GEOLOGY OF NEARBY AREAS
Ricker Mills
Just south of the park at Ricker Mills some of the oldest rocks in the area are exposed in the railroad cut just north of the highway crossing. These rocks belong to a thick sequence of similar rocks which are collectively called the Waits River formation. Studies in other areas indicate that these rocks belong to the portion of geologic time called the Ordovician period which was more than 350 million years ago.
The Waits River formation represents a series of sediments which accumulated at the bottom of a shallow sea during Ordovician time. These sediments included both limy and sandy beds, and fossils may originally have been preserved in some of the beds. Sediments of other types later accumulated over the Waits River formation during a long period of geologic time, so that eventually the formation became deeply buried. (See [Figure 3].) The sea retreated and the rocks were subjected to high pressure and temperatures during a period of mountain-building. The rocks which had been sedimentary were folded and converted to metamorphic rocks by partial melting and recrystallization of the components. As a result the rocks became schists or marbles. Any fossils which may have been present were destroyed or badly altered in the process. This is unfortunate because valuable geologic information as the age of the rocks can be determined from the type of fossils present.
The rocks of the Waits River formation at Ricker Mills are dominantly mica schists with layers containing limy material. These are too impure to be considered marble but enough lime is present so that they react strongly with acid, a test for detecting the presence of lime. The schists principally contain quartz, biotite, calcite (lime) with lesser amounts of muscovite, feldspar and impurities. The rocks weather to dark colors; the gray limy beds are particularly susceptible to weathering and turn dark brown to black on the surface. When more lime is present, the rock weathers to a deep brown porous rock which resembles decayed wood. Some boulders of these altered limestones are found in the glacial deposits in the State Forest.
An additional factor which makes the rocks in the railroad cut at Ricker Mills look “messy” is the iron and manganese staining and the formation of mineral crusts on the surface of the rocks through the action of ground water. Rain water falling on the hills above passes through the soils, dissolving minerals, and precipitating them where the water seeps out and evaporates at the lower level of the railroad tracks.
The schists trend about N.80°W. and dip about 30° to the northeast. Along the length of the rock exposures it can be seen that this dip is not constant but varies from 10 to 30°. The variation in dip gives the schists a wavy appearance.
At two places along the railroad cut, the schist has been intruded by granite. As is shown in [Figure 6], which is a sketch of the rocks exposed on the east side of the railroad, the granite forms vertical dikes. As the schist ends abruptly at the contact of the granite, this indicates that the granite formed after the schist. The granite is nearly the same as the granite in Groton State Forest except that the mica is muscovite rather than biotite. For this reason the granite is lighter in color on fresh surfaces. In general the exterior is dark in color due to the staining of iron from weathering of the mineral pyrite, an iron sulfide, which occurs in small amounts in the granite.
Jerry Lund Mountain
On top of Jerry Lund Mountain occur other outcrops of the Waits River formation and granite. Their exact relationships cannot be seen easily because of the thick vegetation. The Waits River formation on Jerry Lund Mountain is composed principally of quartz mica schist.