Calcareous bodies of Piedade

At the locality (sítio) of Piedade (‘pity’), on the Ponta de São Lourenço, there are numerous calcareous bodies that look like petrified tree trunks and branches. Many of them show concentric layers similar to those seen in certain woody plants. Some of these bodies extend at their lower end into a kind of root, with the shape and direction of the roots of dicotyledonous plants.

The naturalists who have visited Piedade do not agree on how to explain the origin of these bodies. The botanist Richard Thomas Lowe regarded them as sandy-calcareous concretions; Macaulay as a coral formation of the family Alcyonidae; the geologist Smith as mere casts formed in spaces once occupied by plants; and Mousinho de Albuquerque and Thomas Edward Bowdich as the petrified remains of an ancient forest.

This last explanation is the only one that seems acceptable to us, given the shape and appearance of the bodies in question. As for the process of mineralisation, it must have consisted of a simple exchange of organic for inorganic matter, with particles of carbonate of lime taking the place of the plant particles as these decayed. Water, dissolving part of the limestone, would have brought about this exchange by seeping into the cavities left by the decomposing wood.

If we accept that in remote ages the north coast at Piedade extended out into the sea in a gentle slope, bounded by a beach covered with calcareous sand, we may well accept too that the winds, carrying this sand along little by little, spread it over the land of the interior and of the south coast, thus supplying the material needed for this mineralisation.

The ground at Piedade is volcanic in origin, and tuffs and basalt are found in the adjacent cliffs.

In the north-west of Porto Santo there are calcareous bodies of the same nature as those of Piedade.