See Birch, White Flat sliced (half round) for information regarding species and how it is sliced.

This particular example shows the dimensional quality we call “figure.”  All woods have some dimensional or figural quality to them, the intensity and “type” of which is what will change from log to log.  This is a typical figure type we see in this species.  Just remember, the whole log may not develop figure of the same kind or the same intensity throughout.  Those are the fingerprints of nature.