Computer information systems that will archive, query, retrieve, and display geologic information tailored to specific requirements are the foundation for a leap in scientific development similar to that fostered by the invention of the geologic map. In order to achieve this potential, some standardization of the conceptual model for basic elements of geoscience is required to provide a consistent framework for developing interoperable systems. Models for three concepts that are central to geologic science, Earth material, geologic units, and geologic structure, are proposed as a starting point for this framework. An Earth material is a substance defined by chemical constituents, in concert with crystal structure, physical properties, or properties related to the nature and arrangement of constituent particles. Earth material is a mass noun, not countable. A geologic unit is a part of Earth located and distinguished from other parts of Earth based on geologic properties. Geologic units are countable. A geologic structure is a configuration of Earth material within Earth, and may or may not be countable. The existence of a geologic structure requires the existence of some Earth material substrate. A top-level vocabulary defines subclasses of these concepts, and description schemas specify relationships and attributes used to characterize defined classes and instances that extend the top-level subclasses. The geologic models in this paper are consistent with the NADM-C1 model of the North American Geologic-Map Data Model Steering Committee with minor modifications, and addition of detail on a number of important points. A distinct knowledge-representation framework provides a foundation for the geoscience domain models. In particular, modeling of a geologic map as a knowledge representation device is separated from the problem of geoscience knowledge representation in general. Models are presented as Unified Modeling Language static class diagrams.