There is a fairly smooth crust on the Kasei Valles lava flow here, a short distance downstream from a major constriction (lower left part of the image). The constriction would have disrupted preexisting crust, so the smooth crust here must have developed locally on a reasonably stable flow surface. The rise to peak flow would also probably have disrupted older crust.

The flow was not stagnant, because there are some fractures that likely developed when the last flowing lava drained away. This suggests that this is a crust that developed over a fairly significant length of time over a steadily flowing active flow sometime after peak lava flux, and then lava drained from below.

ID: ESP_055335_2015
date: 17 May 2018
altitude: 281 km

https://uahirise.org/hipod/ESP_055335_2015
NASA/JPL-Caltech/UArizona
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