Immediately following depositing, lower pumice was subjected to severe erosion that lead to sedimentation of a large part of this formation. This effect is more intense on the NW slopes, where few in situ occurrences of the initial deposits have remained.

The lower part of the formation comprises fine material, while its higher parts include lapilli and blocks of rounded pumice and lava; percentages and sizes of the latter vary significantly according to the degree of erosion, transfer distance, topographic location etc.

Its structure is generally massive. Various sections of mud flows and debris flows are observed. Calcs of tree branches carried away and entrapped by mud flows have been found in very few sites. Elsewhere (mainly in the Avlaki area), the formation presents significant thickness (up to 40m.) and marked layering.

Paleosoil is developed on the ceiling of the formation; its maximum thickness is 1m. Such paleosoil comprises a characteristic separating layer between Lower and Upper pumice at the northern slopes of the volcano.