The isthmus of Panama probably became a land bridge between 2 million and 3 million years ago. This recent uplift partly explains its rugged topography. In central Panama, near the Panama Canal, rocks can be crudely classified as either dense, relatively impermeable volcanics, or as porous, chemically unstable sedimentary rocks and volcanic mudflow deposits. Weathering of the dense volcanics, which tends to be shallow, sometimes penetrating only a few centimeters into the rocks along fractures, yields soils rich in clay on gentle slopes and shallow, rocky soils on steep slopes. On the other hand, weathering tends to penetrate several meters into the more porous rocks and yields clay-rich soils on both steep and gentle slopes.

Four principal horizons resulting from rock weathering and soil formation. In this case, a basaltic lava flow intruded by a dike of more dense basalt has been extensively weathered to form a weathered bedrock zone where much of the original rock appearance and strength are pre­served. Weathering causes isolation of roughly spherical regions of much less weathered rock called corestone. Continued weathering produces the saprolite which has a soil-like texture but retains the general structure and some of the original rock strength. The amount of corestone, in the weathered material can be used to define the difference between saprolite and weathered rock. Disruption of the saprolite by physical mixing produces the soil.

Soil properties appear to vary systematically with the type of the underlying rock and with the topographic setting. On the broad, flat top of the island, which is underlain by dense basalt, erosion rates are very low. Root exposure is minimal and the soils, cracking red clays with a brown, organically enriched surface horizon less than 5 cm thick, are over a meter deep. In contrast, on the narrow, gently sloping (9°) ridges such as between Miller and Wheeler trails, which are also underlain by basalt, soils are yellow-brown, less than 50 cm thick, and end abruptly in boulder-sized chunks of fresh bedrock with thin weathering rinds. The soils are also rich in clay but contain occasional fresh gravel-sized rocks. Along parts of the narrow ridge between the two main branches of Lutz Creek, basalt reaches the surface and soil is absent. On the steep side-slopes, where gradients can exceed 20°, the soil is of coarse texture, with much material of the size of fine gravel, and less than 50 cm thick. On 50° slopes, weathered basalt reaches the surface, and the soil forms a discontinuous cover trapped in places by the roots of trees.

Siltstones rich in volcanic glass underlie most of Lutz catchment and much of the island. Their soils are yellow-brown silty clays. The thicker soils show cracks up to 2 cm wide and at least 10 cm deep during the dry season. Typically, these soils have a surface horizon 5 cm thick, enriched with organic material. In general, these soils also tend to be thin, their thickness varying systematically with steepness of slope, as occurs elsewhere. On narrow ridge tops with gradients of less than 5%, soils may be less than 20 cm thick. They are 30-50 cm thick on slopes of 20°-30°, but on steeper slopes of 40° the soils are only 10 cm thick. There appears to be saprolite below the soil even on the steeper slopes, perhaps because the parent siltstones are porous and susceptible to chemical weathering. The soils formed on the conglomerate have not been studied sufficiently to characterize, although our one sample appeared similar to the soils formed on siltstones.

On Barro Colorado, plant roots are concentrated in the upper 10 to 20 cm of the soil. Major roots may also penetrate deep into saprolite and extensively weathered bedrock, in areas where these layers are present. On recently fallen trees we have observed roots that extended 80 cm below the surface into the underlying saprolite. Roots are important hydrologically, not only because they increase the permeability of the soil, but also because when exposed they criss-cross the soil surface. These exposed roots act as small dams that retard overland flow and substantially reduce its velocity and erosive capabilities. Rainsplash is an important erosive agent in the tropical forest, and the ponding of water behind root dams decreases the frequency of impact of raindrops on the soil surface.

Barro Colorado has experienced varying degrees of human disturbance. Grinding stones (metates) found in the forest suggest that the hilltop was once subject to shifting agriculture. We have also found charred wood in the soils near the Armour trail #8, by the "big trees"