Instead of using roots and spices, it's made straight from birch tree bark, usually the black birch or sweet birch variety.

As the name implies, a river birch’s native habitat is near river banks, so they prefer moist soil. However, they can tolerate drier soil as the tree matures and the roots become well-established.

They are generally made complete with the rind of one birch tree, and so ingeniously shaped and sewed together, with roots Of the tamarack ... that they are water-tight, and ride upon the water ...