What I think happened is right before Mars lost it atmosphere the oceans and seas of mars rushed over the land and ripped some of these floors into rubble. Lets take a look at one of the images that convinced me of this. This image clearly shows the floor of a crater ripped open like a tin can. Now rocks do not rip open like this only some type of metal.
http://areo.info/mer/opportunity/988/1P215899041EFF76POP2417L5M1_L2L5L5L7L7.jpg
Here is a side view of the end of one floor. It looks like at this end a lot of the floor was washed away.
http://areo.info/mer/opportunity/101/1P137165842ESF2019P2357L5M1_L4L5L5L5L6.jpg
Here is a view looking straight into a small crater but you can see the floor which has a difference appearance than the actual ground.
http://photojournal.jpl.nasa.gov/jpeg/PIA06276.jpg
Here are a couple of images that also look similar to the others:
http://areo.info/mer/opportunity/122/1P139009457EFF2809P2262L5M1_L2L5L5L6L6.jpg
http://areo.info/mer/opportunity/278/1P152861517EFF37KDP2440L5M1_L2L5L5L7L7.jpg
http://photojournal.jpl.nasa.gov/jpeg/PIA07110.jpg
http://areo.info/mer/opportunity/1281/tn/1P241907942EFF8615P2357L5M1_L2L5L5L7L7.jpg.html
http://photojournal.jpl.nasa.gov/jpeg/PIA08753.jpg
http://areo.info/mer/spirit/1466/2P256509790EFFAX27P2574L5M1_L2L5L5L7L7.jpg
http://areo.info/mer/spirit/1466/2P256509936EFFAX27P2573L5M1_L2L5L5L7L7.jpg
Here is an interesting orbiter image. It also shows some sort of metal floor.
http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/PSP/ORB_009600_009699/PSP_009622_1590/PSP_009622_1590_RED.abrowse.jpg
http://areo.info/mer/opportunity/988/1P215899041EFF76POP2417L5M1_L2L5L5L7L7.jpg
Here is a side view of the end of one floor. It looks like at this end a lot of the floor was washed away.
http://areo.info/mer/opportunity/101/1P137165842ESF2019P2357L5M1_L4L5L5L5L6.jpg
Here is a view looking straight into a small crater but you can see the floor which has a difference appearance than the actual ground.
http://photojournal.jpl.nasa.gov/jpeg/PIA06276.jpg
Here are a couple of images that also look similar to the others:
http://areo.info/mer/opportunity/122/1P139009457EFF2809P2262L5M1_L2L5L5L6L6.jpg
http://areo.info/mer/opportunity/278/1P152861517EFF37KDP2440L5M1_L2L5L5L7L7.jpg
http://photojournal.jpl.nasa.gov/jpeg/PIA07110.jpg
http://areo.info/mer/opportunity/1281/tn/1P241907942EFF8615P2357L5M1_L2L5L5L7L7.jpg.html
http://photojournal.jpl.nasa.gov/jpeg/PIA08753.jpg
http://areo.info/mer/spirit/1466/2P256509790EFFAX27P2574L5M1_L2L5L5L7L7.jpg
http://areo.info/mer/spirit/1466/2P256509936EFFAX27P2573L5M1_L2L5L5L7L7.jpg
Here is an interesting orbiter image. It also shows some sort of metal floor.
http://hirise-pds.lpl.arizona.edu/PDS/EXTRAS/RDR/PSP/ORB_009600_009699/PSP_009622_1590/PSP_009622_1590_RED.abrowse.jpg