Creating a Ground Plane
Ground Planes are created at ground level. By default, all new scenes open with an infinite Ground Plane.
To create a Ground Plane, click the Ground Plane tool in the Create palette.
OR
Left-click-hold on the Ground Plane and choose Surface from the menu:
Once the plane appears in your scene you can edit it like any other object. Use the Ground Plane to add ground to your environment.
The Ground Plane is created with a gray color, as are all Bryce primitives. Once you begin assigning textures to objects, Ground objects (as well as all other primitive objects) will inherit the texture assigned to the previous object.
To create a Ground Slab, press Alt+click Ground Plane tool
OR
Left-click-hold on the Ground Plane and choose Volume from the menu:
The Ground Slab will be created with just the top layer showing above the underground:
You can view it better by turning off the underground or moving it up on the Y-axis:
The slab depth can be modified by clicking on the 
button and changing the Y-size from the default 20.48 BU:
Here the Y-size has been changed to 50 BU:
A Ground Slab usually has an opaque material applied to it so it can't affect objects placed within it, however, when a cloud material is applied to it, you've got a fog object that affects the ground below it and any objects placed in it
Examples:
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