A DSNDING B737-300 OVERSHOOTS ITS ASSIGNED ALT IN A NEAR CFIT 30 MI NE OF BOI; ID.

Narrative

DURING DSCNT INTO BOI AT NIGHT IN VMC CONDITIONS ON THE 061 DEG RADIAL WE WERE CLRED TO A LOWER ALT. THE COPLT AND I BOTH HEARD 10000 FT AND I READ BACK 10000 FT TO ATC. ATC LATER SAID ACTUAL CLRNC WAS TO 13000 FT. AT 10000 FT AND ABOUT 30 MI WE GOT A GPWS; 'TERRAIN; TERRAIN;' AND PULL UP WARNING. STANDARD OPERATING PROCS WERE FOLLOWED AND CLB TO 13000 FT WAS ACCOMPLISHED. IT WAS THE END OF A 9 HR 15 MIN DUTY DAY WHICH ISN'T ALL THAT LONG; BUT IT WAS IN MY OWN TIME ZONE AND DAY 5 FLYING A RESERVE SCHEDULE. THERE IS NO QUESTION I WAS TIRED. THAT PARTICULAR RADIO FREQ WAS A LITTLE SCRATCHY. THE LITTLE VOICE IN THE BACK OF MY MIND WAS TALKING TO ME AND IN FACT AFTER RECEIVING THE CLRNC I PULLED OUT THE ENRTE CHART (KIND OF UNUSUAL FOR MOST GLASS PLTS) BUT DID NOT USE IT PROPERLY. I THINK I WAS LOOKING FOR SOMETHING VERY SPECIFIC (AN AIRWAY WITH AN MEA) TO VALIDATE THE LITTLE VOICE AND UNEASY FEELING I HAD. I OVERLOOKED COMPLETELY THE GRID MORA OF '14.2' AND THE BROWN TERRAIN SYMBOL; INDICATING ABOUT 10000 FT OF TERRAIN. (THERE ARE NO NUMBERS ON THE SYMBOL; MAY BE ACR SPECIFIC.) I ALSO CHKED THE MSA ON THE APCH CHARTS (9400 FT) BUT KNEW THESE ONLY GO OUT TO 25 NM. I THINK OVERLOOKING THE ITEMS ON THE ENRTE CHART WAS PARTLY FATIGUE RELATED.