In order to answer this question, we must think about what we know about Sarah, and then make an inference about the shells at the bottom of the bag.
We know that Sarah came from the East Coast. When she first arrives, she brings presents from home for the children: a shell for Caleb and a sea stone for Anna. She speaks lovingly of the sea. In Chapter Four, we are introduced to Sarah’s shell collection. She picks them up one at a time and names them. She shows the children how you can hear the ocean if you hold a conch shell to your ear. She mentions the sea over and over again in the book.
We can infer from this information that Sarah loves the sea and misses it very much. We know that her memories are wrapped up in her shells. So in Chapter Eight, when we learn that her shells are in the bottom of the bag in which she had put the snack she gathered, we can infer that she ran to her room and grabbed them for safekeeping, just in case the squall damaged the house.