**Septarian nodules** ::septaria 
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On the eastern side of Lapari Beach lies a unique geological feature that stands out along the Basque coast: the Deba septaria. These spherical or oval-shaped nodules, usually measuring between 5 and 25 centimeters, reveal an intricate internal network of radial and concentric fractures that were filled with calcite during their formation. This process creates striking white geometric patterns that contrast with the dark clay matrix of the nodule. Some septaria also contain small crystals of barite or pyrite, adding extra mineralogical value.
Since their first scientific description in 1958, the outcrop has suffered significant degradation due to construction work and specimen collecting. For this reason, it is now part of the protected Deba–Zumaia Coastal Biotope, and collecting septaria is strictly prohibited without authorization. Although the filon can no longer be observed directly on the cliff, several specimens are displayed at the Algorri Interpretation Centre (Zumaia) and at the Deba Tourist Office, allowing visitors to appreciate and learn about this fascinating geological phenomenon without compromising its conservation.
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