The lava flow from the Puig Jordà volcano, about 17,000 years ago, spread over the wet meadows containing clay and organic matter in the Tosca Forest area. This interaction between water and lava created small mounds on top of the flow known as tossols.
Tossols form when a lava flow moves over a waterlogged area (see figure 1), composed of saturated sediments and shallow water. As the lava comes to rest, its weight forces water from the sediments into the flow, generating steam that rises through fractures formed as the lava solidifies (see figure 2). When the gas reaches the surface, it triggers a small eruption of steam and the lava fragments it lifts (see figure 3).
Their scientific name, rootless volcanic cones, refers to the fact that they have no conduit through which magma rises; instead, they result from the interaction between lava and wet sediments.
This lava also dammed the headwaters of the Fluvià River, creating a volcanic-dammed lake. Over the years, the accumulation of materials that naturally settled in the valley, together with human activity —which filled in and drained the last wetlands to dry them out for cultivation— shaped what is now the Plain of Bas.
