"Let's model me a mine"
M.E.G.M.S.
Mining & Exploration Geological Modelling Services
Create Boundaries command (Tools menu and toolbar) 
Select this command to select the Create boundaries tool. When the Create
boundaries tool is active the mouse cursor changes to a hammer (
) and "Create Boundaries" is displayed in the MEGS_Log
status bar.
With the Create Boundaries tool you can left click within a borehole field to create a new boundary between units. If you click beyond the end of the borehole then a new unit is appended to the borehole. If you click within a unit then that unit is split into two. The current Insert Mode does not affect the action of Create Boundaries.
The shortcut key for this command is F2. You can cancel the Create Boundaries tool and return to Normal mode by pressing the Escape key.
See About Editing Tools for more information.