The coastal lowlands span more than 480 km around the coast of the island, 15-20 km inland in the north and 3-15 km inland in the south. The plains are a result of erosion, sediments which are washed down from the mountains are deposited in plains. The area is sandy, muddy, or has fertile soils, rocky patches are the exception. Nevertheless, there are small areas of very young limestones, often aeolian formations (eolianite) created by sand which is deposited by the wind. Rivers flowing through such limestone ridges create shallow gorges or even small caves across.
A paper from 2008 presented an inventory of 68 caves along the coasts of Puerto Rico. The caves are found along the coast of the island and as sea caves are created by the erosion of the sea. But there is also the Quaternary eolianite, which is karstified. So only 53 of the caves are erosional and 10 are so-called flank margin caves.