Silurian Period
(443-417 million years ago)

New Hampshire Really Is New

During the Silurian Period, The North American Plate (Laurentia) and the European Plate (Baltica) continued to do their thing - crashing into each other.   They had been doing this since the Cambrian Period, tens of millions of years ago.  In the process, an ancient ocean - the Iapetus Ocean [eye-AP-it-us] - was closing on either side of a volcanic chain of islands.  These islands, and the ocean floor sediments between them and Laurentia, were being steadily pushed up against the shores of the early North American continent.  The islands and hardened sediments finally were attached through a process much like peanut butter being wiped off a knife onto a slice of bread.  This process is known as accretion to geologists.  In this way, these land pieces became the newest part of North America.   

But Wait, There's More

While Laurentia and Baltica were closing in on each other, a third smaller tectonic plate was moving in between them.  This smaller plate was known as Avalonia.  Geologists are not exactly sure where Avalonia came from, but their best guess is that it split from the supercontinent known as Gondwana.  Avalonia consisted of parts of Eastern New England, Nova Scotia, Ireland and the British Isles.  All of New Hampshire's land came in this process of accretion brought about by continental plates colliding with each other.

Of Falls And Formations

 During the Silurian Period, some metamorphic and igneous rocks were formed well below the surface.

Thanks to the passage of lots of time, and the weathering and erosion that goes with it, these rocks are now exposed as surface bedrock. 

The metamorphic Silurian rocks are indicated by the green coloring on the Bedrock Geologic Map of New Hampshire at right.  These rocks are known as the Fitchburg Formation, named for the town in Massachusetts where they were first identified.  These rocks consist of schist [shist], quartzite [kort-zite] and minor carbonate [kar-bo-nate] rocks.  These now very hard and erosion resistant rocks were once the muds (schist), layers of marine skeletons (quartzite) or chalky carbonates that form from naturally occuring chemical reactions in the ocean.  These rocks have a high iron content and appear rusty.  A great place to see some of these rocks are at Smalls Falls.  These falls are visible from I-89 North, between exits 6 and 7.

The igneous Silurian rocks were formed miles underground by a pocket of magma that never made it to the surface.   As a result, it cooled slowly allowing crystals of mica, feldspar and quartz to grow before it solidified.  If you find a rock with these three elements in it, then you have found granite!  As a general rule, the larger the crystal grains are, the longer it took for that magma to cool.