TROPICAL RAINFORESTS: The Understory
Where Are the Rocks?The Amazon basin is formed by the depression of pre-Cambrian bedrock, and is surrounded on three sides by the Andes to the West, and the Brazilian and Guyana shields to the South and North, respectively. The earth of this basin is made up of sedimentary layers of soft clay and sand deposits over 13,000 feet (4,000 m) thick. Therefore, in the Central Amazon basin virtually no rocks are found since they either would have to stay intact on the long journey from the Andes or lie on the river bottom. Since rocks are useful for sharpening machetes along with other tasks, rocks are valuable in the rainforest. In some areas, a person's wealth is determined by the number of rocks the family owns. Barges still carry rocks to parts of the Amazon to sell and trade with native peoples.
Another result from the instability of the clay soils of the Amazon Basin and other tropical regions, is difficulty in constructing roads. Some blame the failure of the Trans-Amazonian highway—sections of which sank into the ground—on the characteristics of these soils.
Characterization of the Amazon Basin in terms of the soil's physical qualities and formations
Continued: Rainforest floor