Please read the questions and logging requirements in advance of beginning this Earth Cache. This cache will require you to visit three locations around town. Please allow at least 45 minutes to complete as there are 2 parts to each of the 3 questions. This is reflected in the D/T rating.
NOT A NIGHT CACHE.
NOTE - logging requirements include completing the tasks & sending the answers, first, before logging a find. Smileys that do not meet the logging requirements will be deleted. If you cannot send the answers first, post a write note.
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LEARNING OBJECTIVES: Downtown Northampton is fortunate to have many buildings and artworks created from geology native to the area. This EC will introduce cachers to some of the local geology and have them identify some unique aspects of the various rocks. Cachers will then make inferences about the palaeoenvironment at the time the rocks were formed.
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Lesson - Geology of Northampton
Part 1 - The basics
Let's start with some basics - the three major categories of rock. They are sedimentary (you can see different layers in a cross-section of the rock, for example red sandstone), metamorphic (formed from sedimentary or igneous rock due to extreme pressure and heat that changes the composition of the original rock, e.g. slate), and igneous (formed from magma when it exits and cools at or above the Earth's surface, eg. granite).
Part 2 - Some local examples seen in Northampton
Map of the north-south bedrock characteristic of the state of Massachusetts.
The geology of Massachusetts includes numerous units of volcanic, intrusive igneous, metamorphic and sedimentary rocks formed within the last 1.2 billion years. The oldest formations are gneiss rocks in the Berkshires, which were metamorphosed from older rocks during the Proterozoic Grenville orogeny as the proto-North American continent Laurentia collided against proto-South America. Throughout the Paleozoic, overlapping the rapid diversification of multi-cellular life, a series of six island arcs collided with the Laurentian continental margin. Also termed continental terranes, these sections of continental rock typically formed offshore or onshore of the proto-African continent Gondwana and in many cases had experienced volcanic events and faulting before joining the Laurentian continent. These sequential collisions metamorphosed new rocks from sediments, created uplands and faults and resulted in widespread volcanic activity. Simultaneously, the collisions raised the Appalachian Mountains to the height of the current day Himalayas.
a) Red Sandstone
The red sandstone used as a building stone in Northampton is a sedimentary rock that was deposited as part of what geologists call the Portland Formation within a valley that formed during the breakup or rifting of the supercontinent Pangea about 200 million years ago. This rift valley is known geologically as the Hartford Basin, which is just one of many such basins now found along the eastern margin of the North American continent. The basin is filled with sedimentary rocks and volcanic basalts of Triassic and Jurassic age, collectively known as the Newark Supergroup rocks.
The Portland Formation red sandstone is classified as an arkosic wacke, which indicates that many of its sand grains are made up of the mineral feldspar embedded in a muddy, Fe-oxide rich matrix. This sediment formed by weathering and erosion of high nearby rocky ridges and was deposited in alluvial fans along the edges of the valley and in river channels, flood plains, and in lakes on the valley floor.
Transport and deposition of sediment by water produced sedimentary structures such as bedding and layering, as well as thin horizontal and cross-laminations commonly present within the red sandstone. Dinosaurs roamed the Mesozoic rift valleys and left their footprints along the muddy banks of ancient rivers and lakes.
b) Muscovite-biotite Granodiorite
The geological definition of a granite is a coarse-grained igneous rock with at least 20 percent quartz (by volume) and more alkali feldspar than plagioclase feldspar. Igneous rocks that have different proportions of those minerals have different scientific names, such as granodiorite (20 percent quartz and more plagioclase feldspar than alkali feldspar). Not everyone uses the appropriate scientific rock name, so many more igneous rocks are called granite than match the scientific definition.
Granite and granodiorite are normally formed from magmas generated by melting continental crust. If the rock that was melted includes a significant amount of shale, or metamorphosed shale, the resulting rock is likely to contain muscovite and biotite, and more plagioclase feldspar than alkali feldspar. Silvery muscovite is easy to see in a granitic rock, so when you see muscovite the rock is probably a granodioite. Black biotite mica makes these rocks speckled.
