Montana's Mesa EarthCache
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An Earthcache located at the Emigrant Rest area on US Highway 89. Local resources at the coordinates posted will broaden your mind on the bluffs and help you answer the questions needed to claim this Earthcache.
Mesas form by weathering and erosion of horizontally layered rocks that have been uplifted by tectonic activity. Variations in the ability of different types of rock to resist weathering and erosion cause the weaker types of rocks to be eroded away, leaving the more resistant types of rocks topographically higher than their surroundings. This process is called differential erosion. The most resistant rock types include sandstone, conglomerate, quartzite, basalt, chert, limestone, lava flows and sills. Lava flows and sills, in particular, are very resistant to weathering and erosion, and often form the flat top, or caprock, of a mesa. The less resistant rock layers are mainly made up of shale, a softer rock that weathers and erodes more easily.
The black-tipped bluffs located on the east side of the Yellowstone River are called Hepburn’s Mesa. The mesa is covered by a basalt lava flow that erupted from a small local volcanic vent that has long since eroded away. Geologically, the lava flow is very young, perhaps having erupted as recently as 2.2 million years ago. Large amounts of iron in the basalt make it dark colored. Some of the iron crystallized as the mineral magnetite, which has magnetic properties, and in large concentrations can cause compasses to give false readings. The basalt flow overlies unconsolidated gravels, which in turn overlie older lightcolored fine-grained sedimentary rocks that were deposited in an ancient lake that once occupied this site. The sedimentary beds of Hepburn’s Mesa contain abundant Miocene fossils, including ancient rodents, moles, and a proto-horse called Merychippus, the first equine to exhibit the distinctive head of today’s horses. On top of Hepburn’s Mesa, overlying the basalt is glacial till of Pleistocene age. The surface of these deposits is hummocky and is littered with abundant glacial erratic boulders. For many generations, Native Americans drove bison off the mesa’s cliffs to obtain food, hides, and other materials important to sustaining their way of life.
In order to claim credit for this Earthcache you will need to email the answers to the following questions to the cache owner. Failure to submit answers may result in your log being deleted.
Question 1: The fine-grained sedimentary rocks were deposited in what kind of lake?
Question 2: Fossils discovered near Hepburn's Mesa indicate that this area had what type of climate during the Mocene time about 11 to 18 million years ago?
Question 3: What is the name of the range that is the western end of the Beartooth Plateau and consists of mailnly Precambrian basement rocks with a local cover of volcanic rocks?
Sources: Wikipedia, geology text books, and various websites.
While adding a photograph is not a requirement for logging this cache, if you have any good ones of the area, please feel free to add them to this cache page.
Additional Hints
(Decrypt)
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