Tephra layers are often used to reconstruct past eruptions. However, accurate interpretation of tephra layers depends on their degree of preservation. We asked: how much volcanological information do terrestrial tephra layers typically retain? We addressed this question with a study of the tephra layer produced by the May 1980 eruption of Mount St Helens, comparing historical records of deposit thickness and grain size distribution with measurements made four decades after the eruption. Using published isopach maps as a guide, we looked for the tephra layer in locations 15 – 600 km from Mount St Helens, selecting sample locations we judged to have high preservation potential. We found that preservation of the 1980 tephra deposit was often extremely good: observed thickness, mass loading and grain size distributions were similar to equivalent measurements made in 1980. However, we also observed high variability in preservation on small spatial scales, and our results indicate that landscape-scale volcanological reconstruction is sensitive to sample number and location, even when many sites display excellent preservation. Tephra preservation is clearly a complex and contingent process influenced by climate, biology, topography, parent material (i.e., grain size, morphology and geochemistry) and time. The relative importance of these factors will vary from place to place. We propose that the only way to tease apart these factors – and to fully understand the information content of tephra layers - is through direct observation of tephra deposits (whether natural or experimental) over extended time periods.