Landslide Deposits

What the map says

Slumps, rock falls, and earthflows that range from active to inactive. Material in the deposits ranges in size from clay and silt to blocks several feet in diameter (near outcrops of the Fountain Formation and Dakota Group)

1:100,000 sheet: Colton, Roger B., 1976, U.S. Geological Survey Geologic Investigations Series I-855-G (3 crags). A contact line at that scale is about 100 m wide on the ground.

Deposits composed of unsorted and unstratified debris of various sizes as well as almost intact blocks of bedrock that slid and flowed down slopes are locally common in the map area. Younger landslide deposits commonly have hummocky surface morphology and are bounded upslope by crescent-shaped headwall scarps and downslope by lobate toes. Older landslide depos- its commonly lack scarps and have more muted surface morphology. Unit Qls locally includes material displaced by rotational debris slides, translational rock and debris slides, debris flows, and possibly minor earth flows (as defined by Cruden and Varnes, 1996). Some landslides in basement rocks may have formed in rock weakened by fault- ing and deep-seated gravitational spreading. The sizes and lithologies of the clasts and the particle sizes of the matrices of these deposits reflect those of the displaced surficial deposits and bedrock units. Some of the largest landslides and landslide deposits form on unstable slopes that are underlain by shale of the Belden Formation (*b) and sedi- ments of Dry Union Formation (Nd). The large landslide deposit just south of Pine Creek formed largely by the remobilization of surficial deposits, but it probably also includes the underlying sediments of the Dry Union Formation (Nd). The relatively fresh landslide scarps in this deposit suggests that some of the movement occurred during the Holocene. Some landslide deposits with relatively smooth topography may be as old as early Pleistocene. Estimated thickness possibly as much as 60 m

1:100,000 sheet: Kellogg and others, 2017, U.S. Geological Survey Scientific Investigations Map 3382 (2 crags). A contact line at that scale is about 100 m wide on the ground.

Deposits of unsorted and unstratified debris, on slopes or at base of slopes, that are commonly characterized by hummocky topography. Many of the landslides and landslide deposits form on unstable slopes that are underlain by shale, siltstone, and claystone on east flank of hogback belt and on slopes of North Table Mountain and South Table Mountain near Golden. Younger deposits are commonly bounded upslope by crescent-shaped headwall scarps and downslope by lobate toes. Unit locally includes material displaced chiefly by rotational rock slides, rotational earth slides, debris slides, earth flows, and earth slide–earth flows as defined by Varnes and Cruden (1996). Some deposits probably are formed by translational slides and rock or earth creep. Sizes and lithologies of clasts and grain-size distributions of matrices of these deposits reflect those of the displaced bedrock units and surficial deposits. Landslide deposits are prone to continued movement or reactivation due to natural, as well as human-induced, processes. Deposits in mountainous terrain derived from Precambrian quartz-feldspar gneiss (Xf) and biotite gneiss (Xb) locally contain blocks of rock as long as several meters. Landslide deposits derived from Pierre Shale (Kp), Laramie Formation (Kl), and Denver Formation (bKd) are rich in clay. Some of this clay is expansive and locally may have high potential for shrinking and swelling (Shroba, 1982). Deposits on gentle slopes along and east of hogback belt locally include minor sheetwash and creep-deformed deposits. Deposits in mountains locally include minor rock-fall deposits. Estimated thickness 5–50 m

1:100,000 sheet: Kellogg and others, 2008, USGS Geologic Investigations Series 3000 (3 crags). A contact line at that scale is about 100 m wide on the ground.

Slumps, debris flows, earthflows, rockfall avalanche deposits, and similar large masses of locally derived debris moved downslope by gravity.

1:100,000 sheet: Trimble and others, 2003, U.S. Geological Survey Geologic Investigations Series I-856-H (1 crag). A contact line at that scale is about 100 m wide on the ground.

Locally includes talus, rock-glacier, and thick colluvial deposits Lithology: unconsolidated colluvium.

1:500,000 sheet: Horton and others, 2017, U.S. Geological Survey Data Series 1052 (2 crags). A contact line at that scale is about 500 m wide on the ground.

Served by Macrostrat (CC BY 4.0). The wording is the map's own, quoted as a fact about the mapped unit, not about any one wall.