The very top of this image shows the edge of ancient lava flow that went no further across the plain. It’s also a natural boundary in the reflectivity (albedo) of the surface.

ID: ESP_055232_1595
date: Wednesday, 9 May 2018
altitude: 254 km (157.48 mi)

https://uahirise.org/hipod/ESP_055232_1595
NASA/JPL-Caltech/University of Arizona
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