Sol 4165, October 12, 2015. 
More scouting the outcrops on the Marathon Valley floor. The rocks of greatest interest are those that have a faint reddish tinge in false-color Pancam images (such as these by Holger Isenberg). These are believed to contain clay minerals, which can tell about the water-history of these rocks. (Click to enlarge.)
Opportunity raw images, its latest mission status, a location map. and atmospheric opacity, known as tau.