We couldn't help notice this snag covered with an incredible amount of burls on our spontaneous trip over the weekend to Olympic National Park. Incredible! Burls are caused when the grain of a tree grows in an abnormal manner, and are commonly found in the form of a rounded outgrowth on a tree trunk or branch that is filled with small knots from dormant buds. Many wood carvers love to work with burls because the grain is so unique.