mound, embankment, the earthen altar to the god of the soil
壝 centers on a raised earthen structure: a mound or embankment, especially one used as a sacred altar to the god of the soil.
壝 combines 土 (earth) with 遺, which likely contributes the sound and the idea of something left or remaining, perhaps referring to a lasting earthen mound.
The earth radical 土 on the left marks this as an earthen structure. The right side 遺 suggests something left behind or remaining. Picture a mound of earth left as a permanent altar to the soil god.
For イ, imagine an earthen altar so old that even the god says 'Eee!' in surprise: Eee -> イ.