Bark

Oak

A tree trunk's bark forms cracks or plates or peals as the tree grows bigger from the inside.  Many of the patterns created are similar, but often they can be distinct enough to help identify a tree.

Bark comes in many colors, ranging from white to light gray to dark grey to light brown and dark brown.  Some have distinct shades of green, tan, orange, reddish or maroon; or actual patches of the colors.  In the extreme this can give the tree a mottled appearance.

Siberian Elm

Splits in the bark can be small wrinkles, or shallow furrows, or deep fissures.  In some cases the tree appears almost smooth.  In other cases the pattern created by the ridges and furrows is very deep.  It can resemble long scaly plates, short blocky plates, or even give the tree a shaggy appearance.  The edges of the plates can be smooth, or irregular, or curled.  In some trees the bark exfoliates, peeling or flaking off the tree.