Southwest 8 Boundary Stone
The boundary stones are the oldest federal monuments and were placed between 1791 and 1792 to mark the territory formed by donation of land by the states of Virginia and Maryland to form the new capital city: Washington, D.C. Originally 40 stones were placed to demark the perfect diamond shape, which has only becomes less perfect upon the state of Virginia receiving back the land it had donated. Not all of the stones are nowadays to be found on their original spot, some were replaced due to construction or to avoid private property. More or less eroded by the climate and the ravages of the time, 36 stones stand today.
The stones come from Aquia Creek in Stafford County, Virginia. The quarry is located on a small island on Aquia Creek, the rock formations in the quarry are about 100 to 130 million years old. This island was purchased by Pierre L’Enfant who intended to use the freestone that was quarried there to build the new capital city about 40 miles to the North. Conveniently located in the vicinity of the Potomac, the quarry had already been used for decades, its 100-130 million years old rock formations supplying stone for local needs. The workers were able to chisel the stone using handpicks, because it was easily carved without breaking. Until the completion of the original U.S. Capitol in 1825, the freestone continued to be quarried in Aquia Creek. Nowadays, the quarry is a historic nature preserve, the freestone's place in construction was taken by other, more resilient rocks, such as marble, granite and limestone.
Sedimentary rocks can be divided into 2 groups:
- Non-clastic sedimentary rocks: There are no clasts present. These rocks were formed by either precipitation of chemical substances originating from water (chemical sedimentary rocks – limestone, dolomite, chert), or by water evaporating from organic material, such as remains of plants or living organism (coal, evaporite or travertine).
- Clastic sedimentary rocks: These are formed by accumulating of eroded parts of existing stones. Clastic sedimentary rocks can be reinforced or not reinforced.
They are further classified according to the size of the grain.
Note that Sandstone, Arkose and Greywacke are all consolidated sedimentary rocks.
Arcose contains at least 25% of feldspar, because it forms by weathering of feldspar-rich igneous or metamorphic rock, such as granitic rocks. Because of its origin, it’s a durable, hard rock, typically of red colour.
Greywacke contains at least 15% clay, it’s a hard, durable rock that, because of its varied mineral content, comes in a variety of colours, such as grey, dark grey, black, brown, beige, cream, yellow, white, pink, red, or green.
Sandstone is a sedimentary rock formed from existing rocks by erosion. The process of forming sedimentary rocks from the eroded material is known as diagenesis. This material accumulates, is later buried, compacted and cemented into a sedimentary rock. This can happen both on the land and in the sea.
The main component of sandstone are sand grains, which are mostly smooth, rounded and of uniform size. These grains contain up to 10% psefitic components and up to 20% clay components, also matrix of silt and clay and varying percentage of mastic. The colour of sandstone is defined by the admixtures. Iron oxide colours the sandstone into the tones of red, glauconite into green.
Sandstone that contains more than 90% quartz clasts, is called quartzite. In this rock, the grains are covered by cementating quartz. Quartzite is usually of very light colour and also durable.
1. Visit the boundary stone at the listed coordinates. If you wish, upload a photo from your visit with your log.
2. Do you think the freestone is more suitable for construction indoors or outdoors? What does the the boundary stone’s appearance tell you to support your answer?
3. Sedimentary rocks only form on the land. True, or false?
4. Describe the colour and size of grains of the boundary stone.
5. The freestone was formed in which geological period? Was it devonian, cretaceous or pleistocene?
6. Send your answers via a private message. You can log your find as soon as you have sent the answers, you don't need to wait for a reaction. If your answers aren't satisfactory, you'll receive an explanation.
Rocks and Minerals
Building Stones of Our Nation's Capital
This cache was nominated for the NOVAGO's 2020 Best Camouflage Award
Bragging rights go to: