The "containers" seem to be made up of the dead creatures around them.
This scenario seems to be repeated over and over:
Which is when mars lost it's atmosphere and it's planet's surface destroyed, the dead creatures were so massive that the martians sculptured stories to tell about it and past down the information from generation to generation! So now Curiosity Rover is taking photos of those very sculptures that tell the horrid story of a planet's surface ruined forever ! Forcing in my opinion the Martians to go underground!
The analysis I use for the photos and imagery are not mean't to be exacting, only to get you closer to understanding that there is intelligent life on mars (martians). Many of the rocks you may think are rocks, are in fact highly designed sculptures, mechanical objects etc.
Nasa is trying to leak out anomalous imagery with out of course talking about it public! Though most images have been doctored, Nasa is trying to humbly and quietly nudge us along with out going too public about it vocally. Though it is work and painstaking effort to find objects in the images that have not been altered in some way!