Terrain Texturing using the four swatches

My sim is currently a flat, sandy island, with the sand level at 21 meters. I lowered part of the center to below the waterline down to 0 meters in order to build underwater caves. I want the ground and sides of the sunken section to be lava rock, so I set the low swatch to the Library lava rock texture, with the four corners set to min 20m, max 60 meters.

When I apply, all the sand turns dark, as if the lava color is bleeding through from below. I lowered the min value downwards, all the way to .5m. As I lower the value, the darkening of the sand at 21m reduces but never totally goes away.

What don't I understand? Any help would be appreciated.

How it works, as I understand it...

4 textures, low to high.
Each bleeds into the next, and you do not control that.
You set a lower level and an upper level.
System sets a mid level half way between low and high.
These three levels are where the textures blend.

No one has sufficently fine control over terrain texture blending for what you are attempting to have reliable results. Even from one login to the next there can be subtle differences as to exactly where the client tries to make the transition.

Simply do the sand as the lower level, and line the cave walls and floor with rock-textured prims. You'll use more prims, but have far less aggrivation.

Thank you both for your comments. I'll follow the prim approach. I had hoped that the blending wasn't quite that unpredictable.