DRG A PREFLT WX BRIEFING, THE PLT WAS ADVISED THAT VFR FLT WAS NOT RECOMMENDED DUE TO FORECAST FRONTAL ACTIVITY MOVING SOUTHBOUND ALONG HIS ROUTE. SEVERE TURBULENCE & ICING HAD BEEN REPORTED JUST TO THE NORTH OF HIS COURSE. HE FILED A VFR FLT PLAN & TOOK OFF AT 0730. AT 0931, THE PLT CONTACTED ARTCC. HE SAID HE WOULD BE UNABLE TO CONTINUE VFR TO HIS DESTN &REQUESTED VECTORS TO BILLINGS, MT, TO AWAIT WX IMPROVEMENT. AT 0935, BEFORE VECTORS WERE ISSUED, THE PLT CANCELED HIS REQUEST SAYING HE WOULD NOT NEED THE VECTORS AS THERE WERE 'BIG HOLES . . . BELOW US NOW.' SHORTLY THEREAFTER, RADIO & RADAR CONTACT WERE LOST. A SEARCH WAS INITIATED, BUT THE ACFT WAS NOT LOCATED UNTIL 10 DAYS LATER. IT HAD CRASHED INTO THE WALL OF A LARGE CANYON IN MOUNTAINOUS TERRAIN NEAR THE POINT WHERE RADAR CONTACT WAS LOST. A PLT, WHO WAS DRIVING IN THE AREA, REPORTED THE SKY WAS OBSCURED WITH 1/4 MI VIS, BUT FURTHER SOUTH, THE WX IMPROVED TO UNLIMITED VIS & A CEILING OF 2000 TO 3000'. THE PLT'S LOG WAS NOT FND, BUT ON 1/26/84, HE RPRTD A TOTAL TIME OF 400 HRS ON AN APPLICATION.