Dakota Group

What the map says

Total thickness about 88 m near Ft. Collins, 84 m near Boulder, and 81 m near Milliken. Consists of two formations South Platte Formation?Light- to dark-gray well-sorted fine-grained crossstratified and ripple-laminated sandstone, siltstone, and gray c

1:100,000 sheet: Colton, Roger B., 1976, U.S. Geological Survey Geologic Investigations Series I-855-G (34 crags). A contact line at that scale is about 100 m wide on the ground.

Light-gray to brown-gray, fine-grained, ripple-marked sandstone and interbedded gray carbonaceous shale in upper part, and chert-pebble conglomerate and sandstone in lower part; base is major regional unconformity. In eastern foothill belt, the Dakota consists of South Platte Formation in upper part and Lytle Formation in lower part. Sandstones are strongly cemented with silica and form prominent ridges; blocks of weathered Dakota sandstone litter slopes surrounding outcrops. The Dakota was deposited in fluvial and swamp environments during a major marine transgression. Thickness ranges from about 88.4 m (southeast) to 74.7 m (northeast) to 76.2 m (northwest) to 75 m (southwest) across the quadrangle Lithology: sandstone, shale, conglomerate.

1:100,000 sheet: Workman and others, 2018, U.S. Geological Survey Scientific Investigations Map 3399 (3 crags). A contact line at that scale is about 100 m wide on the ground.

Consists of South Platte Formation and underlying Lytle Formation. South Platte Formation contains two or three yellowish-gray, well-sorted, cross-stratified, porous, fine- to medium-grained quartz sandstone sequences separated by dark-gray, silty, hard, locally carbonaceous shale, interbedded with thin quartz sandstone beds and gray to white refractory clay or porcellanite layers [see Bryant and others (1973) for more detailed description of members of South Platte Formation]. The South Platte is about 67 m thick. Lytle Formation consists of yellowish-gray, medium- to fine-grained sandstone and conglomerate; locally contains reddish iron stain. Well-rounded clasts in conglomerate composed of quartz, quartzite, chert, and some petrified wood; conglomerate generally near base of unit. The Lytle is about 24 m thick. In Fraser basin, combined thickness about 75 m (Taylor, 1975)

1:100,000 sheet: Kellogg and others, 2008, USGS Geologic Investigations Series 3000 (11 crags). A contact line at that scale is about 100 m wide on the ground.

Consists of an upper interbedded sandstone and shale unit (the South Platte Formation) about 61 to 91 m thick, and a lower conglomeratic sandstone formation (the Lytle Formation) about 12 to 24 m thick. Forms the most prominent hogback ridge along the mountain front, the so-called “Dakota hogback.” The South Platte Formation contains refractory clay.

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.

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.