No, not "really", though there's a optical illusion trick...here it is:
http://www.terra-rium.de.vu/The reason it is not possible is that a heigthfield is 2d representation of a terrain on the x- and y-axis and that the brightness of the grays determine how much elevation there is. Each pixel represents a certain amount of meters. This way the renderer can resolve the terrain and is also the reason why your grand canyon looks so different than 'the real thing'.
With an overhang there will be different heighths in the same pixel and thus impossible to resolve.
TG2 works in a totally different and way more powerful manner like PG explained shortly.
Martin
