Haystack Mountain SP Rock Weathering & Topography EarthCache

Haystack Mountain State Park:  Rock weathering and topography.

Rocks that formed the eastern edge of the ancient North American continent 600 million years ago today underlie the western part of Connecticut.  Although they are generally ‘granitic’ in composition, they are composed of schist, marble, and gneiss, including amphibole gneiss.  This EarthCache will focus on the general weathering properties of these old rocks, in particular, comparing the rate of weathering of marble with that of granite.  To do this we will travel to Haystack Mountain State Park in Norfolk, CT. 

Purpose:  This EarthCache is published by the Connecticut Geological and Natural History Survey of the Department of Environmental Protection.  It is one in a series of EarthCache sites designed to promote an understanding of the geological and biological wealth of the State of Connecticut.

Supplies:  You will need a copy of this login to answer the questions once on site and be able to take a few photos on site.  Spoilers may be included in the descriptions or links.

Directions:

Location:  Norfolk CT          

Activity 1.                    N.42^o00.2111’, -073^o12.577’

This puts you in the parking area of Haystack Mountain State Park.  Follow a wide trail that leaves the southeast corner of the parking area (Figure 1).  This will eventually turn toward

Figure 1.  Glacial striations on rock at beginning of trail.  Glacier moved parallel to the striations which were made by gouging of rocks frozen into base of glacier as it moved over the bedrock and toward the southeast.  A skeptic might say that because these striations are parallel to the road they probably were made by a bulldozer or some other heavy equipment.  However, less photogenic striations parallel to these were found also on rocks off the road.

the north, start uphill and will be marked with a white blaze.  This will meet a yellow blaze trail that will take you to the top.  Watch for abundant poison ivy at the top.

The rocks you will pass (Figure 2) on the ascent are gray and dark gray colored amphibole gneiss with layers of black amphibolite₁.  These rocks were part of the old

North American continent and originally may have been volcanic rocks before being

metamorphosed.  In numerous places short white veins of quartz have been deposited in what

http://www.ct.gov/dep/cwp/view.asp?a=2716&q=325136&depNav_GID=1650

were cracks in the rock.  Notice that the quartz protrudes from the rock.  This is because quartz does not weather as rapidly as amphibolite does.

A.                                                 B.                                                   C.

Figure 2.  Amphibole gneiss.  A.  Outcrop along steep part of trail. Gneiss is a banded metamorphic rock.  The banding is manifest in this image by the layering and the differential (enhanced) weathering of some of the layers.  B.  Amphibole gneiss block about 6” high used in constructing the tower.  Banding is evident by the lighter and darker colors, which are caused by varying proportions of the minerals that compose the rock.  Darker layers contain more amphibole; lighter layers contain more plagioclase feldspar.  C.  Amphibole gneiss outcrop near top of trail.  Banding of the gneiss is apparent.  Notice that the bands are folded (just below and to the left of the compass, which is 3.5” long, in center of image).  White mineral is quartz, which fills veins and small gashes (tension fractures) in the rock.

An observation tower has been built on the top of Haystack Mountain. The stones used to construct the tower are composed of amphibole gneiss. The tower rises above the highest tree-top and provides a panorama view of the surrounding area (without the tower, trees would obscure the view).  The view to the west is of particular interest to this EarthCache (Figure 3).  In the far distance is Bear Mountain (elev. 2316’), which is the highest mountain top in Connecticut.  To the west of (behind and not visible in picture) Bear Mountain is Mount Frissell (elev. 2453’), the top of which is in MA.  The highest ground elevation in CT (2370’) is on the flank of Mt. Frissell at the state border. 

Notice in particular the valleys.  One can be seen in the distance in front of Bear Mountain.  That valley extends north-south, but a spur heads toward the observation tower and is seen in the center foreground of Figure 3.  Looking at the geologic map (Figure 4) one must be impressed that the valley in the left foreground coincides with the area underlain by marble rock (light blue on map) and other calcareous rocks.  Calcareous rocks are not resistant to ravages of the environment (we will confirm this in Activity 2) and hence weather away, creating valleys.  If you have access to the geologic map of the entire state you can will see  that the valley in front of (east of) Bear Mountain similarly is underlain by marble and calcareous rocks.  Indeed, western Connecticut contains numerous “marble valleys” which are underlain relatively easily-weathering marble.

