Some cool surface grinding images:
Minox Contax Racetrack Playa
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Racetrack Playa in Death Valley National Park.
For most of the year Racetrack Playa in Death Valley, California is a dry lake, a completely flat expanse of cracked white clay. There are a few scattered stones on the surface, from a handful of centimetres in diameter to half a metre. Some of the stones have trails hundreds of metres long displaying that they have moved across the ground – but how?
Nobody has noticed 1 in motion, but geologists have tracked the stones progress for years, usually in March.
The most likely explanation entails the spring weather. Rain and melting snow from the surrounding hills leaves a lot of of the rocks partly-submerged in enormous, shallow pools. As temperatures fall at evening, ice can form a collar around the base of a rock. This creates enough buoyancy for strong winds to overcome friction with the lake bed. This is only possible simply because of the flatness of the Playa, which allows wind to gust at 90 mph close to the ground.
In any case, the movement probably on lasts less than a minute and might only happen every many years. This is probably why the sailing stones of Racetrack Playa have remained a single of the world’s more elusive climate phenomena.
-The Guardian
Sharan Miniature Contax I on expired Minox Minocolor 100 film.