It may look alarming and appear to be a crack. However, the 'cracks' are actually called Checking.
The following photos are from two of our previous inspections (Click to enlarge):
WHAT IS CHECKING?
Checking is often times found in big wood timbers such as deck posts or wood beams in older basements. Checking is a natural process that occurs when wood dries and cracks in a long, parallel pattern along the grain of the wood. As the wood dries, the outside dries quicker than the interior causing differential shrinkage and subsequently causing checking.
IS CHECKING A CAUSE FOR CONCERN?
>Typically,< this is not a structural issue, only a cosmetic issue.
For detailed information about checking, read more from 'Wood Science Consulting'.