Sidewalk Pattern

This pattern is on some sidewalk slabs. It looks like oxidized concrete. I love the directionality of the marks and wonder what caused them. I love walking past this because the visual effect of walking towards it is quite fun! You feel that perhaps the sidewalk is moving. No, it's not just the hot southern sun that makes me think that. It is the effectiveness of this pattern & its unusual location on the sidewalk. 

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