Muscovite-biotite granodiorite can be found in many buildings in Northampton. Local sources of this rocks are found in both eastern and western Massachusetts. Some of the muscovite-biotite granodiorite from western MA has been metamorphosed and has aligned minerals, a gneissic fabric. The Williamsburg granodiorite is distributed in small bodies around Williamsburg and adjacent towns. It also forms the popular rock climbing cliff face (Chapel Ledges) near Chapel Falls between Williamsburg and Ashfield.
c) Gneiss
A gneiss is a type of metamorphic rock with aligned minerals in broadly spaced zones. Mineralogical or lithological layering is commonly present. Minerals that are platy (e.g. micas) or elongated (e.g. amphiboles) are aligned due to deformation of the rock during metamorphism. Think about how floating logs might be aligned by the current in a river to help you understand why minerals might be aligned during metamorphis.
Gneiss is found many places in Massachusetts, particularly in the older, higher-temperature metamorphic rocks of the Berkshires, and also in the higher-temperature metamorphic rocks in central Massachusetts. There tends to be less mica in gneiss than in schist, so local metamorphic rocks that have been quarried for more massive building stones are likely to be gneiss.
d) Red, Green and Black Slate
Slate is a low-grade, mica-rich metamorphic rock that splits easily into thin, planar slabs that can be used as roofing shingles. The principal minerals in slate are quartz, muscovite, and chlorite. Uniform, clay-rich shale is the metamorphic protolith, which leads to the abundance of flat, sheet silicate minerals muscovite and chlorite. Alignment of the sheet silicates during metamorphism gives slate its good cleavage.
Slate is normally green due to chlorite, red due to hematite, or black due to graphite. Most of the slate used in Northampton comes from Vermont, where the slate is mostly red or green in color or a purplish mixture. If you see black slate, it may have come from Pennsylvania. Slate roofs are expensive and heavy, but the long life of slate roofs (~100 years) make them a good long-term investment.
Slate can also be used as a paving stone, if thicker slabs are prepared. A few vestibules in downtown Northampton are paved in slate.
e) White limestone
White limestone is a valuable sedimentary rock, composed mainly of calcium carbonate (CaCO3) plus a minimum percentage of impurities.The lower this proportion, the whiter the limestone will be.The limestone is a grainstone containing visible fragments of bryozoans, gastropods, crinoids, and other fossil shells. See below for examples of these fossils.
Gastropod
Crinoids
Bryozoans
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Responses to submit - if submitting responses for a partner, be sure to include their geocaching name with your response. EACH PERSON, however, must post their own photo demonstrating they were on site (example provided).
Use the lesson and your own observations to help answer these questions.
1. Near the posted coordinates - to demonstrate you were on site, take a photo of you / your geocaching name in front of the nearby Town Center information placard (see example). Post with your log.
Location #1 - at the posted coordinates - a vegetated bioswale
2. Look closely at the composition of the bioswale (focus on the rocks).
a) Based on the lesson, identify - with a reason based on your observation - the name of local stone that's been used here.
b) Explain why you think the City of Northampton used this particular stone for the bioswale (in other words, what geological properties does it possess that make it "ideal" for the purpose used here?)
Location #2 - WP 2 - Edwards Church
3. Look at the white limestone at the main entrance. Note that the church front facing Main Street is not used as the main entrance to the building. The limestone columns needed for the earthcache are accessible from the accessibility ramp to the right of the entry, when facing the building.
a) Identify 2 different fossils and send me the photos (the instant message feature works best for this purpose).
b) What do these fossils suggest about the palaeoenvironment (definition: an ancient environment that has been preserved in an natural archive such as sediments and rock at some point in the past) local to the area?
Location #3 - WP3 - Hope Sculpture
4. Look very closely at the layers that comprise the base of this sculpture.
a) What type of rock this is - sedimentary, metamorphic or igneous?
b) Identify the geological evidence (i.e. give examples from your observations of the sculpture) that supports your selection.
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Thanks for visiting and I look forward to reading your responses.
Finds that are not supported by complete and reasonable answers will be deleted.