B757 FLC INITIATES TURN TOWARDS LOWER TERRAIN AFTER L30 ISSUES AN ALT BELOW SID MCA.

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ATC CLRNC WAS 'IDALE 1 DEP' TO 'MAINTAIN 7000 FT.' WE DEPARTED OFF RWY 25R AND; USING LATERAL NAV (LNAV) SYS; CLBED TO 7000 FT MSL WHILE TRACKING THE SID COURSE. WHEN HANDED OFF TO LAS VEGAS DEP; WE WERE ACKNOWLEDGED AND TOLD TO 'MAINTAIN 7000 FT;' BUT NO TRACK DEV OR VECTOR OFF THE SID. IDALE SID -- WITH A COALDALE TRANSITION -- REQUIRES A MINIMUM ALT AT IDALE INTXN OF 9000 FT. TERRAIN RISES STEEPLY BTWN IDALE INTXN AND SHEAD INTXN. THE CTLR WAS NOT AUDIBLY COMMUNICATING WITH ANY TFC CONVERGING WITH OUR TRACK; BUT CONTINUED TO WITHHOLD FURTHER CLB; OR TURN US AWAY FROM THE RISING TERRAIN AHEAD. AS WE REACHED IDALE INTXN AT 7000 FT; HE CLRED US TO CLB TO 10000 FT MSL. I DISENGAGED LNAV; TURNED THE AIRPLANE 20 DEGS L; AND CLBED TO 10000 FT OVER LESS STEEP TERRAIN. WE HAD ACTUALLY BEEN 'CTLED' OVER IDALE INTXN AT 7000 FT -- FULLY 2000 FT BELOW THE PUBLISHED MINIMUM ALT. CONDITIONS WERE 'SEVERE CLR' AND SMOOTH. WE WERE HANDED OFF TO ZLA WITH NO INDICATION OF WHY THE DELAYED CLB FROM LAS VEGAS DEP CTL (IE; NO 'THANKS FOR THE HELP' OR ANY INDICATION OF ABNORMAL CIRCUMSTANCES). IN IMC; THIS COULD HAVE BEEN A 'CFIT.' MANY NEW SIDS AND STARS RECENTLY ESTABLISHED AT LAS. COULD THE CTLRS NOT BE PROFICIENT? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT REVIEWED EVENT WITH CHIEF PLT. ATC RESPONSE WAS NOT KNOWN. RPTR CONCERNED THAT ATC DID NOT VALIDATE WITH THE FLC IN A TIMELY MANNER THAT THE IDALE XING RESTR WAS CANCELED AND WHAT ATC WAS PLANNING. NOT KNOWING; THEY TOOK THEIR OWN ACTION TO FLY TOWARDS LOWER TERRAIN UNTIL THE TFC EVENT CONCERN WAS RESOLVED WITH ATC. THE RPT RECOGNIZES THAT LAS ARPT HAS MADE SIGNIFICANT ARR/DEP ROUTING CHANGES AND 'THINGS STILL NEED TO BE WORKED OUT.' RPTR BELIEVES THAT CHANGING THE ARRS AND DEPS TO CROSS OVER THE SAME GENERAL POINT IN SPACE WAS NOT A BENEFIT TO THE USER; BUT ADDED TO COCKPIT AND ARPT COMPLEXITY.