Caves?
More likely dark dust gathered on the sheltered side of the hills. The northern feature has been disturbed and elongated by a major dust devil trail.
That's how I see it. In a few months we may get a ground-level view.
Don't expect miracles from these hills, BTW. They are simply more of the same Gusev fill that has been uplifted a few hundred metres by ridge-forming processes (global compression). They are still over 1.5 billion years old and only marginally eroded, so we may still not be seeing below the layer of basalt.
You never know, though, with Mars...