Sunday, December 20, 2009

New Marble Outcrop in Brookfield, CT

Generally speaking, I'm not a fan of man-made landscape alteration, but fresh highway roadcuts provide a great unweathered view of the local geology.
A few weeks ago, after years of proposals followed by years of construction, the northern extension of "Super 7" (US route 7) north of Danbury CT finally opened - and the exposures of the Stockbridge Marble along the road are spectacular!  The Stockbridge Marble was formed as the rocks of the carbonate bank (limestones deposited off the coast) of ancestral North America were metamorphosed during the Taconic Orogeny ( see blog on 12/2/2009 and the diagram below).

The rocks were further deformed, and actually overtured, in the subsequent Acadian Orogeny. The extent of those deformations are evident in the complex folds pictured here.
Carbonate rocks, including marbles, generally weather faster than silicate rocks, and in SW Connecticut and SE New York many of the regional valleys are underlain by less resistant marbles.  Along route 7 between Danbury and New Milford the Stockbridge Marble underlies the Still River valley, and is bounded by harder rocks on either side of the valley.  The same rock underlies the Saw Mill River valley in New York  where is known as the Inwood Marble (there's a "Marble Avenue" exit on the Saw Mill River Parkway in Pleasantville/Thornwood, and the Shop Rite plaza in Thornwood is built on an old marble quarry there - great outcrops of snowy white "snowflake" marble are still visible on the quarry walls behind the stores, but they're weathering fast!).
Riders of the Hudson Metro North commuter rail can get a good look at the Inwood Marble at the Marbledale stop on the way into Manhattan.

Tuesday, December 8, 2009

Earliest Sunset of the Year

Well, we've made it.  Tonight is the earliest sunset of the year!
The daylight period is still getting shorter (most folks know that shortest day is the Winter Solstice around December 21), but not a lot of people can explain tonight's early sunset.  It turns out that the rate at which the Sun travels across the sky is not constant - the tilt of Earth's axis and its elliptical orbit conspire to push the Sun ahead of our clocks, and then slow it down again, twice every year.  Astronomers call the difference between time told by the Sun (apparent solar time) and clock time (mean solar time) the "equation of time".(If you're interested, you can get the sunrise and sunset times for your location at the US Naval Observatory site.)
The chart on the left above, called the analemma, combines the equation of time with the position of the Sun relative to the equator.  Click it for a larger view, and notice that through most of the fall the Sun has been running ahead of the clock, but in December it began to slow dramatically.
It's the Sun slowing down relative to the clock that's moving the daylight period later into the day even as the days get shorter!
This photo composite was made by Tom Matheson over the course of a year, snapping a picture of the Sun at exactly 8 AM (by the clock) each day.  Here is a labeled image of  Tom's photo.
(This blog isan edited  re-post from December 2008)

Wednesday, December 2, 2009

Rocks on the Shore of Lake Champlain Tell an Interesting Story

An early May paddle on Lake Champlain turned into an interesting geology field trip (for me at least ;-) ) when we passed this rocky shoreline north of Burlington, VT.  The rocks here record a significant event in the formation of North America.
To understand what happened here, imagine a deck of cards spread out on a table. Now imagine that you use your arms to bring the cards together into a pile. As the cards slide together some will end up on top of others, and the mass of cards will become shorter in the horizontal direction, but thicker in the vertical direction. In a similar way, colliding crustal plates (“drifting continents”) produce thick masses of earth’s crust pinched between them.  Around 440 million years ago, eastern North America was deformed as it collided with volcanic islands to the east as the ocean between North America and Europe was closing (and as the super continent of Pangea was assembling).  This event, called the Taconic Orogeny, resulted in the rocks of western New England piling up and forming the Taconic Mts. (which have been reduced from their former grandeur by 400 million years of erosion!)
So what's happening in this photo?

Essentially you're looking at the boundary between 2 of the cards you imagined above.  The rocks in the top half of the photo have been thrust westward (to the left) up and over the rocks at the bottom of the image along a "low angle thrust fault" (traced with a red line).  It's hard to judge just how far the top rocks have moved relative to the bottom rocks right here, but along a major thrust fault east of here, the displacement is estimated to be on the order of 50 miles! (See this Earth Science Picture of the Day).  And you can see that the rocks themselves have been squeezed, too - notice that the shortening and thickening is evident on a very small scale in the deformation of the light colored vein at "A".
You can see more pics of this outcrop, and pics of the entire paddling adventure if you have nothing better to do.
And here's a pretty nice cartoon of the Taconic Orogeny from oldest at the top to present day at the bottom.