This is a two stage Earthcache – as such, there is no physical cache. Instead after examining both the gneiss stone markers at both the posted coordinates and waypoint 1 you will answer 4 questions and message me the answers. This Earthcache highlights the interesting contrast between how Marker 10 and Marker 11 have changed over time. Both markers are Baltimore Gneiss, both quarried and placed around the same time, but both where placed in different environments.
Like the previous Earthcache that was here, I too recommend driving to both locations. Marker 10 is on Mainstreet Ellicott City and I have posted Parking Lot coordinates in the waypoints. There is ample room to pull of the road to observe Marker 11, but I recommend not visiting this marker at late night due to its proximity to homes.
National Road Milestones
At the posted coordinates under the railroad bridge you will find the 10 mile marker stone from the Old National Pike. This marker was placed here when the Ellicott brothers built a road from Baltimore to their new settlement here called Ellicott's mills. When completed in 1880, this stretch of highway continued west to Frederick and then onto Cumberland where it joined the National Road.
The Old National Pike Milestones were laid on 1805 beginning at the Baltimore courthouse, spaced one mile apart and placed on the north side of the National Pike heading 45 miles west. Owned by the state of Maryland on the edge of the right-of-way, they are completely accessible to the public. Dimensions of the stones vary from section to section but generally they are about 12” inches wide, 8” inches deep, and about 30” inches tall.
The distance of the stone's facing the road; "38 M to B" (38 Miles to Baltimore). The other three faces bear no inscription. The stone material also varies along the route, but the first 39 stones are of Baltimore gneiss from the Ellicott City area. A few of the original gneiss Mile Markers are missing or have been relocated, the large majority are still there in their original location where they were placed 200 years ago.
Gneiss is a Metamorphic Rock
Metamorphic rocks start out as some other type of rocks (see rock cycle diagram below), but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors.
Gneiss forms through recrystallization of pre-existing rocks under high temperature and pressure. Feldspar is the most important mineral that makes up gneiss along with mica and quartz. Gneiss can be formed from a sedimentary rock such as sandstone or shale, or it can be formed from the metamorphism of the igneous rock granite. During metamorphosis the rock remains completely solid with varying pressure and magnitude. This leads to newly formed minerals. Gneiss is characterized by its banded alternating layers that are composed of different light and dark minerals.
Weathering
All materials that are exposed to the outdoor environment are subject to degradation caused by natural weathering processes. Gneiss generally weathers rather slowly, though some minerals in the stone might be affected by weathering more than others.
Physical or Mechanical weathering:
Thermal :
Extreme heat, from the sun’s rays to the hot summer temperatures, cause the rock to expand. Likewise colder temperatures cause these same rocks to contract. These movements alter the integrity of the granite causing stress and eventual breakdown.
Exfoliation or Unloading :
The overlying rock has put on tremendous pressure to the underlying rock. Once that pressure is released, the surface rocks expand and fracture parallel to the surface in flat sheets
Frost wedging :
Moisture captured in fissures and crevices freeze and thaw repeatedly fracturing the rock into angular pieces
Chemical weathering.
Dissolution:
Dissolution changes rock formations by removing the minerals that are soluble. Some minerals dissolve in water and can be carried further downstream where they may be deposited as they precipitate or crystallize.
Oxidation:
Certain rocks can ‘rust’. Rust is an iron oxide, a usually reddish brown oxide formed by the reaction of iron and oxygen in the presence of water or air moisture. Other iron bearing silicates containing aluminum can also undergo oxidation and hydrolysis which forms iron oxides and clay materials.
Hydrolysis:
Hydrolysis is the process that feldspar undergoes and is weathered to form clay. Nearly all the common rocks of the Earths crust will weather to form clay with the exception of quartz. (Clays make up nearly half the sedimentary rocks on Earth)
Biological Weathering
Lichen:
Lichen grow on the rock secreting an acid which dissolves the minerals in the rock leaving crevices. This facilitates the physical weathering mentioned above.
Vegetation:
In addition to lichen created acids, other vegetation create acids as they decay along with animal carcasses.
REFERENCES:
1. Old National Pike Milestones, Maryland's National Register Properties, Maryland Historic Trust, Department of Planning, mht.maryland.gov
2. Baltimore to Frederick town turnpike milestone - 10 M to B, Patricia Bixler Reber, Forgotten history of Ellicott City & Howard County MD, November 20, 2017, historichomeshowardcounty.blogspot.com
3. That’s a Gneiss Stone!, Karin Kirk, Use Natural Stone, Nov 3, 2017 , usenaturalstone.org
4. Ellicott City - National Road Milestones, Earthcache by Bluesnote, December 29, 2015, geocaching.com
5. Weathering, Wikipedia, the free encyclopedia, wikipedia.org
6. Old National Pike Milestones, Waymark by Navy72, 5/2/2011, waymarking.com
LOGGING REQUIREMENTS:
To log this Earthcache: Read the geology lesson above. Answer all four questions posted below and send them via e-mail or messenger contacts on my Geocaching profile. The chart below is for your use only and is not a requirement to complete this earthcache.
QUESTION 1. From your observations do you believe that iron is present in the bands of these markers?
QUESTION 2. Which marker do you believe is in a harsher environment?
QUESTION 3. Which milestone has had the most deterioration to its inscriptions? Why do you think that is?
QUESTION 4. Which type of weathering has had the most impact on these stones? What evidence leads you to that conclusion?
OPTIONAL PHOTO: Posting a photo that readily indicates that you (and anyone else logging the find) are at the locations.
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