Etymology: The outside character means cliff.
The inside character means spring and by extension, source / origin / primitive.
Thus, we get a primitive spring gushing from a cliff.
Its current meaning plain, may have come through the related concept of primitive land.
Or it could be a borrowed meaning.
Mnemonic: CLIFF WITH SPRING OVER TIME TURNED INTO A PLAIN.
原始 (genshi): atom
草原 (kusahara): grassy plain
(Based upon Henshall, 1998)