18 May 2008
In an earlier post, I shared some pictures and interpretations from the first day of a two-day whirlwind tour of New York geology. Now without further ado is day 2!
Our first stop on Day two was Stark’s Knob, a tall outcrop of basaltic rock jutting out of the middle of an otherwise unremarkable bit of New York countryside. The basalt is in the form of rounded blobs or “pillows”, indicating that it was extruded from an underwater vent. The pillows at Stark’s Knob have no bubbles in them, which means that they were probably formed deep underwater where the pressure was so high that the gas could not expend. How did a pile of volcanic rock from the bottom of the ocean end up in the middle of the New York countryside? It turns out this outcrop was scraped off of the ocean bottom and carried on top of the surrounding crust just like the crud that accumulates on top of a spatula used to scrape a dirty pan.
Stark’s Knob is also famous for its decisive role in the Battle of Saratoga in the American revolution. The hill was fortified by General John Stark and prevented the British army from escaping north to Ticonderoga and Canada.
Our next stop was at Lester Park near Saratoga Springs. There, by the side of the road, there are some large slabs of exposed limestone with bizarre concentric rings embedded in them. These rings are fossils of some of the oldest forms of life on Earth; stromatolites. Stromatolites are colonies of cyanobacteria (blue-green algae), and the oldest fossils date back 2.7 billion years. The ones that we saw were “only” about 490 million years old, but it was cool to see them nonetheless.
Our next stop was one of my favorites. We pulled off to the side of a highway to look at a remarkable road-cut: the rocks were distinctly different from the dull-grey sedimentary rocks that are so common in New York. These rocks were sprinkled with garnets so that they looked like petrified raspberry muffins, and they sparkled with flecks of mica and chips of quartz. The presence of garnet and mica, along with the undulating folds in the rock’s texture told us that these were metamorphic rocks from deep in the crust. A billion years ago, all of the continents merged together to form a single landmass called the Greenville super-continent. (you may have heard of the Pangaea super-continent, but that was a mere 250 million years ago) These pretty garnet studded rocks that we stopped to look at along the side of a New York highway were once 35 kilometers beneath the Earth’s surface, forming the roots of a mountain range bigger than the Himalayas.
The final stop in our tour of New York geology was a location along the Mohawk river. This river once drained the enormous lake formed by the receding glaciers of the last ice age, and we visited a location where an ancient waterfall once stood. This fall would have dwarfed Niagara, and it left its mark on the solid bedrock, carving strange cylindrical holes called potholes. These are formed when sediment-laden water swirls with tremendous force and wears away the rock. Words don’t do these justice, and neither do my pictures, but here are a couple to give you an idea:
Thus concluded our tour of New York geology. It’s amazing what diversity you can find within one state if you know what to look for. If you are interested, there is a great series of books on roadside geology that will show you all of the most interesting stops in your own state. (if you live outside the US, there are probably similar books available, but I don’t know much about them…)