DUE TO THE NEED TO BE ABLE TO SEE THINGS IN THE DISTANCE, THIS CACHE MAY NOT BE DOABLE AT NIGHT.
Butte
In geomorphology, a butte is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. Buttes characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock. The resistant layer acts as a caprock that forms the flat summit of a butte. The caprock typically consists of either sedimentary rocks such as sandstone and limestone; or lava flows. The word butte comes from a French word meaning knoll (but of any size); its use is prevalent in the Western United States. Due to their distinctive shapes, buttes are frequently landmarks in plains and mountainous areas.
Mesa
A mesa is an isolated, flat-topped elevation, ridge or hill, which is bounded from all sides by steep escarpments and stands distinctly above a surrounding plain. The term mesa was adopted from the Spanish word mesa, meaning "table". Mesas characteristically consist of flat-lying soft sedimentary rocks capped by a more resistant layer or layers of harder rock. The resistant layer acts as a caprock that forms the flat summit of a mesa. The caprock typically consists of either sedimentary rocks such as sandstone and limestone; or lava flows.
Rule of Thumb
The basic rule of thumb for deciding if that hill is a butte or a mesa is as follows: If the width of the top is less than the height of the hill, it is a butte. If the width of the top is greater than the height of the hill, it is a mesa. Of course, people will name things as they will, so some place names may not follow this rule!
Local Geology
The GEOLOGIC ATLAS OF TEXAS, BROWNWOOD SHEET indicates that the youngest rock in this area is the Edwards Limestone (Ked if you’re looking at a geologic map). It is underlain by the Commanche Peak Limestone (Kc), which in turn is underlain by the Walnut Formation (Kwa). All of these rocks are Cretaceous in age (ie, from roughly 100 million years old to about 150 million years old.)
The Edwards Limestone is composed of limestone and dolomite (a calcium magnesium limestone), with chert nodules. It is thinly bedded to massive (ie, very thick bedded) and resistant to weathering. The lower part often forms vertical or near vertical scarps when eroded.
The Commanche Peak Limestone is composed of limestone with very thin beds of shale. When eroded, it tends to form moderate slopes unless directly protected by the overlying Edwards. Its thickness can vary from 15 to 80 feet.
The Walnut Formation is composed of interbedded (ie, alternating) beds of limestone and claystone. Because of this, parts of the formation are somewhat resistant to weathering while others are soft and easily eroded. The Walnut thickness varies from 100 to 150 feet.
Principle of Original Horizontality
The Principle of Original Horizontality states that layers of sediment are originally deposited horizontally under the action of gravity. It is a relative dating technique. The principle is important to the analysis of folded and tilted strata. It was first proposed by the Danish geological pioneer Nicholas Steno.
The Law of Superposition
The Law of Superposition is one of the principles of geology scientists use to determine the relative ages of rock strata, or layers. This principle states that layers of rock are superimposed, or laid down one on top of another. The oldest rock strata will be on the bottom and the youngest at the top.
Area Geomorphology
The Hill Country, also known as the Balcones Canyonlands, is composed of rugged topography, with flat-topped hills separated by steep canyons. The Hill Country starts just west of the Balcones Escarpment. These buttes and mesas are some of the examples of the flat-topped hills.
Faulting occurred in Central Texas millions of years ago, when the coastal plains to the east bent downward while the more stable central Texas interior of the Llano Uplift remained relatively stable. The Balcones Escarpment and the Hill Country are the result of this fault episode.
The east side of the fault system moved downward by as much as 700 feet. After millions of years of erosion, the soft sediments in the Hill Country area were eroded away, exposing the hard, Lower Cretaceous limestones and dolostones. This erosion created hills and steep canyons in the Hill Country.
Waypoint 1: Stop at this waypoint and look to the Northeast. Is that hill a butte, a mesa, or neither? Explain your answer.
Now move to Waypoint 2 and look to the West. Is that prominent hill a butte, a mesa, or neither? Explain your answer.
Cap Rock
Based on the local geology given above, what you can see looking at the two different hills, the Principle of Original Horizontality, and the Law of Superposition, which formation do you think the cap rock is (Edwards, Commanche Peak, or Walnut)?
Pictures
Pictures of your visit are appreciated but not required.