B733 FLC OVERSHOT MANDATORY XING ALTS ON SLC ILS RWY 34R APCH.

Narrative

ATC CLRED US TO CROSS PLAGE AT OR ABOVE 9000 FT AND CLRED US FOR THE ILS RWY 34R APCH. AFTER XING PLAGE AT 9000 FT; I VERBALIZED TO THE CAPT THAT THE NEXT ALT WE WERE CLRED TO ON THE APCH WAS 7100 FT. HE DIALED IN 7100 FT ON THE MCP AND STARTED DOWN. THE NEXT FIX AFTER PLAGE IS SCOER AT 7100 FT. THE NORMAL ALT AT PLAGE IS 10500 FT; BUT ATC MAY AUTHORIZE 9000 FT. I HAD LOOKED AT THE APCH PLATE AND THOUGHT THAT WE WERE PASSING SCOER SINCE WE WERE ALREADY AT 9000 FT. HENCE; I THOUGHT THE NEXT ALT SHOULD BE 7100 FT. WE SHOULD HAVE BEEN MAINTAINING 9000 FT UNTIL SCOER. INSTEAD WE STARTED OUT OF 9000 FT JUST AFTER PASSING PLAGE AND ARRIVED AT ABOUT 7700 FT JUST PRIOR TO SCOER. IT WAS AT THIS POINT THAT THE CAPT REALIZED OUR MISTAKE. THE SAID 'UH OH' AND LEVELED OFF UNTIL SCOER. HE SAID THAT WE SHOULD NOT HAVE STARTED DOWN YET. I UNDERSTOOD HIM AND REALIZED THAT WE HAD SCREWED UP. IN THE CHAIN OF EVENTS: 1) ATC CLRED US TO 9000 FT INSTEAD OF 10500 FT TO START THE APCH. 2) I LOOKED AT THE NEXT ALT ON THE APCH AFTER 9000 FT AND SAW 7100 FT. I DID THIS WITHOUT VERIFYING WHICH FIX WAS NEXT. 3) I VERBALIZED 7100 FT TO THE CAPT WHICH CAUSED HIM TO THINK WE SHOULD START DOWN. I FEEL LIKE A DUMMY FOR MAKING SUCH A SIMPLE AND OBVIOUS MISTAKE; BUT IN RETROSPECT; I COULD SEE HOW IT COULD HAPPEN TO ANY PLT GIVEN JUST A MOMENTARY LOSS OF POSITIONAL AWARENESS. OTHER FACTORS: ONLY MY SECOND TIME TO SLC AND LAST TIME WAS SEVERAL MONTHS EARLIER. CAPT ALSO HAD NOT BEEN THERE RECENTLY. OTHER NOTE: ATC NEVER MENTIONED THE EARLY START OF DSCNT. I'M NOT SURE WHY. WE WERE VISUAL AND PERHAPS HE WAS THINKING ALONG THOSE LINES IN WHICH CASE ALT WAS OUR CHOICE.