Cuesta for the Hogback EarthCache

Cuestas and Hogbacks

Hogbacks or cuestas are formed when tilted sedimentary rock beds are eroded. Harder rock layers resist erosion, leaving a steep face with the hard rock on top and softer layers beneath. One side of a hogback is the planar but tilted hard rock layer while the other side is steep and crumbly due to erosion of the soft layers.

Technically, the term hogback is usually used when the layers are tilted steeply so that both sides of the ridge have about the same slope. In contrast, a cuesta involves layers tilted more gently, such as we see at this Earthcache site [Wikipedia].

The Morrison, Colorado area is well known for the Dakota Hogback (whose layers happen to include dinosaur fossils). Another great example of this type of structure can be seen near Perry Park. Here, several erosion-resistant layers protrude at an angle from the surrounding terrain. Parallel ridges are formed as the soft rock between erodes away.

The cuestas themselves are on private property, but you get an excellent view from several points along a short stretch of the highway.

Strike and Dip

Geologists use the terms strike and dip to describe rock layers. The strike of a planar feature such as the gentle side of these cuestas is the direction of a horizontal line on the surface of the feature [Wikipedia]. If you were to walk along the slope without changing altitude, your average straight-line path would be the strike. When looking at a cuesta, a contour line on a topo map is the strike of the feature. Geologists measure strike as an angle between 0 and 90 degrees either east or west from true north. A strike of N15°E, for example, indicates a line 15 degrees east of true north as shown here.

The dip of a slope is its inclination from horizontal in a direction perpendicular to the strike. Think of the direction of the dip as the path a ball would take if placed on the surface -- straight downhill. A 0-degree dip is horizontal and a 90-degree dip is vertical. The dip includes a letter (N,S,E,W) that gives the rough downhill direction.

To determine the strike and dip, you can actually go to the ridge and measure them. In this case, however, the cuestas are not accessible so we must use another method, such as

• Refer to a topo map. The strike is the direction of the contour lines. To determine the dip, compute the elevation difference and horizontal distance between two points on the slope and use trigonometry to compute the dip. Dip = arctan(elevation difference / horizontal distance). Example:

• You can position yourself in line with the cuesta ridge and use a compass to determine the direction to and along the ridge. Measure the dip by viewing the slope through a protractor. Example showing a cuesta with a 12° dip:

To log this Earthcache

Look to the west-northwest from the listed coordinates or one of the other nearby pullouts on the highway and note the parallel cuestas you see. Send email with answers to questions 1 and 2.

1. [Email] How many parallel cuesta ridges can you see in this general direction?

2. [Email] Estimate the strike and dip on the gentle side of the cuestas. Use either method above, or just eyeball it. To make it easier, the listed coordinates are directly in line with one of the cuestas.

3. [Log] Post a photo of yourself and your GPS with the cuestas in the background. This serves as proof that you actually visited the Earthcache site.

Logs without supporting documentation and photo will be deleted!

Congratulations to Thystle and Justified for FTF.