Assignment 3 Q. 1). Examine the sketch of the 1925 Gros Vente (\"grow vant\") la
ID: 294304 • Letter: A
Question
Assignment 3 Q. 1). Examine the sketch of the 1925 Gros Vente ("grow vant") landslide, and refer to your book. (8 points) A. How did the structural geology and the rock types influence the likelihood of a slide? B. How did water affect this slide? Consider both rainfall/snowmelt and the location of the river. C. Why did the slide move left-to-right (in the sketch) as opposed to in and out of the page or right-to-left? D. How and why did the landslide eventually cause a flood? Gros Ventre River (a) Before slide Water-saturated shale beds lubricated base of slide Former land surface Slide debris in valleyExplanation / Answer
A) The beds are steeply dipping and the beds on the left hand side dip towards the river. Shale is a rock which is highly porous but impermeable and is overlain by Sandstone which is a compact rock that is highly porous and permeable. The shale is underlain by Limestone which is a compact rock and has relatively very less porosity and almost impermeable. The presence of shale itself spells trouble to the geology of the area because of its ability to store water due to its high porosity which later results in swelling of the rock grains.
B) The water that falls as precipitation in the form of snow and rain percolates through the highly porous and permeable sandstone and thus saturating the highly porous shale bed below. Shale has an inherent property of swelling when its grains are water saturated. This water saturated shale then acts as a lubricated base for the overlying sandstone to easily slide upon. The river is also in direct contact with the shale bed at its base and thus saturating the shale bed.
C) The slide moved left to right because the beds are dipping towards the right and the force of gravity influences the beds to slide along the dip of the beds. It would not move in any other direction precisely because sliding takes place along the dip of the bed and not against the dip, and is always towards the valley.
D) The water-saturated shale acted as a lubricating base for the overlying sandstone, thus resulting in the sliding of the sanstone bed in the direction of dip. This sliding inundated the river with a whole load of sediments (sandstone) that the water levels rose, thus causing the flood.