Ängsö’s bedrock consists of primary rock. The bedrock has clear traces of the Ice Age, with rounded, smooth rocks and ice grooves.
The soil is moraine that was formed when the ice melted. About 4,000 years ago Ängsö began emerging from the water. When the island slowly edged up out of the water, the earth was affected by wave action. As a result, fine particles were washed away (what is known as “surging”) and remained on the lowest land surfaces. Today Ängsö is characterised by elevated areas with bare primary rock and surged moraine and fine soils in low-lying areas.