Sunday, June 21, 2015

Stone Church in Dover Plains

My wife and kids don't often hike with me but today was Father's Day so we went off to a place called Stone Church in Dover Plains - an easy walk to a very cool place.

So what is Stone Church?  It's a natural rock formation near Dover Plains in eastern Dutchess County where a stream has cut out and eroded a neat opening in a cliff which evokes the arched window of a Gothic cathedral.

The trailhead is easy to miss.  There's a historical marker off Route 22 in the middle of Dover Plains showing the location (there's no direct parking, check out this brochure for parking locations).


Interested in Sassacus and the Pequot War?  Read more about it here.

Walk up what looks like a driveway...


And you'll be at the start of the trail...


Beautiful granite steps lead down to a mowed path, lined with young maple trees, through a meadow...


Lots of milkweed, a beautiful wild plant I have an affinity for...


The path then winds through some woods (with mature maples lining the trail)...


A nice interpretive sign...


And I learned something I didn't know.  Hudson River School artist Asher Durand once sketched Stone Church (c. 1847)...


The trail continues along the creek...


Around a corner is the opening - the Stone Church...


This is why it's not a good idea to go for a hike on a Sunday afternoon.  There were many people here (too many for me to properly enjoy it - I'll have to return on a week day sometime soon).  My wife at the entrance (other people are strangers)...


The view inside...


Really cool place.  I will definitely have to go back and explore more when there are less people around and when I'm prepared to wade into the water to check out the back of the cave (there's supposedly a 30-foot waterfall that we heard, but didn't see).

The rock, by the way, is a garnet mica schist (look closely at the rocks on the trail and you'll see little red lumps - those are garnets).  This relatively soft metamorphic rock was eroded when the stream, once likely flowing over the cliff as a waterfall, found it's way into a crevice and enlarged it over time to the feature seen today.

Well worth a visit (just not when I'm there!).

Sunday, May 31, 2015

Manhattanhenge

So yesterday (May 30), and again on July 13 of this year (2015), the setting Sun will line up with Manhattan's roughly east-west street grid - an event dubbed Manhattanhenge (making the connection, of course, to Stonehenge).

Here's a picture I saw on my newsfeed this morning.


So why does this occur when it does?

As the year progresses, the rising and setting position of the Sun moves as well.  If you went out every morning at sunrise, and plotted the position of the Sun against the horizon each day, you would see the following.


Obviously, Manhattan's streets aren't aligned in a true east-west fashion or else Manhattanhenge would occur only on the equinoxes (March 20 and September 23, 2015) when the Sun is rising due east and setting due west.

The summer solstice this year is June 21.  The two Manhattanhenge dates (May 30 and July 13) occur 22 days before and 22 days after the solstice.  So what is the direction of the setting Sun on those dates?  These can be obtained from the U.S Naval Observatory Astronomical Data Services website (a great website for observational astronomy nerds like me, by the way).

For Manhattan, the azimuth (compass direction in 000°-360° where 000° is north, 090° is east, 180° is south, and 270° is west) direction of  sunset is 300° on both of these dates.


Let's take a typical cross-town street - the famous 42nd Street.  What is its western azimuth direction?  It's 300°, of course.


This works particularly well for Manhattan because of the clear view across the Hudson River toward New Jersey and the tall buildings framing the view.

It also brings up an interesting issue for archaeoastronomy - the study of astronomical alignments in archaeological structures (like Stonehenge). Manhattan's street grids were built according to the geographic alignment of the island and the Manhattanhenge effect is completely coincidental.  But what about ancient structures that appear to have alignments with the Sun, Moon, or other astronomical objects (prominent stars and constellations).


If you can find such alignments in ancient structures, were they intentional or coincidental?  That's not always an easy question to answer, I'll defer further discussion to the next post.

Thursday, May 28, 2015

Niagara Falls Erosion - Part II

In my previous post, Niagara Falls Erosion - Part I, I discussed the geologic setting of the Niagara Escarpment and how the average rate of erosion of the falls since the last Ice Age has been about 3 feet a year.  This is based on the total distance of movement of Niagara Falls from the Niagara Escarpment since the last Ice Age.


Today I'll look at this in a little more detail.  The first accurate survey of the exact position of the falls was done by the famous New York State geologist James Hall back in 1842.  In 1905, Grove Karl (G.K.) Gilbert, who worked for the United States Geological Survey (USGS), studied the rate of recession at the falls since Hall's measurements by resurveying the falls, and came up with an average rate of 5.3 feet/year (about 1.6 m/yr) - read his report here.

