This image is mesmerizing, and even a bit surreal. It's as if we're looking at an x-ray of the surface of the Earth – getting a glimpse of pattern and morphology underneath a thin agricultural skin. There is something fascinating about the superposition of distinctly human-made geometric forms, the straight lines, right angles, and near-perfect arcs, with the difficult-to-describe shapes from nature. And it's not just a juxtaposition of pattern but of time as well, combining the not-so-distant geologic past with the present.

These ghostly patterns in rural Wisconsin are remnants of rivers that traversed this area some 15,000 years ago as the last ice age was ending. The patterns suggest a braided river morphology, which, unlike the meandering rivers present in the region today, are characterized by multiple water-filled channels that coalesce and bifurcate in intricate patterns.

Note the sandy (and likely gravelly) bars in between and defining the channels in the braided river example in the right image below. The whitish blobs, for lack of a better term, in the top image from Wisconsin are very likely preserved remnants of similar sandy/gravelly bars in the shallow subsurface. It's not clear from this image exactly why these features show up like this at the surface. From talking with a geologist* who knows much more about this area than I do, it's probably subtle differences in the soil type.

Why did the type of river morphology change so drastically? This area of the upper midwest was very close to the southern edge of the ice sheet that covered much of northern North America during the last ice age. As the climate warmed and the ice sheet began to recede, the meltwater fed energetic river systems. Glacial-outwash–fed rivers in our modern world are known to transport a lot of sediment and develop these braided patterns. Over time, as the ice sheet receded farther and farther north, the river systems began to adjust to their current slower and less sediment-choked condition.

These features show up especially well in the older, black and white aerial photos. The image at the top is from the freely available Terrabrowser (for Mac only). I know that aerial photos can be viewed in Google Earth as well, but I did not try that. Give it a shot and see what you can find!

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*Images: (1) USGS aerial photo of Lower Chippewa Valley, Wisconsin / from Terrabrowser; (2) Lower Chippewa Valley; Flash Earth permalink (3) New Zealand; Flash Earth permalink

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* Huge thanks to geoscience blogger Matt Kuchta at Research at a Snail's Pace for* first posting this several months ago and then entertaining my multiple questions about the features. Make sure to check out his excellent blog.*