Accordingly to Carvalhido (2008) the main distinctive features of the Local Natural Monument of Magmatic Drops in Canto Marinho landscape, of medium scale, such as granitic balls, and of detail, such as mineralogical varieties or folds, are due to the first stages of the closing of the primitive ocean, the Rheic (the records of its biodiversity and dynamics can be seen in the Local Natural Monument of Rheic Remains in Pedras Ruivas), initiated about 400 million years ago.

The crenulations and folds, also in sheaths, which give a sinuous undulation to the whitish-yellow rocks that make up much of the local substrate (quartzite), were generated in the margin lands, in the first collision phase of the primitive continents, by shortening the space, heating and dragging.

With the new impulses, the amalgamation originated the fusion of rocks and the formation of magma whose cooling, at several kilometres of depth, originated the granites that currently form the hills of St. Luzia and Arga, but also the elevation of the coastal hills of Montedor and Gelfa.

Migrating like hot air balloons through the quartzite, giant drops of magma flowed from the main magma chamber that originated Montedor (Local Natural Monument of Ancient Beaches of Alcantilado de Montedor), and through fractures consolidated here, at about 700 m.

After about 300 million years of erosive processes and tectonic uplifting, we can see the minerals witnessing the hot contact of magma (contact metamorphism) such as garnet and staurolite, and the granite balls, some with more than 10 tons, which were interpreted as a deposit (tsunamite) generated by the oldest tsunami recorded in the Portuguese seismic catalogues, from 66 BC (Baptista & Miranda, 2009).

References:

Baptista, M. & Miranda J. (2009). Revision of the Portuguese catalog of tsunamis. Natural Hazards and Earth System Sciences, 9, pp. 25-42

Carvalhido (2018). Livro de Pedra, Monumentos Naturais Locais de Viana do Castelo – Catálogo. Câmara Municipal de Viana do Castelo, 2ª edição