This rate, however, drastically slowed over the 20th century.  The building of huge hydroelectric power plants in both the U.S. and Canada means that anywhere from one-half (daytime) to two-thirds (nighttime) of the water in the river about Niagara Falls is diverted away for power generation.  Today, the average rate of erosion is estimated to be less than 1 foot per year (0.3 m/yr).

Here's a diagram showing erosion of the Canadian Horseshoe falls over the past few hundred years (both estimated and directly measured).  It's noticeably fast.


As you can see by looking at a map of the falls (or an aerial view), the reason there are two waterfalls currently is because the falls have intercepted Goat Island with the Canadian Horseshoe falls on one side and the American falls on the other.


Prior to 600-800 years ago (depending on the exact rates of erosion), there was only one waterfall on the Niagara River.  In a couple of thousand years, after erosion moves the waterfalls up past Goat Island, the two falls will again merge into one (really, what will happen is that the Horseshoe falls, which is eroding much faster will move past Goat Island leaving the slower American Falls behind, high and dry).

After about 15,000 years, the falls will have moved far enough upstream that it will lose the resistant caprock, only cutting through shale (Salina Shale).  This will turn it into a series of rapids and we'll lose the spectacular waterfall (well, we'll all be dead, but our descendants, assuming humanity is still around, will lose the falls).

Enjoy it while you can!

Tuesday, May 26, 2015

Niagara Falls Erosion - Part I

In my last post, I mentioned that the Army Corp of Engineers "turned off" the American side of Niagara Falls back in 1969 to deal with erosion issues.  So how fast do the falls erode?

Here's a cross-section of the geology at the falls.  There is one important (for our purposes) rock formation here - the Lockport Dolostone (also called Dolomite but I prefer Dolostone) which is underlain by a few different sandstone and shale units.

Dolostone is similar to limestone.  Limestone, a very common sedimentary rock, is made of the mineral calcite - CaCO3 (calcium carbonate).  Dolostone, on the other hand, is basically limestone with some magnesium substituting for the calcium in the formula (which creates the mineral dolomite) - CaMg(CO3)2.  This generally makes it a harder, more resistant rock than limestone.

Outcrop of Lockport Dolostone commonly seen in the area

The shales underneath are much softer and more easily eroded.  Here's a local example of similar differential erosion near Lockport, NY (Whirlpool Sandstone and Queenston Shale - see cross-section above).


Since the Lockport Dolostone is so hard and resistant, and is slightly tilted (dipping to the south), it doesn't weather as much as the surrounding shales and erodes to form a prominent cliff called the Niagara Escarpment.


The Niagara Escarpment runs over a large area from western New York, through Ontario, along the Upper Peninsula of Michigan, and then into Wisconsin (red line in map below).


We're only concerned with the escarpment where it crosses the Niagara River between Ontario, Canada and New York State (note the dipping Lockport layer).


The Great Lakes and Niagara River formed after the most recent Ice Age (the sequence of their formation is a bit complicated so I'll avoid that here).  Basically, Niagara Falls formed at the Niagara Escarpment around 12,300 years ago and has been eroding upstream since then carving the Niagara Gorge through the Lockport Dolostone.  The Dolostone ledge forms the resistant caprock to the falls (without this resistant rock unit, the Niagara River would descend from Lake Erie to Lake Ontario in a more gentle slope with no major waterfalls).

The American Falls pouring over the Niagara Escarpment

Since Niagara Falls is currently around 11.4 km (7.1 miles for those of us in the U.S.) from the Niagara Escarpment, and took 12,300 years to erode that far, how fast is the average rate of erosion here?


(7.1 mi / 12,300 yr) = 5.8 x 10-4 mi/yr x (5,280 ft / 1 mi) = 3 ft/yr

Three feet every year - that's pretty fast!  The problem, of course, is that this is an average rate of erosion over 12,300 years.  Why might this be a problem?  How about an analogy...

If I got into my car and drove to Niagara Falls right now (350 mi), it might take me 6 hours.  A reasonable assumption is that I took the New York State Thruway and traveled at 58 mph (350 mi / 6 hr).  But maybe instead I drove at 70 mph for 5 hours and spent an hour leisurely eating dinner at a rest area.  Or maybe I drove at 90 mph for 1 hour, got pulled over for a ticket for a half hour, and then drove 58 mph the rest of the way paranoid about getting another ticket.  Average rates don't always tell us specifically what was going on during each moment of time.

So, what was going on during the past 12,300 years?  Was Niagara Falls eroding at a constant rate the whole time or has the rate varied?  We'll examine that in my next post.