This is part of the first batch of community made vanilla maps. This map was made by Lailisa #0000.
It is possible to corner jump your way back once the wooden planks fall off, but it is very risky. It is recommended to wait until the shaman builds.
You can build a bridge over the gap; however be careful not to block off mice trying to walljump up the side pillars or stranded on the ice blocks, especially if you are trying to save them.
Shamans may also consider replacing the initial wooden planks with a more stable bridge.