Rock Slide on the Enola Low Grade Trail EarthCache

THE 2018 ELGT ROCK SLIDE

Seattle, Washington is also known as the Rain City. The annual average rainfall there is 152 days and they typically get 37 inches of rain a year. In 2018 Lancaster County had a higher rainfall average than Seattle, setting a record for wettest year in county on record. Lancaster County's annual rainfall in 2018 was 153 days. In an average year this county gets 41 inches of rain, but in 2018 Lancaster ended the year with 61 inches of rain.

On August 2, 2018 a rock slide occurred here bringing huge rocks and boulders smashing down onto the Enola Low Grade Rail Trail. Some of the rocks fell onto live wires which sparked a fire in the trees. Since the area was so saturated with rain the fire was put out before emergency crews arrived. Even though there were no injuries, emergency crews had to rescue 15 people stuck on the other side of the collapsed rocks and take them back to their cars. The trail was closed for 7 days while clean-up was performed by Manor Township crews.

Representatives from the Pennsylvania Department of Environmental Protection evaluated the cause of the collapse and deemed it weather related. Years of water filling crevices between the rocks had taken its toll on the cliff side. The process of water freezing and expanding in the winter, and thawing and contracting in the spring loosened the cliff formation. With the ground being heavily saturated from record rainfall, it broke free from the extra weight.

CLASSIFICATIONS OF ROCK SLIDES

ROTATIONAL SLIDE: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide

TRANSITIONAL SLIDE: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting. A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass.

BLOCK SLIDE: This slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass.

ROCKFALL: Falls are abrupt movements of masses of geologic materials, such as rocks and boulders, that become detached from steep slopes or cliffs . Separation occurs along discontinuities such as fractures, joints, and bedding planes, and movement occurs by free-fall, bouncing, and rolling. Falls are strongly influenced by gravity, mechanical weathering, and the presence of interstitial water.

TOPPLES: Toppling failures are distinguished by the forward rotation of a unit or units about some pivotal point, below or low in the unit, under the actions of gravity and forces exerted by adjacent units or by fluids in cracks.

DEBRIS FLOW: A debris flow is a form of rapid mass movement in which a combination of loose soil, rock, organic matter, air, and water mobilize as a slurry that flows downslope.

DEBRIS AVALANCH: This is a variety of very rapid to extremely rapid debris flow.

EARTHFLOW: These have a characteristic "hourglass" shape. The slope material liquefies and runs out, forming a bowl or depression at the head.

CREEP: Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Movement is caused by shear stress sufficient to produce permanent deformation, but too small to produce shear failure.

LATERAL SPREADS: These are distinctive because they usually occur on very gentle slopes or flat terrain with the dominant mode of movement being lateral extension accompanied by shear or tensile fractures. The failure is caused by liquefaction, the process whereby saturated, loose, cohesionless sediments are transformed from a solid into a liquefied state. The failure starts suddenly in a small area and spreads rapidly. Often the initial failure is a slump, but in some materials movement occurs for no apparent reason.

CAUSES OF ROCK SLIDES

There are multiple types of causes of landslides, the three that cause most of the damaging landslides around the world are these:

Water: Slope saturation by water is a primary cause of landslides. This effect can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and water-level changes along coastlines, earth dams, and the banks of lakes, reservoirs, canals, and rivers

Seismic Activity: Many mountainous areas that are vulnerable to landslides have also experienced at least moderate rates of earthquake occurrence in recorded times.

Volcanic Activity: Landslides due to volcanic activity are some of the most devastating types. Volcanic lava may melt snow at a rapid rate, causing a deluge of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volcanoes, devastating anything in its path. These volcanic debris flows reach great distances, once they leave the flanks of the volcano, and can damage structures in flat areas surrounding the volcanoes.

ROCK SLIDE MITIGATION

LOGGING REQUIREMENTS:

To log this Earthcache: Read the geology lesson above. Answer all four questions posted below and send via e-mail or messenger contacts on my Geocaching profile.

QUESTION 1. What did Lancaster County experience in 2018 that contributed to this rock fall?

QUESTION 2. Examing the rock slide at the posted coordinates and compare it to the rockslide classifications listed on this cache page. What kind of rock slide is this?

QUESTION 3. Of the three common causes of landslides listed, which one is the cause of this rock slide?

QUESTION 4. Do you see any type of mitigation that could prevent further rock slides or injuries to people here?

OPTIONAL PHOTO. Posting a photo that readily indicates that you (and anyone else logging the find) are at the location.

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