The first part of Carson's topic sentence Soon after the spraying had ended is a transitional clause that looks back to the previous topic: DDT spraying. Topic sentences often begin with such transitional clauses referring to the previous paragraph. The second part of the topic sentence there were unmistakable signs that all was not well shapes and controls what follows. This kind of bridging helps the reader follow Carson's argument. Notice, too, how Carson further helps the reader follow her argument by providing a more focused version of the topic sentence later in the paragraph All the life of the stream was stilled. This sentence tells us exactly what Carson meant by all was not well.