Missionary Ridge is on the east side of the Animas Valley. It contains and displays an impressive sequence of Permian through Cretaceous sedimentary rocks.
Logging Requirements: (My preference is that you send the answers through the new "Messenger" feature)
1. Where on the ridge is the landslide located
A. Southern edge near the top (Dakota through Morrison)
B. Central portion mid-way down slope (Morrison through Dolores formation)
C. North portion near the top (Dakota through Morrison)
D. North portion near mid-way down slope (Morrison through Entrada)
2. Why has the rockfall debris area become such a large area on the ridge (geologically)? Based on your answer, would you consider the formation "competent" or "incompetent"? (see definitions below)
3. What percentage of the ridge (vertically) has become involved in the rockfall at the time of your visit?
Geologic Formation Descriptions - from bottom to top:
- Cutler and Dolores Formations- two distinct shades of red - because of the oxidized iron (hematite) shows evidence of arid continental conditions when this was part of ancient supercontinent Pangaea.
- Entrada Sandstone - white - preserves a Jurassic field of sand dunes which were later inundated with shallow waters and streams (see next layer)
- Wanakah, Junction Creek Sandstone, and Morrison formations - gray - these three formations are indistinguishable from each other from across the valley. The Morrison formation is composed of mudstones and thus they erode at a greater rate, causing them to be subject to erosion and undercutting (and thus rockfalls). These formations came about because of stream or shallow water sediment deposition.
- Burro Canyon and Dakota Formations - these erosion-resistant layers cap the ridge. They are the main reason that this ridge stands above the Animas Valley.
In 1998, a large block of the Dakota Sandstone fell and broke apart, scouring away trees as it fell. During subsequent spring seasons after heavy snow, the Morrison Formation mudstones became saturated an can
flow downslope as mudflows and debris flows now that the snope is not protected by caprock and vegetation.
Definitions:
Rockfalls - Rockfalls are an evidence of erosion. They are more visible than some other versions of erosion because of the size/magnitude of the geologic features (boulders) involved. Rockfalls are caused when large quantities of rock fall freely from a cliff face. "A rockfall is one or more fragments of rock that are detached by sliding, toppling, or falling. The rockfall then falls along a vertical or sub-vertical cliff, proceedsd own slope by bouncin and flying along ballistic trajectories or by rolling on talus or debris slopes." (Matthews, "Geology of the Silverton-Durango Train Route") Sometimes, rockfalls fall, bounce, roll, or slide down a slope and form a tallus slope at the bottom of the rockfall.
talus/scree - simply, a pile rocks of large to small size that accumulate at the bottom of an incompetent cliff/slope as a result of rockfalls.
competent cliff - a cliff (formation) that contains well consoladated rock is stable and therefore not likely to be susceptible to rockfalls. Mechanical erosion such as from roots or rain doesn't have a significant effect on this type of formation.
incompetent cliff - a cliff (formation) that contains largely unconsoladated rock and therefore is susceptible to rockfalls. Mechanical erosion shuch as from roots or rain has a significant effect on this type of soft-rock formation.