To the North, above and behind you, the Basalt (Lava) scarp of the top of Grand Mesa is visible. You are standing on a “ridge” hundreds of feet below that. Further below you is a lake with yet another “ridge” beyond that which bounds the South side of several lakes. Far off in the distance to the East you can again see the top of Grand Mesa with what appears to be broad “stair steps” working their way down the south flank of the Mesa. The North side is similar. Most of the 300+ lakes for which Grand Mesa is know are “Trapped” in these stair steps. These geological "steps" are known as “Slump Blocks” – also referred to as “Toreva Slump Blocks” after the town of Toreva AZ near which they were first described.
The slump blocks surrounding Grand Mesa appear to have resulted from retrogressive rotational failure, a type of mass movement that occurs when landslides enlarge opposite to their direction of movement by slumping of successive blocks from the mesa edge (fig. 1). Blocks of relatively rigid cap rock have been transported by sliding in the underlying claystone. Individual blocks moved by rotation and translation. Rotation accounts for backward tilting of the blocks (Yeend, 1969) and translation (forward movement/slippage) accounts for much of the separation between them. . . . . . Slump blocks below the rim of Grand Mesa exist in practically all stages of their evolution, from incipient slumps that have moved less than a meter to old, degraded slumps that have moved hundreds of meters from their original positions and have subsequently been weathered and eroded. U.S. GEOLOGICAL SURVEY, “GEOLOGIC MAP OF SLUMP-BLOCK DEPOSITS . . . (etc)” Open-File Report 96-017 By Rex L. Baum & Jack K. Odum
The major forces at work here is that the underlying claystone becomes saturated with water, softens, and erodes away until it becomes unable to support the much denser basalt cap on top. The claystone begins to slowly slip away carrying the cap with it. The “valleys” between the “ridges” trap water which becomes many of the lakes you can see from this view point.
We would like to acknowledge the support of the rangers at the Grand Mesa National Forest Visitor Center, in particular the resident geologist, Mike Wiley, who first showed us the diagram above and answered our questions about what we were seeing.
To gain this Earthcache
You may log your find immediately but to complete the process you must answer the following questions and e-mail them at the same time to TEAM kekj . Please do not post the answers in your log: It will be deleted!
1. What is another name for “Slump Blocks” ?
2. How many major “steps” do you estimate you can see far off to the East?
3. What is the color of the rock on which you are standing?
4. Do you think the rock you are standing on is Basalt (Lava) or Claystone?
5. At which point on the sketch in Fig. 1 do you believe you are standing, A, B, or C?
Look for our nearby “Grand Mesa's Cap – Exposed” Earthcache (GC5D5FT) for more info on how the basalt cap got high up here! "Grand Mesa's Descent into Failure" (GC5D5FT) shows a very early stage slump block just beginning to form!