This paper begins with a discussion regarding the nature and complexity of understanding in the conceptually confined domain of earth science, here limited to its "geological" aspects. There then follows a report on a study of how the ideas about a range of concepts relating to the Earths structure and processes held by individual children from Key Stage 2 (7-11 years) in one primary school in north-east England reveal hierarchical levels of descriptive and causal understanding. Such patterns, described here as "alternative frameworks", can be used to inform our understanding of young children's learning in earth science and, unless taken into consideration, represent an additional "critical barrier" to learning in this domain to those previously identified by research. Implications of these findings for further research and for classroom practice are addressed.