Earth and Atmospheric Sciences, Department of



Maria A. Oliveira

Date of this Version



Progress in Physical Geography: Earth and Environment 44:6 (December 2020), pp. 870–889.

doi: 10.1177/0309133320927629


Copyright © 2020 Maria A Oliveira, Esteve Llop, César Andrade, Cristina Branquinho, Ronald Goble, Sónia Queiroz, Maria C Freitas, and Pedro Pinho. Published by SAGE. Used by permission.


Tsunamis and storms cause considerable coastal flooding, numerous fatalities, destruction of structures, and erosion. The characterization of energy and frequency associated with each wave contribute to the risk assessment in coastal regions. Coastal boulder deposits represent a physical proof of extreme inundation and allow us to study the effects of marine floods further back in time than instrumental and historical records. Age estimation of these deposits is challenging due to lack of materials (such as sand, shells, corals, or organic matter) that retain information about the passage of time. Lichenometry, a simple age estimation method, which is cost effective, quick to apply, and nondestructive, is here proposed as a solution. A lichen growth model for a calcium-tolerant lichen species was developed and used to estimate the age of a boulder deposit related to extreme marine inundation(s) in Portugal. Estimated ages indicate several very recent events (< 700 years) for most of the boulders’ stabilization and agree with results obtained with optically stimulated luminescence of marine sands found beneath boulders. Frequent and recent boulder transport implies a storm-origin for this deposit. These conclusions contrast with other works describing identical deposits that are attributed to paleo-tsunamis. This study presents a methodology using lichenometry as a successful alternative for age estimation in rocky coastal settings. These results offer an alternative explanation for coastal boulder deposits found on the west coast of Portugal.