A Connecticut geology exercise, with auxiliary lessons in local history and recording skills.
Connecticut's brownstone is a sedimentary rock that was deposited during the Mesozoic Era in eastern North America. It is an iron-stained sandstone, fine to medium-grained, with quartz and feldspar sand grains cemented by calcite (along with minor hematite and quartz), and smaller amounts of other minerals. Our brownstone was extremely popular for buildings in the 1800's, especially in New York City, and for monuments everwhere. Granite tends to be medium to coarse grained, with mainly feldspar and quartz that crystallized in a strong interlocking pattern, plus some biotite or muscovite mica. Much of what is called "granite" is actually granitic gneiss, a metamorphic rock with bands of minerals, which was once commonly quarried in Connecticut. The other common building stone, marble, is composed mostly of calcite crystals. Limestone is sometimes used, which does not show individual crystals but looks somewhat like concrete without any aggregate pebbles. Slate has also been used for tomb stones and for buildings. It is easy to identify by its flat smooth surface (as in chalkboards) , dark gray or greenish colors, and hardness greater than marble or brownstone.
Granite and brownstone have very different physical and chemical features. Those differences are especially caused by the differences between calcite cement (in brownstone) and crystalline feldspar (in granite). Another difference is the layering from original mud and sand layers in brownstone. Granite does not start out with layers, but some layering (called foliation) can form by heat and pressure on the granite deep within the earth. Brownstone was never buried deeply enough to become foliated, but its sedimentary layers allow it to be split into thick smooth slabs before cutting.
Calcite is only a 3 on Moh's hardness scale. That allows brownstone and marble to be easily cut by carbide saws, and to be shaped and carved with steel tools. Ca and CO2 that are dissolved in groundwater have precipitated in sand to cement the grains into stone. That means calcite can also be re-dissolved, especially by water that is acidic (as in rain made more acidic by pollution). Old brownstone (and marble) building stones and monuments often show chemical weathering from solution of their natural cement. The older the stone, the more this is apparent.
The minerals of granite are hard; feldspar is a 6 and quartz a 7. Steel (pocketknife blade) is between 5 and 6, and a nail (soft steel) is around 5. Granite has to be cut with special diamond blades and blasted with carbide sand to carve out letters. The crystalline network of grains also make a much stronger rock than do the round grains of brownstone. And, feldspar and quartz resist weathering much better than calcite, which you can observe and measure in building and monument stones. Still, very old granite can show weathering that highlights its grains.
You can show students several methods of science as well as help them to learn about earth materials and construction techniques for buildings and monuments. As does a geologist, a student should go outdoors to make observations and interpretations of natural features and events, keep notes of those observations, and interpret them in a report back at the "office."
First, do an indoor session with samples of brownstone (and/or marble), and granite (or granitic gneiss). If possible, also use larger pieces of pure calcite, quartz and feldspar. A handlens and small knife (or nail) are basic tools for such study. Ask students to observe the physical characteristics of their sample; color, grain size, any mineral that can be identified, etc. Do two tests; scratch with the steel (harder of softer?); and look for calcite by an attempt to make any calcite present effervesce with a weak acid solution. Geologists use a 10% solution of HCL, but strong vinegar is easier and safer, and usually works OK (especially if warmed).
Discuss Moh's hardness scale; sedimentary grains and crystalline grains; how and why rocks are cut for monuments and buildings (include aesthetic reasons); and how to make notes about rocks and minerals. All scientists keep careful notes about what they are doing and what they are seeing; observation is the first key to science.
At a cemetery, identify the types of stones used for monuments (Or examine the sides of old stone buildings). Do the features of stones vary with age? Granite has been used much more since the Civil War than it was used earlier. Do not encourage physical tests here, of course. It is better to bring your own reference stones for identification by comparison. Look carefully at lettering and other carvings on the stones. Devise a scale of the degree of weathering; none, a little, a fair amount, lots, and total obliteration. After 20 or 30 observations, students can make a graph of age vs weathering for monument stones. Such graphs are analysis, the second key to science.
Back at the classroom, students become geological consultants who are reporting (another key activity) on the best types of stone for monuments. The reports will include the third key, their interpretation of the data (observations) relative to what they know about weathering processes and mineral components of the rocks. The reports themselves are the last and possibly most important key: Science MUST include disseminating or publishing, or it is not science.
Point out that harder stones cost more to cut and carve. Questions: Is it important for stones to last many centuries, or would it be OK if a family uses stones that last, say, only 50 years or so? Can you think of some ways to help to preserve monuments from destructive weathering?
Using a geological map of the state, discuss locations where granite, brownstone (and marble) occur and could be or are quarried. The are probably a lot of places where such stones occur but have not been quarried. What are the reasons for a good location of a stone quarry? Think about transportation problems and topography, and other land uses that may conflict.
If feasible, visit a quarry. Some states have few or no active monuments quarries left in operation, but members of your state geological survey might know about good closed quarries that are safe and accessible. Portland, Connecticut has a large water-filled quarry that supplied much of the brownstone of 19th century North America, plus a small operating quarry that has recently opened nearby.
MOH'S HARDNESS SCALE, 1 to 10 from soft to hard. The difference between 9 and 10 is greater than the difference between I and 9! Many field geologists carry around some of these objects and minerals to perform a simple field hardness test: which scratches which? The test is especially useful to identify just a few white minerals that look alike: quartz, feldspar, and calcite.
1. For Ct. rock sets, geological and topographic maps, bulletins on the geology of Ct., and other books: Call or visit the Ct. State Geological Survey bookstore at 79 Elm Street in Hartford (860-424-3555).
2. AGI Data Sheets: American Geological Institute, 5205 Leesburg Pike, Falls Church,
VA 22041. A small pocket-sized reference, updated every few years.
3. The Face of Connecticut: People, Geology and the Land, by Michael Bell
Connecticut Department of Environmental Protection, paper.
4. Guidebook for Field Trips in Eastern Connecticut and the Hartford Basin (Nancy McHone, editor) Connecticut Department of Environmental Protection, paper, 1995
5. For geological field supplies, two mail-order catalog companies are:
35 Pollock Road
P.O. Box 1301
Riggins, Idaho 83549-1301
1004 South Mebane Street
P.O. Box 2096
Burlington, NC 27216-2096