Main Features
The island has moderately gentle topography, with relief of 145 m from 26 m (lake level) to 171 m a.s.l. at the radio tower (STRI 1987). Most slopes are gentler than 10o.
The topography is clearly related to the geological structure (Figure 2.3). To the west of the main Lutz-Drayton fault system the topography is cuesta-like, with the top NE corner of the dipslope around the radio tower, and a general gentle dip to the SW. The dip slope appears to be structurally controlled by the upper surfaces of andesite flow sheets. The dip slope has a large upper plateau surface that is fringed by slight steps to the south and west, with gently graded treads separated by low, slightly steeper and moderately bouldery risers. There is a further low bouldery riser down to the lowland on the Caimito marine sediments in the west and southwest of the island. The streams in both the andesite dip slope and western and southwestern marine sedimentary terrain downslope are only slightly incised.
The scarp element of the main cuesta consists of the steeper terrain on the Bohio Formation in the north and northwest of the island, and also along the fault system down the centre. The topography consists of spurs running down from the edges of the andesite dip slope. The spurs run down to the lake edge in the north. In the east, they run down to the lower terrain on the Caimito marine volcanic deposits. The eastern scarp swings round to SW in the south and there are low but steep-sided NW-SE and N-S aligned spurs crossing the eastern end of the Wetmore trail. This part of the Bohio terrain has the most active dissection on the island, with steep, root-stepped slopes. The valleys are backsapping rapidly, partly by piping and roof collapse, and this has left live roots suspended across fresh gullies. The geomorphic activity in this area may result from localised recent minor tectonic tilting.
The Caimito volcanic landscape to the east of the Lutz-Drayton fault system also has a slight cuesta -like form, but this is lower and even less pronounced than in the west, and the dip is due south. The scarp of this cuesta forms the slopes down towards the lake from the northern side of the eastern Thomas Barbour trail. There is a discontinuous midslope ledge, on which pale heavy clays occur (see Barbour soil class in Chapter 5). The gentle but bouldery dipslope flattens out to the south, on the Harvard peninsula.
Age of surfaces and paleoclimates
The central andesite plateau can be interpreted as a structurally controlled cuesta dipslope, with its elevation and form determined by the configuration of the lava flows. Alternatively it may be an old erosion surface, which has been slightly tilted by minor tectonic uplift after pediplanation (Hare & Gardner, 1985; Johnsson & Stallard, 1989).
However formed, the surface appears to be old judging from the depth of sola and advanced weathering of the clasts in the subsoil stone sheets.These may be inherited residual corestones in the andesite, or result from size sorting of regolith materials during lateral colluviation or mainly vertical size sorting by biopedoturbation. Whichever process or combination thereof emplaced them, the coarse stones the plateau sheets must have been harder at the time of deposition than they are now. Their softening and rubefaction occurred post-emplacement, and imply a substantial period of regolith stability.
Buried stone sheets are sometimes interpreted as indicators of palaeoclimates that had less but more intense rainfall, sparser vegetation, and more active colluviation than at present. However, active colluviation in the past need not necessarily be of palaeoclimatic origin. Pollen and phytolith records indicate that there was more or less continuous forest cover in the whole of the Canal Basin of Panama throughout the Late Quaternary (Bush & Colinvaux, 1990; Colinvaux, 1996), suggesting that climatic variation on BCI may not have been severe. Tectonic uplift, and the consequent rejuvenation of topographic dissection by the depression of local base levels, and reactivated colluviation are possible non-palaeoclimatic explanations of these sheets.