Wild Life: Charlotte Rocks!
by Tate Bushell,
August 12, 2010, page 20.....

Look closely and you’ll find some of Charlotte’s history written in the rocks. It predates European settlement, the glaciers and even the existence of mammals. Some of these rocks sit stacked in two pillars at the entrance of Charlotte Park and Wildlife Refuge on Greenbush Road. The stories in these ancient texts tell of both their creation and their current influence on the landscape. Rocks are sometimes difficult to read so, here, I will translate.
 
The main characters are quartzite and limestone. These two common types of local bedrock were both formed in an ancient ocean, the Iapetus (i-AP-pe-tus), around 550 million years ago. Half a billion years ago the planet looked very different than it does today. Firstly, life was found only in water. Insects, some of the first terrestrial animals, didn’t evolve until 170 million years after the Charlotte rocks were formed. Also, tectonic plate arrangement was different, which created a unique set of continents and oceans – there was no Atlantic Ocean, and there was no North America. It’s hard to believe, but the quartzite and limestone rocks, in fact all of Vermont, sat underneath the warm and shallow Iapetus Ocean just below the Equator.

The specific requirements needed for these rocks to form tell us about the environment at the time. First let’s think about the quartzite, the rock with a pink hue. In shallow regions of Iapetus, sand accumulated and was compressed to make sandstone. This sandstone underwent another heating and compression process (metamorphosis), which turned it into quartzite. It is thought that some of this sand was periodically uncovered by water, in an environment such as a tidal flat. Perhaps this and other ancient environments resembled the sandy beaches and deltas of Lake Champlain today.

The limestone rock is evidence of a different aquatic environment. Deeper, but shallow enough to support light-dependent animal communities, these waters were teaming with shelled organisms; microscopic zooplankton were most plentiful, followed by clams, mussels and coral. As these animals died, their shells rained down to the sea floor. Over millions of years an estimated two miles of calcium-rich shells collected on the ocean shelf and, by the collective weight of new sediments over topping them, fused into limestone rock. Scattered among these broken bits of shell were intact clam shells that left impressions visible in the rocks today. A large limestone slab mounts each of the park’s pillars, and with a brief search you will uncover clam impressions half a billion years old!

Rocks vary in chemical composition and therefore have unique physical properties that influence our entire landscape. The grey limestone (CaCO3 or calcium carbonate) is easily eroded by water, while the pinkish clear quartzite (SiO2 or silicon dioxide) is very resistant. Under the influence of rain, wind or glaciers this differential erosion produces a variety of landform features. Examples are the sheer walls of limestone sliced by rushing water left overlooking the Winooski River and Mt. Philo’s quartzite cap, which will persist into future millennia.

Many of our plants and plant communities exist only where limestone is abundant. In water, CaCO3 breaks down into a calcium atom and a molecule called carbonate, both of which benefit the chemistry of life. Calcium, which is needed for plant growth, acts like a vitamin to promote the growth of plants sensitive to calcium deficiency. For example, our famous sugar maples find their competitive edge in forests that sit over nutrient-rich limestone. The carbonate is a natural acid neutralizer that creates a local environment suitable for plants that grow poorly in acidic conditions. As such, farmers traditionally apply lime (pulverized limestone) to better the soil where they grow crops.

This story is of earth recycling. The quartzite, made from sand, is now slowly breaking down, and will eventually return to the ocean as zillions of sand particles. The limestone started as invertebrate shells and is now slowly feeding nutrients to our forests and wetlands. These non-living pieces of our landscape have complex but direct relationships with the living, often on the molecular level. I marvel at the long journey that, say, a cluster of calcium molecules takes as it moves from a piece of limestone rock to the shell of a chicken egg. Knowing that my own chickens are, in part, made from the stuff of rocks reminds me of the role rocks play in the story of life.
   
Tate Bushell (tbushell@uvm.edu) is a student in the Field Naturalist Graduate Program at the University of Vermont.Charlotte Conservation Commission collaborates with talented area naturalists to provide WILD LIFE nature notes, helping residents understand and appreciate wild nature close to home. Coordinated by Linda Hamilton and Alicia Daniel. For more information about the Conservation Commission, see charlottevt.org.