Geology
The formation of the Amazon floodplain is linked to sea level fluctuations during the Pleistocene (~2 million years ago - 10,000 years ago). The Amazon basin has a very low elevation. More than a million sq km lies less than 100 m above sea level. During the Pleistocene, when sea level was low, incisions were made into the middle and lower Amazon River and its tributaries. When sea level was high, the outflow of the river and tributaries was blocked. When sea levels were low (during cold periods), the rivers eroded deep valleys into soft Cretaceous/Tertiary sediment, as well as sediment deposited by the river during previous periods. When sea level rose during interglacial warm periods, the valleys would be filled with sediment again.
Because of the high amount of fluvial activity in the Amazon, most of the substrate in the várzea is sediment, washed down from the eastern slopes of the Andes Mountains. This substrate is made up of alluvial and fluvial Holocene sediments (less than 10,000 years old). Substrate varies slightly depending on the particular várzea, but this is the general pattern. The fluvial sediments at each várzea are heavily influenced by seasonal and/or daily (tidal) flooding. See hydrology for more information about flooding.
Extensive geologic information is not easy to obtain for the várzea, or for the Amazon in general. This is due to the vastness of the Amazon Basin, as well as dense forest cover.
River Solimoes, near Tefe, Brazil
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