dissertation

Dissertation sur la nature morte

Enter the email address you signed up with and we’ll email you a reset link. More information on many of these authors, and on other books of theirs, is contained dissertation sur la nature morte their individual webpages, which can be found by searching on their names at the top-level of the Author and Book Info . Sailing stones, also known as sliding rocks, walking rocks, rolling stones, and moving rocks, are a geological phenomenon where rocks move and inscribe long tracks along a smooth valley floor without human or animal intervention. Trails of sliding rocks have been observed and studied in various locations, including Little Bonnie Claire Playa in Nevada, and most famously at Racetrack Playa, Death Valley National Park, California, where the number and length of tracks are notable.

The Racetrack’s stones speckle the playa floor, predominantly in the southern portion. Stones with rough bottoms leave straight striated tracks, while those with smooth bottoms tend to wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone’s wake. Trails differ in both direction and length. Rocks that start next to each other may travel parallel for a time, before one abruptly changes direction to the left, right, or even back to the direction from which it came. At Racetrack Playa, these tracks have been studied since the early 1900s, yet the origins of stone movement were not confirmed and remained the subject of research for which several hypotheses existed.

The first documented account of the sliding rock phenomenon dates to 1915, when a prospector named Joseph Crook from Fallon, Nevada, visited the Racetrack Playa site. Naturalists from the National Park Service later wrote more detailed descriptions and Life magazine featured a set of photographs from the Racetrack. In 1952, a National Park Service Ranger named Louis G. Kirk recorded detailed observations of furrow length, width, and general course.

Bob Sharp and Dwight Carey started a Racetrack stone movement monitoring program in May 1972. Eventually, 30 stones with fresh tracks were labeled and stakes were used to mark their locations. Each stone was given a name and changes in the stones’ positions were recorded over a seven-year period. Sharp and Carey also tested the ice floe hypothesis by corralling selected stones. A panorama of the Milky Way with the tracks of sailing stones below: Notice the stone on the right side. Two of the next six monitored winters also had multiple stones move.

No stones were confirmed to have moved in the summer, and in some winters, none or only a few stones moved. In the end, all but two of the monitored stones moved during the seven-year study. Karen did not move during the monitoring period. However, Karen disappeared sometime before May 1994, possibly during the unusually wet winter of 1992 to 1993. Karen was rediscovered by San Jose geologist Paula Messina in 1996. Professor John Reid led six research students from Hampshire College and the University of Massachusetts Amherst in a follow-up study in 1995. Physical evidence included swaths of lineated areas that could only have been created by moving thin sheets of ice.