2d game foreground image based on terrain shape

Hey,

I want to make foreground plane based on shape of randomly generated terrain (view from side) to display texture under ground

Here's simple image that should help understand what I mean, bold line is shape of terrain. http://i47.tinypic.com/wsjk07.jpg

Any ideas how to make this at runtime (texture will be given, I only need properly shaped plane or other way to make this)?

Thanks in advance

Regards, Michal