In Walden "The approach to this was through a succession of descending grassy hollows, full of young pitch pines, into a larger wood about the swamp." -
In this passage, "pitch" means

a. The quality of a sound governed by the way a vibration producing it; the degree of highness or lowness of a tone

b. a legal delivery of the ball by the pitcher

c. the steepness of a slope especially a roof

d. a sticky resinous black or dark brown substance that is semiliquid when hot, hard when cold. It is obtained by distilling tar or petroleum and is used for waterproofing.