Helena Has Its Faults EarthCache
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A belt of seismicity known as the Intermountain Seismic Belt
extends through western Montana, from the Flathead Lake region in the northwest corner of the state to the Yellowstone National Park region where the borders of Montana, Idaho, and Wyoming meet. Helena sits right in the middle of all this activity.
****NOTICE: THE REQUIREMENTS FOR THIS EARTHCACHE HAVE BEEN UPDATED AND MADE LESS DIFFICULT - PLEASE READ DESCRIPTION BELOW****
In geology, a fault is evidence of movement in the planet's crust, usually featuring fractures in rock. Since faults do not usually consist of a single, clean fracture, the term "fault zone" is used when referring to the zone of complex deformation that is associated with the fault plane.
The two sides of a non-vertical fault are called the "hanging wall" and the "footwall." By definition, the hanging wall occurs above the fault and the footwall occurs below the fault.
The "sense of slip" is defined by the relative movements of geological features present on either side of the fault plane. The sense of slip defines the type of fault.
Faults can be categorized into three groups based on the sense of slip: a dip-slip fault, where the main sense of movement (or slip) on the fault plane is vertical. Where the main sense of slip is horizontal the fault is known as a transcurrent or strike-slip fault. Oblique-slip faults have significant components of both strike- and dip-slip.
STRIKE-SLIP FAULTS
A strike-slip fault is a fault on which the two footwalls slide past one another laterally with very little vertical motion. The San Andreas Fault is the most famous American strike-slip fault.
DIP-SLIP FAULTS
Dip-slip faults can be again classified into two types: "reverse" and "normal". A normal fault occurs when the crust is extended. The hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is called a graben. An upthrown block between two normal faults dipping away from each other is called a horst. The feature that results between these two is called a fault scarp. Evidence of this is where the ground surface has dropped three to four meters above a “normal fault” as a result of earthquakes.
A single graben or multiple grabens can produce a rift valley. A reverse fault is the opposite of a normal fault — the hanging wall moves up relative to the footwall. Reverse faults are indicative of shortening of the crust. A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°. Thrust faults typically form ramps, flats and fault-bend (hanging walls) folds. Thrust faults are responsible for forming nappes (formed during continental plate collisions, when folds are sheared so much that they fold back over on themselves and break apart) and klippen (the remnant portion of a nappe after erosion has removed connecting portions of the nappe, usually resulting in isolated hill or table) in the large thrust belts. Chief Mountain, in northern Montana, is an example of a large klippen.
In Helena, evidence of seismic activity can very easily be found. There are strike-slip faults, grabens, horsts, ramps, flats, fault-bends, nappes, and klippens.
**** LOGGING REQUIREMENTS ****
In order to log this earthcache as "Found," you must perform two steps: first, travel to ONE of the following coordinates and take a picture of yourself or your GPSr, with the dominate feature that indicates seismic activity. Post this image with your online log.
~ N 46 38.149 W 111 51.577
~ N 46 42.130 W 111 54.040
~ N 46 44.447 W 111 57.775
~ N 46 43.124 W 112 05.082
~ N 46 41.509 W 112 03.760
~ N 46 39.374 W 112 07.598
~ N 46 35.712 W 112 06.760
~ N 46 38.796 W 111 047.686
The second step is to then email the cache owner with
the correct identity of the feature(s) you have discovered and why you came to your conclusions.
** Please note that the cache owner reserves the right to delete any log(s) that do not comply with the requirements stated above.**
Additional Hints
(Decrypt)
[All features are accessible by public roads and can be photographed without crossing private land - be sure to look for nearby geocaches when at these locations!]