A natural stone arch is a rock exposure that has a hole completely through it formed by the natural, selective removal of rock, leaving a relatively intact frame. This is usually caused by erosion. Erosion can selectively remove rock both macroscopically and microscopically. Rock of any type (with the sole exception of a pure crystal) is a complex matrix of small, interlocking, solid particles. These particles are mostly microscopic fragments of various mineral crystals known as grains. Under high temperatures and pressures, some of the crystalline grains fuse, especially the smaller ones, and act as cement between the larger grains.
Macroscopic erosion occurs when joints or fractures are first induced in this rock matrix through some (usually catastrophic) process, and then widened through a variety of other processes. This splits the rock into distinct macroscopic pieces that can then move relative to each other under the forces of gravity or water pressure.
Microscopic erosion occurs when certain processes dissolve the crystalline cement, thus destroying the rock matrix and allowing other processes to disperse the remaining loose grains. Both types of erosion occur separately and in combination on all rock exposures. Only under very special circumstances will a natural arch form.
This arch was formed over geologic time as water from a small pond fed by the existing creek gradually weakened the underlying rock. Both Macroscopic and microscopic erosion played a part in forming this arch. Annual freezing and thawing sped up this process. Freezing water expands and would help split the rock, or make small cracks larger. Microscopically the water from the creek would dissolve the weaker rock and over a period of between 5 to 20 thousand years, we see the arch as it stands today. If one looks carefully, chunks of the crumbly eroded black lamprophyre rock can still be found in the debris under the arch, leaving the pinkish syenite rock in place as the impressive arch.
The creek that formed the archway still flows through it. Under and a few meters behind the archway is a small waterfall off to the left of the main trail. Very refreshing after an uphill hike. The trail continues up to the top of the arch, but to get there, you'll have to climb up some tree roots.
There is no dock near the trailhead, only a gravely shoreline, but there is a sign at the trailhead. Please use caution when travelling across the lake, winds can quickly build up on the Arrow lake causing rough passage by boat - small craft warnings often occur. The trail itself, switchbacks up the mountain for about 2km to the arch.
To claim this as a find you MUST visit the archway and email me the answer to the following two questions;
1) - Estimate the thickness of the archways rock at the apex.
2) - What is the elevation on top of the arch?
Uploading photos to the cache page is the best way to say thank you to the cache developer. It also encourages others to visit the location and helps to show changes throughout the seasons. Thank you.
西脇市