... our edible earth

Alan Lee What is it you are actually walking on when you walk through the forests of south eastern Peru, which to confuse you here I will also call the forests of western Amazonia? All those plants and trees are of course growing on soil. Where does it all come from? What’s it all about and most importantly, for me, what does it mean for the birds that are eating it?

Geologically, the area constitutes the Andean foreland zone, which includes emerging arches and subsiding basins - basically ripples in the earth’s crust caused by tectonic activity resulting from the Pacific plate pushing into South America – which has resulted in the formation of the Andes – and the terrible Earthquake experienced earlier this year off the coast of Peru. The Andean foreland basin in Peru belongs to the world’s largest Cenozoic (think dinosaurs) sedimentary basin and consists of 3 different “intraforeland” basins - the Pastaza-Maranon, Ucayali and the Madre de Dios basins. What you are standing on at any of the Rainforest Expeditions lodges is sand and soil (fluvial sediments) deposited in a complex array of present and previous river floodplains, deposited in the basins mentioned above. The oldest, unflooded sediments are called terra firme soils that are severely weathered and leached due to the humid and warm tropical climate. However, the floodplain soils of the main river channels usually have relatively high fertility and some parts of the Peruvian Amazon have been suggested to be “rich” with regard to mineral nutrients. Fluvial sediments of suspension rich rivers, such as the Tambopata, of Andean origin are rich in their extractable mineral nutrient content, especially calcium. However, rivers that emerge from the northern Amazon i.e. Venezuela and Suriname are black-water streams low in mineral content as they originate from a different geological background – the Guianan Shield. So maybe maybe that is why the soils there are not as attractive to birds to eat as a result and so we don’t get clay licks there like we do in Peru. So what is all this old mud sitting on? Believe it or not, the entire western Amazon basin used to be a giant inland lake called Lake Pebas, so large that it was subject to tidal influences. And if you are going to visit the big clay lick at TRC, if we are to believe what we are told, then that big red cliff was actually formed by intertidal deposits – i.e. it was the shoreline of that great lake. With tectonic upliftment though, that lake disappeared millions of years ago. So what is interesting for me is that the parrots and macaws at TRC are eating ancient sea bed, full of foraminifera and other tidal creatures, while the parrots and macaws at Posada and Refugio are eating old river deposits. So maybe the birds are just making a trip to that ancient beach – after all, the seaside has always been a popular recreational area and maybe that even explains why parrots near the sea don’t have to visit clay licks? So, next time you are at the clay lick and waiting for the birds to arrive, give a thought to the complex history of the bank of earth in front of you – a grand fossil beach that could be millions of years old at TRC, and maybe the bottom of an old oxbow lake at Posada and Refugio.