The Ravines of Fray Bentos
The yellowish brown ravines that predominate on the coasts of the Uruguay River, become very defined and observable at the height of the city of Fray Bentos. It is "rough" or dust contributed by thousands of years of wind agents (winds). These powders of very fine granulometry, are called geologically "loess" and the territory of the Argentine Republic is where they predominate, although there are also in China and some parts of Europe, where they originated due to transport by the winds during very long periods of weather.
These ravines have given a very special feature to these coasts, to such an extent that in the European cartography of the mid-1600s, the one of "ravines of Fray Bentos" is already used as toponymic. They are heights of more than 15 meters that allows to observe them with clarity to all those who arrive by waterway from the south.
The presence of these ravines has turned them into true "strong points" where the fluvial dynamics have created inlets and semicircular entrances between them. As much as the ledges of the ravines are coincident in good depths of the river, the inlets have been used as sheltered and protected sites for the boats. This has been evident in various sites used by man for the installation of towns or industrial establishments that needed ports. Such are the cases of the city of Fray Bentos itself, the natural port where the Liebig's Establishment was installed and the outgoing where the natural port of the M'Bopicuá establishment was located.
THE FORMATION OF THE RAVINES.
As we extract from the publication GEOTECHNICAL CHARACTERISTICS OF THE LOESSES OF ARGENTINA, by the authors Ricardo J. Rocca, Emilio R. Redolfi and Roberto E. Terzariol (you can read and remove on the page "On the ground Fray Bentos), the beginning of the loessoid sedimentation has been related to a phase the Andean orogeny of the Late Miocene (about 10 Ma, millions of years) that by modifying the Andes mountain range, generated a barrier to the humid winds of the Pacific, retaining moisture in its Chilean western slope These are the old deposits found in the deep subsoil.The most recent deposits correspond to the Late Pleistocene / Holocene and constitute the primary loess.
Original sources of loess and means of transport (Zárate, 2003). It was Teruggi (1957) who gave the first understandable explanation with a scheme that is currently considered over simplified. In the case of the Pampean type, particles of volcanic origin originated in the Central Andes and Patagonia. They were transported by erosive agents such as glaciations and fluvial to the north of Patagonia. From there the particles were transported by the action of the winds from the S - SW towards the center of the country, in a period dated by thermoluminescence of the materials, of about 30,000 years.
The sand particles were dragged by saltation covering a sandy plain in a dry and desert environment, while the same winds and rains produced the sedimentation of dust particles in suspension. Said material, covering the surface, a loess belt was formed behind the sea of sand of 2,000 km in length and 250 to 300 km in width (Krohling 1999). "This sedimentation is what can be observed in large large-scale deposits of Toscas, for example, in the areas of Paraná (Río Paraná, Argentina) and Fray Bentos (Uruguay River).