meadow, original, primitive, field, plain, prairie, tundra, wilderness
厡 centers on an open, natural expanse of land: a meadow, plain, prairie, or tundra. The meanings "original" and "primitive" extend from the idea of untouched, wild land as the source or original state.
厡 is a variant form of 原. 原 combines 厂 (cliff) with 泉 (spring), originally depicting a spring emerging at the base of a cliff, which came to mean a flat open area or source. 厡 retains the same components but arranges them slightly differently.
Under a cliff (厂), a spring (泉) feeds a wide open meadow. The untouched wilderness is the original, primitive landscape.
For ゲン, imagine a vast green meadow stretching from the cliff: green -> グリーン, and the first syllable グ cues ゲン.