You're right, r.lewis, it's just fine-grained m...

You're right, r.lewis, it's just fine-grained material sluffing off from upper layers. The source could be the one semi-contiunous dark layer and some of the broken darker blocks above it. Maybe this dark material is interstitial hematite that has been winnowed out from these sources.

Thanks for the colour photo, Microkid. As both your and Horton's photos seem to be close to true colour, the dark material at Burns Cliff is different in composition than the material forming the pasty flows.