The studies whose main results are reported in the present paper were carried out in November-December 2000 and in January 2001 at the request of Ponta do Oeste, S.A., aiming at the stabilization of the rocks that build up the cliff, in order to consider and propose the adequate safety measures required by the construction works of commercial infrastructures and other support facilities to the Marina da Ponta do Sol, planned to be implemented between Ribeira da Tabúa and Ribeira da Caixa. In a first stage, the works being carried out included observations taken from the sea of the cliff, looking at the general lithology and morphology, main joint and fault systems, hydrological features, as well as vegetation cover type and density. In a second stage, on land and close to the cliff, detailed mapping of the relevant features being observed, such as, lithological types (lapilli tuff layers and basalt flows), recurrence patterns of both lithologies, rock alteration and stability, rock layers thickness estimation and water circulation patterns.. The cliff that exhibits an attitude E-W, approximately, between Tabúa and Lugar de Baixo, could be segmented in five sections distinguished by their main characteristics, and referenced as ZI, Z2, Z3, Z4, e Z5. On the other hand, each one of these sections could be subdivided in two major sub-zones, an upper one and a lower one, separated by a narrow platform or landing found at half height of the cliff front, that is common to all zones, despite being discontinuous in the sites of transition from zone to zone. Vegetation covers do occur particularly in this major narrow platform referred to, but occurs as well in other sub-parallel much more narrow platforms or landings developed on the cliff front, from its base up to its top. From the results of the studies being carried out some proposals and recommendations were put forward to be taken into account in the project preparation and for the construction works required for the cliff stabilization.
The studied cliff is located between the mouth of Ribeira da Tabúa, Concelho de Ribeira Brava, and the mouth of Ribeira da Caixa, Lugar de Baixo, Concelho da Ponta do Sol (figure 1). In places, the cliff displays altitudes slightly higher than 100m above sea level and the general inclination of its slope is about 75°, although in some segments of the cliff the cliff's slope is nearly vertical. Geologically the cliff is included in the peripherical volcanic complex, β2, of post-Miocene age, that belongs to the second phase of the Madeira's Island formation (Zbyszewski et al, 1975). In the complex referred to remnants of volcanic eruption sites do occur, that can be observed both in the eastern and western sectors of the cliff, at the cliff top.
In morphological terms, the geological formations that build up the long cliff under study, develop discontinuous sub-vertical to vertical slopes separated by narrow landings (figure 2).