That is a long sentence with many subordinate phrases, however it contains just one main subject and one main verb. The main subject is "beach" and the main verb is "stretched". The inverted word order might perhaps create some confusion for you. The normal word order would be "A beach stretched" One of the meanings of the verb "to stretch" is "to extend or spread out over space or time. The reason the word order is inverted is to create a poetic effect.

"Stretched a beach" In front of her was an area of sand/stones near the ocean or a beach for short. "a tonic to hear" It gives a positive feeling to hear; it's comforting to hear I hope I was able to help! :)