Redevelopment of Place Van Zeeland, Belgium - Google SketchUp Workshop: Modeling, Visualizing, and Illustrating - page 69

FiG 4.8 Surface generated by the Sandbox tools.
With complex surfaces, the Sandbox tool often generates faces outside the
perimeter of the desired surface. To delete these faces, you can enter the
surface group and, from the menu bar, choose View > Hidden Geometry. This
allows you to see any hidden edges that lie outside the contour surface. You
can then delete these using the Eraser tool.