The Ouray 50 starts at Fellin Park and joins the 100 milers for the last 50 of their race.  (See last page of turn-by-turn document below.)  With over 20k feet of elevation gain, this is one of the most difficult 50 milers in the world.  Single track trails include:  Weehawken/Alpine Mine Overlook, Hayden/Crystal Lake, Old Twin Peaks, Silvershield, Perimeter Trail/Lower Cascade, Chief Ouray Mine, and Old Horsethief/Bridge of Heaven.