Left mound radical - the side-form of
阜, picturing stacked earth/terraces seen from the side. The indexing radical, anchors
阪 in the terrain/elevation family:
阶 step,
险 dangerous,
阻 obstruct,
陡 steep. Marks
阪 as a feature of sloped land - a hillside, the original meaning preserved in the city name
大阪 Ōsaka (literally 'big slope').