Kink Folds on East Cuesta Ridge EarthCache

This Earthcache is located within the Los Padres National Forest along Forest Route 30S 11 (Mt. Lowe Road) that leads from Highway 101 along the top of the East Cuesta Ridge. The graded road is gated but public use (walking or bike) is permitted.

The Site

Observe the rock exposures in the road cut located at the coordinates. Try to visually trace out the path of the layers. Can you see the complex folding? Did the rocks form originally in this manner, or is the pattern the result of a tectonic event (mountain building) long after the rocks were deposited?

Geologists interpret the Earth and its history by carefully recording data and by following some simple rules. The relevant data here are related to the type of rock (sedimentary shale), and the orientation (sometimes called the "attitude" or strike and dip) of the bedding planes. The applicable rules are “original horizontality” and “superposition”. Simply put, sedimentary rocks are deposited as horizontal layers, with the oldest on the bottom and the youngest on the top.

Fold Structures

The shale layers visible in this road cut change orientation abruptly, and when traced, reveal a complex fold structure. The two basic fold structures are upright anticlines and synclines. An anticline is a fold structure whose shape is concave downward with younger rocks on the outside. A syncline is a fold structure whose shape is concave upward with younger rocks on the inside. More complicated forms develop with changing stress fields within the crust. These include plunging, inclined, overturned, and recumbent folds.

Because of the rule requiring original horizontality of the shale layers, we conclude that a force must have acted on these layers sometime after deposition of the shale.

Types of Deformation

Forces acting to deform rocks within the earth's crust are related to plate interaction or simple burial. The confining force of burial acts equally on all parts of the affected rock mass and therefore can compact rock mass to reduce volume, but does not lead to structural features like anticlines and synclines. Differential force, referred to as tectonic stress, is caused by plate motion. The three types of differential force are compression, tension (or extension), and shear.

Compression, commonly linked to convergent boundaries, occurs when forces act to squeeze the rock mass more-or-less equally along a line or axis of principle stress. A body affected by compression will be shortened perpendicular to this axis of principle stress. The opposite of compression, known as extension or tension, involves forces pulling in opposition along the principle axis. Here the resultant affect is a lengthening parallel to the stress. Finally, shearing stress develops when forces exist that are opposing but parallel.

Brittle vs Ductile Deformations

Generally something that is brittle will not deform, or deform very little before it breaks, whereas something that is ductile will deform a lot before it breaks. When a rock breaks, it is called brittle deformation. Any material that breaks into pieces exhibits brittle behavior. When rocks bend or flow, like clay, it is called ductile deformation. In the graphic above, the second row of images represent various types of ductile deformations, and the third row shows some types of brittle deformations.

Complex deformation can occur in these areas as branching fault lines converge and diverge throughout the affected area and local regions within zones of shearing experience compression and or extension. Geologists have recognized the relation between these complex shear zones and transform plate boundaries and have coined the terms transpression and transtension to better describe the unique nature of the stress field in these regions.

To log this Earthcache, send me a message with the following information:

1. The text "GC25RVF: Kink Folds" on the first line.
2. The number of people in your group.
3. Was the crust shortened, lengthened, or sheared in this area?
4. Was this area affected primarily by brittle or ductile deformation?

A limited amount of parking (Waypoint PARK1) is available on a turnout off of Highway 101, accessible from the northbound lanes only. There is a graded road leading from the turnout to the top of the East Cuesta Ridge. You will need to climb over a locked gate. The Earthcache is about a 1.5 mile hike or bike ride from the gate.

Image credits: Earth's Dynamic Systems by Hamblin/Christiansen, Prentice Hall; Understanding Earth by Press/Siever, W.H. Freeman

Sources: Special thanks to instructor Jeff Grover of the Cuesta College Geology Department for providing the content.