Tuesday, September 29, 2015

née du feu et de la glace

Canoe Island, née du feu et de la glace. The San Juan's location along a major fault line, subduction zone and site of ancient continental ice sheets, makes for some exciting and complex geological history. French speaking naturalists of the late 18th and early 19th century played a large role in the development of glacial theory that scientists use today.

Glacial and interglacial cycles are an example of just such a French influenced theory. This theory attributes the advancement and retreat of continental ice sheets to long lasting cycles of higher or lower average global temperatures. The groundwork for this theory came from Saussure and Charpentier, french speaking naturalists, who studied glaciers of the Alps.

Horace Bénédict de Saussure was a Swiss-French physicist and alpine traveler. He was the third to climb Mont Blanc in Chamonix, France, 1787, and proposed that glaciers moved erratic rocks throughout the countryside. He provided extensive observations on the topography of snowy portions of the Alps as well as alpine botany.

Jean de Charpentier was a Swiss-German geologist who studied Swiss glaciers. He documented erratics between 1818 and his death in 1855. Many of the erratics were found in places that current glaciers didn’t extend to. He believed that the glaciers used to be much bigger but wasn’t sure how. The eventual theory of glacial and interglacial cycles, first proposed by Karl Schimper and published by Louis Agassiz, accounts for this conundrum.

This theory is also how geologists studying the San Juans account for the many glacial landscape features found in the islands. How neat that the understanding of San Juan Island geology and glacial history has such an accent français!

Ice Sheets that covered much of North America, including the San Juan Islands, during glacial periods over the last 2.6 million years