martedì 12 giugno 2012

The Wave - L'Onda

Le chiamano The Wave, l'onda, ma con i cavalloni marini queste curve di arenaria del Paria Canyon, al confine tra Arizona e Utah (USA), non c'entrano nulla. Secondo gli esperti all'incirca 190 milioni di anni fa, quelle che vedete erano semplici "dune" di sabbia, che nel corso dei secoli si sarebbero indurite e compattate fino a formare una suggestiva distesa di "onde" dall'aspetto stratificato. Gli agenti erosivi hanno poi completato l'opera dando origine a un paesaggio che richiama ogni anno centinaia di fotografi da tutto il mondo. Solo in qualche caso questi finti flutti si riempiono d'acqua vera: dopo i temporali, quando nelle cavità della roccia si formano delle piccole pozze.

The Wave is a sandstone rock formation located in the United States of America near the Arizona and Utah border on the slopes of the Coyote Buttes, in the Paria Canyon-Vermilion Cliffs Wilderness, on the Colorado Plateau. It is famous among hikers and photographers for its colorful, undulating forms, and the rugged, trackless hike required to reach it.
“The Wave” consists of intersecting U-shaped troughs that have been eroded into Navajo Sandstone of Jurassic age. The two major troughs, which comprise this rock formation, are 19 meters wide by 36 meters long and 2 meters wide by 16 meters long. Initially, infrequent runoff eroded these troughs along joints within the Navajo Sandstone. After their formation, the drainage basin, which fed rainwater to these troughs, shrank to the point that that the runoff became insufficient to contribute to the cutting of these troughs. As a result, the troughs are now almost exclusively eroded by wind as indicated by the orientation of erosional steps and risers cut into the sandstone along their steep walls. These erosional steps and risers are oriented relative to predominate direction of the wind as it is now naturally funneled into and through these troughs.

The Wave exposes large, eolian sets of cross-bedded sandstone composed of rhythmic and cyclic alternating grainflow and windripple laminae. The rhythmic and cyclic alternating laminae represent periodic changes in the prevailing winds during the Jurassic as huge sand dunes migrated across a sandy desert. The thin ridges and ribbing seen within The Wave are the result of the differential erosion of rhythmic and cyclic alternating grainflow and windripple laminae within the Navajo Sandstone. These laminae have differing resistance to erosion as they have been differentially cemented according to variations in the grain size of the sand composing them. The soft sandstone, including the ridges and ribbing, of The Wave is fragile. As a result, a person needs to walk carefully to not break the small ridges.
In places, The Wave exposes deformed laminae within the Navajo Sandstone. These laminae were deformed prior to the lithification of the sand to form sandstone. Judging from their physical characteristics, this deformation likely represents the trampling and churning of these sands by dinosaurs right after their deposition. Dinosaur tracks and the fossil borrows of desert-dwelling arthropods, such as beetles and other insects, have been found within the Navajo Sandstone within the North Coyote Buttes Wilderness Area.

The Wave is here

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