Just read this from New Scientist: New Mars Rock Hints at Short-Lived Lakes

According to Philip Christensen, from Arizona State University in Tempe, the dark layer in Endurance is most likely to be a sandstone made of grains of basalt. This suggestion is based on data from a remote sensing instrument on Opportunity called Mini-TES, which uses infrared radiation to identify minerals.

It returned a signature matching basalt, a kind of volcanic rock. The team speculate that the layered outcrop is a sandstone, comprised of fine particles deposited either by wind or water.

"Finding basalt sandstone is not that surprising - there's basalt everywhere " says Christensen, who leads the Mini-TES science team. "The interesting thing is that it argues this may have been a lake, but it must have been cold and it can't have been there for a long time."This is because basalt is unstable in the presence of water at surface conditions. It could survive immersion in water that was near to freezing for hundreds, perhaps thousands of years. But over geological time periods, it would eventually weather away.

Small, short-lived lakes on a deep pile of basaltic sand dunes. Sounds like a ...?

jms