Marble valleys tend to have water wells that produce hard water because of dissolved calcium and magnesium from the rock.  Soils tend to have a neutral pH because of the natural lime in the environment.  This affects the environment producing unique habitats.

Figure 3.  View, looking toward the west from observation tower on top of Haystack Mountain.  Bear Mountain forms right skyline; it is about 13 miles distant.  Note the valley in the center foreground.

Figure 4.  Geologic map of Haystack Mountain and the area to the west.  Geologic maps indicate what kind of rock (the formation) underlies any area shown on the map.  Light blue area on this map is underlain by marble and/or calcareous rocks and form the low areas on the map.  The green areas north, south and to the east (including Haystack Mountain) are underlain by relatively resistant (to weathering) “granitic” rocks, including schist and gneiss.  Haystack  Mountain is located on the eastern border of the map, just below center.  Width of map (east-west) is approximately 3 miles.

Activity 2   To confirm our observation that marble weathers faster than granitic rocks, on your way home stop and visit a cemetery, preferably an old one.  We stopped at a cemetery near the base of Haystack Mountain.  Where ever you visit, observe how different types of gravestones have fared in the weather.  Look for two kinds of grave markers:  those made

 A.

B.

Figure 5A.  Marble gravestone.  Note it is white or light gray.  When this stone was cut it had sharp letters that were easy to read.  Today the letters are rough and have rounded edges.  This must be caused by the rock’s reaction to the forces of nature.  Note that what look to be gray or black minerals are actually lichen and are not a part of the marble.  Lichens on the marble make it look very similar to the granite.  B.  Detail of granite (actually this is a granite gneiss) gravestone (left). This stone is more than 100 years older than the stone made of marble (above), but note its letters are almost as sharp as the day they were cut.  Granite is composed of light gray minerals (feldspar), gray minerals (quartz) and black minerals (biotite mica).  Picture on right compares the degree of sharpness (a function of how much weathering has occurred) of a marble stone on the left with a granite stone on the right.  The letters carved into the granite are still sharp;  those carved into the marble are beginning to be illegible.

of marble and those made of granite or granitic gneiss (see Figure 5). Gravestones composed of each rock type are fairly abundant in many cemeteries.

Marble is usually white or light gray (See Figure 5A, B).  It is composed of calcium-magnesium carbonate (the mineral dolomite) or, more rarely, calcium carbonate (the mineral calcite).  Both dolomite and calcite are soluble in rainwater (which is naturally slightly acidic).  Dissolution of the gravestone in water roughens what were originally smooth surfaces and blurs what were originally sharply cut letters.  Notice the inscriptions on older marble gravestones are less distinct than on the newer ones.  On some marble gravestones you may notice long blade-shaped crystals that have not weathered as much as the marble and hence stand out in relief.  They are composed of the mineral tremolite, a calcium bearing amphibole.

Now look for granite gravestones (Figure 5C, D).  Granite is composed of the minerals quartz, feldspar, and mica.  It may be gray or pink (usually more flesh colored) or a mixture, depending on the nature of the feldspars. Granite usually has an even texture which may be finely- or coarsely- crystalline. Granite gneiss is usually gray and has a foliation (layering).  Notice that on the granite gravestones, although they may have lichens growing on them, the letters are all sharp and easy to read.

The lesson to be learned here is that rocks used for headstones weather differently:  marble weathers much more rapidly than granite.  One can easily imagine that the same applies to rocks in their natural environment and that it might affect the topography, such as creating lower topography (valleys) in areas underlain by marble.  Indeed, that is what we saw from the top of Haystack Mountain.

To log this EarthCache:

1.  Determine the coordinates and take a picture of the bench-mark (brass disc cemented into the floor) on the observation deck of the tower.

2.  Submit a picture of you and/or your companions on the steps of the tower.

3.  Provide one explanation for the following two observations:

            a.  Marble headstones have a worn appearance whereas granite headstones look

unaffected by weather (aside from lichen growth on their surface).

b.  In western Connecticut, valley land-forms are associated with areas underlain by

marble and calcareous rocks whereas high elevations are associated with areas underlain by “granitic” rocks.

Difficulty rating:1,

Terrain rating: 2 (The trail is steep in places near the top).