This earthcache will take you across state lines, thus it is an “interstate cache.” You will observe Granite and Gneiss in their native “rock face” format and finished rock work in the nearby towns. This earthcache will take at least 1 hr and require several miles of driving, and some walking too!
Your big goal:
Identify rocks you see as being either Granite or Gneiss, then being able to identify the size and regularity of the grains so as to determine which rocks you see are the famous “world class” Westerly Granite.
History:
Did you know that Westerly Granite is used worldwide, and is considered the “gold standard” of granite? MIning began in 1846. Smith Granite can count Westerly Granite in monuments and buildings in 36 states including such famous places as Gettysburg, National Mall, and Central Park NY. Thanks to the railroad, the rock was able to be shipped west, increasing its demand by other states. At its peak, granite mining shaped this region, with more than 50% of the population working in the granite mining industry. Mining continued in a significant way until the mid 1950s in this region.
Geology:
Southwest Rhode Island, over the border into south eastern Connecticut are made of Gneiss and Granite. Specifically, the Westerly Granite and Narragansett Pier Granite belong to a series of granitic rocks from the Pennsylvanian and Permian.” (Wikipedia “Geology of Rhode Island). This granite is prized because it is massive, fine grained (and thus easy to carve, much like marble), and nicely colored. The USGS considers it to be the North American standard for granite because of its homogeneous character. Other granites are thus compared to Westerly granite (Skehan).
Terms:
Granite Gneiss - Gneiss that is banded originally igneous granitic rock, metamorphisized (put under pressure)
Westerly Granite is Described as gray with tints of blue to pink (you will see gray, bluish gray, pinkish gray, and reddish gray), equigranular and fine-grained calc-alkaline quartz monzonite to granodiorite. It is usually appearing as a dike, and is slightly younger potentially than the Narragansett Pier Granite, though both are considered part of the same granite group (Narragansett Plutonic Suite), and even sometimes interchangeably named. Westerly Granite is a superior statutory granite since it is fine grained, equigranular (grains of the different minerals are equal sized), and generally massive, meaning there aren’t “sheets” of foliation -- like you often see in Gneiss and other rocks that are metamorphic. (Feininger)
For sake of this earthcache we’ll refer to the pink and red forms of Westerly Granite as being equivalent to Narragansett Pier Granite.
Narragansett Pier Granite is slightly older, some geologist think, than Westerly granite, though they are considered part of the same eval group (Narragansett Plutonic Suite). These granites are pink or red. In some places, the grain can be slightly courser than the gray or blue Westerly granites.
Logging Requirements:
- GZ -- find the granite in the building. Determine if it is Gneiss or Granite. If it is granite, which color (red. Pink, blue, gray) and thus what term would we use to identify it? Would you consider its grain size to be a) irregular and larger (4-6 mm) or b) regular and fine grained (2-3 mm)?
- Stage 1 - Quarry Pond. This shopping area is on the edge of the old quarry (one of them).
- Stage 2 / 3 -- visit either the Columbus Statue (#2) or (former) Industrial Trust Building (#3). Determine if it is Gneiss or Granite. If it is granite, which color (red. Pink, blue, gray) and thus what term would we use to identify it? Would you consider its grain size to be a) irregular and larger (4-6 mm) or b) regular and fine grained (2-3 mm)?
- Stages 4 /5 - visit both sites (parking coords park and ride for Stage 4, roadside for Stage 5). These are roadcuts. Determine if it is Gneiss or Granite. If it is granite, which color (red. Pink, blue, gray) and thus what term would we use to identify it? Would you consider its grain size to be a) irregular and larger (4-6 mm) or b) regular and fine grained (2-3 mm)? Look for Dikes at both locations and ID them if present.
Note: Reference Points show some other monuments north west and north east a few miles that can be substituted for Stages 2 or 3.
By the time you’re done, you’ve trained your eye (at least a little) to be able to identify quality granite when you see it!
Sources:
Feininger, Thomas, US Geological Survey “Westerly Granite and Related Rocks of the Westerly-Bradford Area: Field Trip G” scholars.unh.edu/.
Skehan, James William. Roadside Geology of Connecticut and Rhode Island. Mountain Press Pub, 2008.
History of Westerly Granite Mining quarriesandbeyond.org
Providence Journal - Examples of Westerly Granite in Rhode Island www.providencejournal.com
Narragansett Pier Granite - mrdata.usgs.gov
Granite Gneiss - www.dws.gov.za
Geology of Rhode Island - en.wikipedia.org