The participants in this study belonged to a professional development school that embraced the responsibility and challenge of improving students' mathematical thinking. In this study, experienced teachers' pedagogical reasoning was made visible as they analyzed pieces of student written mathematical work in an approximation of practice designed to support professional noticing. Researchers then worked to characterize participants' professional noticing using the lens of responsive teaching. Results indicate that experienced teachers' decisions about how to respond to students' mathematical thinking fall on a continuum and often shift in responsiveness across pieces of student written work. The findings of this study provide guidance for teacher educators who work to develop K-12 educators' responsive teaching practices and have practical implications for the use of approximations of practice to develop responsive teaching practices.