New Hampshire Geology New Hampshire Bedrock Map

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.

Silurian New Hampshire

During the Silurian Period, New Hampshire continues its northward drift from its position near the South Pole in the Precambrian Period.  Since New Hampshire is located halfway between the Equator and the North Pole today, there was still a long way to go.  Fortunately, New Hampshire has had over 400 million years to accomplish this move!
(Click on the image for a closer look.)
Image Credit: Christopher Scotese;

The Recipe For Granite

Granite Recipe

(Click on the image for a closer look.)|
Diagram credit: Daniel E. Reidy

The Fitchburg Formation

NH Bedrock Map
On the Bedrock Geology Map of New Hampshire below, look for the metamorphic Fitchburg Formation (green area) and igneous rocks (red, near seacoast) formed during the Silurian Period.  Where is your town in relation to these rocks?

(Click on the image for a closer look.)|
Map credit: New Hampshire Geological Survey