Table: southern_sixers
Columns: rank
peak_name
elevation_ft
range
isolation

Question: WHAT IS THE ELEVATION OF THE UNAKA MOUNTAINS, IN ROAN HIGH BLUFF PEAK, AND ISOLATION LARGER THAN 1.54?
SQL: SELECT AVG("elevation_ft") FROM southern_sixers WHERE "range" = 'unaka mountains' AND "peak_name" = 'roan high bluff' AND "isolation" > 1.54