taru-adjective, adverb taking the 'to' particle
densely wooded; deeply forested
Literary or poetic description of a place thick with trees; often used with と (e.g., 森森と茂る).
The mountain was thickly wooded with trees.
Reduplication of 森 (しん, 'forest'), emphasizing a state of being thickly forested. The reading しんしん is the on'yomi doubled.