Scientific observation is central to classroom inquiry and children's investigations and explanations in science. Young children can struggle with observation, and research has shown that professional scientists who engage in complex observation tasks, observe detailed patterns when they have well-developed disciplinary knowledge. However, fewer studies address how this observational expertise develops and its specific role as a component of disciplinary knowledge within a larger complex knowledge system. Grounded in an existing theory of conceptual change, Knowledge in Pieces (KiP), we view knowledge as a complex system consisting of both perceptual and inferential parts. We demonstrate how these perceptual and inferential parts can be related to each other in the developing knowledge systems of learners engaged in scientific observation in field geology. In the analysis we examine in-service science teachers' observations of bedrock while they were aiming to generate an understanding of the relevant historical geological processes. The analysis documents the moment-to-moment complex relationships between the perceptual and inferential parts of a knowledge system and thus offers an empirical account of how observation is situated within a knowledge system. The results challenge the notion that scientific observation is a simple skill by demonstrating how discipline-specific knowledge is mobilized during scientific observation in a field-based setting. This work has implications for science education instruction.