Symbolism, Imagery, Allegory
The thing that Sarah misses the most about Maine—besides her family and friends, that is—is the ocean. Although she can't take her brother or aunts to the Midwest with her, she can take a part of the sea in the form of her shell collection:
"A scallop," she told us, picking up the shells one by one," a sea clam, an oyster, a razor clam. And a conch shell. If you put it to your ear you can hear the sea." She put it to Caleb's ear, then mine. Papa listened, too. Then Sarah listened once more, with a look so sad and far away that Caleb leaned against me.
"At least Sarah can hear the sea," he whispered. (4.3-4)
The shells are a way for Sarah to feel connected to her hometown and to feel like she can still access it in some way. They're also a way for her to share the ocean with Anna and Caleb, who have never seen the ocean and don't even know how to swim until Sarah teaches them in the cow pond.
They might be delicate, but Sarah's shells are really like a bridge, helping her stay connected to and share her home from the landlocked prairie where the Wittings live.