Stratigraphy

Describe the stratigraphic order, not the colors or layout. Screen reader users care about sequence and relationships (oldest → youngest), not vertical placement or color blocks. State the scope and context first. Say what the diagram represents (location, geologic feature, and purpose) before listing layers. Follow a consistent direction. Use a clear convention such as “from bottom (oldest) to top (youngest)” and stick to it. Stratigraphy is inherently sequential. Name formations and ages; summarize thickness only if meaningful. Include formation names and geologic periods. Numeric thicknesses or fine grain detail should be summarized unless they are the instructional focus. Do not include every visual symbol unless it conveys meaning. Fossils, unconformities, or major boundaries should be mentioned; decorative textures should not. Use a short alt text + long description when needed. Complex stratigraphy usually exceeds reasonable alt‑text length; a long description (or adjacent text) is appropriate.