New Hampshire Geology New Hampshire Bedrock Map

The Cambrian Period page 2
(540-490 million years ago)

Going Down?

trilobiteDuring the Cambrian Period, the early North American and European tectonic plates were colliding.   These early plates were called cratons.  A craton is the original building block of a tectonic plate that gets added on to or has sections separated from it later in Earth's geologic history.

As the two plates approached each other, the edge of the North American craton started to slide under the European craton.  This is a process called subduction.

As subduction occurs, the diving plate plunges into the Earth's upper mantle, where temperatures are much hotter.  The higher temperatures melt the subducted plate, creating magma.  Hot magma is less dense than the cooler tectonic plate and it starts to rise to the surface.   Warmer things like to rise and cooler things like to fall. This is called convection.  When magma reaches the surface in an eruption, geologists call this molten rock lava.

Going Up?

As the North American plate subducted under the European plate, a chain of volcanic islands formed from the rising magma.  The islands formed in an arc in the middle of the fast closing ocean between the two land masses.  This tropical volcanic island arc would closely resemble the Caribbean Islands of today.

The Big Squeeze

Even while this volcanic arc of islands was forming, the tectonic plates continued to plow towards each other.  Eventually, the ocean would close and the islands would be crushed between the plates.  These 'pasted-on' smaller land masses are known as exotic terranes.  Exotic means 'from somewhere else'.  Almost all of New Hampshire is composed of exotic terranes. In fact, the only 'native' rock is from the volcanic rock created in the Cretaceous period. The eroded remnants of these once tropical volcanic islands are among the first parts of New Hampshire to appear on North America.  

These islands still exist as the hills of the Bronson Hill Complex.  This chain of hills stretches from the Connecticut Coast, up the eastern side of the Connecticut River Valley to about Littleton, NH where it curves off to the east and crosses over to the Maine border and beyond.

Elsewhere on the North American craton, this same collision pushed up the Taconic Mountains and the Early Appalachian Mountains.

The Bronson Hills

Looking at this Bedrock Geology Map of New Hampshire, developed by the New Hampshire Geological Survey, the Bronson Hills can be seen as a narrow band of  purple splotches and streaks visible running from the Connecticut River Valley in the southwestern part of the state, running up to Littleton and hooking north and east into Maine.