![[Home]](/images/zh.gif)
HomePage - RecentChanges - Random - ^zhurnaly - RSSOn 3 May 2001 the Caltech Alumni Association sponsored a field trip to Great Falls National Park on the Potomac River upstream from Washington, DC. Retired geologist E-an Zen gave a brisk walking commentary:
Those logs? The flood of 1996 Left them as driftwood way up on that cliff. The valley was submerged. Just think, When 30 inches of snow melted, all Within a day or two, the runoff had To make it through this gorge. The crest Rose 70 feet or more and moved Those meter-sized boulders. Amazing, yes?
And potholes! Let me show you how the stream Made vortices against the side of this Formation. Alcoves here were cut so fast --- An inch a year --- that you could almost watch Them form if you were patient. Fingernails Grow just as rapidly. A big pothole Develops in a human life. That's quick!
If you come back here in 10,000 years You'll see something quite different. The land Is lowering about a millimeter Every century. This boulder that We're standing on came down the river in A flood about a quarter million years Ago; it moved some fifty miles. And see The patch of lichen here? It grows so slow That by its size you know it's centuries Old now. So we can tell this rock has not Been much disturbed; beryllium that's formed By cosmic rays which hit the surfaces Provide us with a measure of its age.
And note this mica schist: it must have been Pressed under 10 kilometers at most Of overburden, 900 Centigrade, To partly melt the crystals. That suggests A geothermal gradient far more Than anything that's normal nowadays In Appalachian land. We're looking at A half a billion years of history.
Kinda puts today's problems into a slightly larger perspective. (Many thanks also to Alumni Association Deputy Director Arlana Silver and to Dr. Dallas Peck, for organizing and running the expedition.)
TopicPoetry - TopicProfiles - TopicScience - TopicPersonalHistory
(correlates: BigLessons, UpcSatori, WhatCounts, ...)