leaving the sinuous ridge. This image from Google Earth shows the ridge in a perspective view.
Hmmm, inverted topography ... perhaps a future theme for Pathological Geomorphology?
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Images: (1) Anderson & Bell, 2010, Geologic mapping and characterization of Gale Crater and implications for potential as a Mars Science Laboratory landing site / link. (2) Table Mountain, CA -- near Jamestown, up the road from Modesto / Flash Earth permalink. (3) Google Earth / see Flash Earth link for XY information.
