Cliff notch formation along the sub-tropical La Paz Peninsula, Southern Baja California, Mexico

• Deep, andesitic notches in Baja Mexico are at the high tidal level.
• Spectrometry and spectroscopy showed that chemical weathering is of minor importance.
• Salt weathering is more important than wetting and drying.
• Rates of notch formation and collapse increase with wave exposure.

Most work on cliff notches in the Tropics and Subtropics has been on calcareous substrates that experience chemical or biochemical corrosion and the erosional and protectional effects of boring and encrusting marine organisms. There are deep notches in coarse andesitic lahar deposits along the subtropical western coast of the La Paz Peninsula in Baja California, Mexico. Many notch-forming mechanisms cannot operate here. There are no visible marine organisms at the cliff foot, the waves are generally too weak to dislodge rock fragments, there is little loose material for abrasion, and the substratum is resistant to corrosion. To determine the processes of notch formation, and their rates of erosion due to weathering and debris removal, rock samples were alternatively immersed in water for 90 min and then dried in air for one or two weeks to simulate conditions in the upper high tidal zone, corresponding to the deepest part of the notches in the field. These experiments ran for 17 months and used either fresh water to determine the efficacy of wetting and drying, or synthetic sea water to include the additional effect of salt weathering. The results suggested that salt weathering is more effective in this environment than wetting and drying and, because one week of drying was more effective than two weeks, that optimum notch development is probably closer to the neap than to the spring high tidal levels. As the experimental rock breakdown could also result from chemical weathering, rock samples were analyzed using inductively coupled plasma optical emission spectroscopy and mass spectrometry. The results suggested that, despite some Mg and Ca leaching, chemical weathering is not important in this area. Although the dominant notch-forming mechanism is salt weathering, field evidence implied that its efficacy rapidly increases with exposure to wave action, probably because of more frequent and stronger spray and splash and possibly because of the premature removal of loosened, weathered grains.