Not knowing the exact values you are working with. One thing you can try is to decrease the overall brightness to 60 to 70 % using adjustment ( brightness contrast) then use Adjustment (Levels); Push the White slider over to the left till it touches the edge. This will Force the contrast in the "HIGH" end of the bump. If you go with Midgardsormr suggestion of doing the areas separately then to blend set the mountains above the low lands and use levels to push the blacks up to turn all the darker greys to black ( may need to adjust the grey slider to preserve your "HIGH Areas" then set the Blend Mode to Screen to blend the dark areas of the Top Mountain layer into the Mid range of the Lowlands level below ! Directional is the way to go for more realistic Shadows. I learned that late last month !