BH250-128g
Title
BH250-128g
Subject
Glen Wood Shale
Description
Glen Wood Shale
BH250-128g is a sample of the Glenwood Formation collected in the Sogn Valley along Highway 14 in southeastern Minnesota. At this locality, the formation is concealed beneath soil and vegetation, so it is necessary to dig into the roadside ditch to expose the outcrop. The rock is a soft, fissile, greenish-gray shale composed primarily of clay minerals with minor silt-sized quartz and other detrital grains. It was deposited during the Middle Ordovician in a quiet, shallow-marine environment as rising sea level flooded the sandy seafloor represented by the underlying St. Peter Sandstone. The Glenwood Formation is overlain by carbonate rocks of the Platteville Formation.
Although relatively thin, the Glenwood Shale is environmentally and hydrogeologically important. Its low permeability restricts groundwater movement between the permeable St. Peter Sandstone and the overlying carbonate rocks. Water commonly moves laterally along the sandstone–shale contact, producing seeps, springs, and wet zones where the contact intersects the land surface.
BH250-128g is a sample of the Glenwood Formation collected in the Sogn Valley along Highway 14 in southeastern Minnesota. At this locality, the formation is concealed beneath soil and vegetation, so it is necessary to dig into the roadside ditch to expose the outcrop. The rock is a soft, fissile, greenish-gray shale composed primarily of clay minerals with minor silt-sized quartz and other detrital grains. It was deposited during the Middle Ordovician in a quiet, shallow-marine environment as rising sea level flooded the sandy seafloor represented by the underlying St. Peter Sandstone. The Glenwood Formation is overlain by carbonate rocks of the Platteville Formation.
Although relatively thin, the Glenwood Shale is environmentally and hydrogeologically important. Its low permeability restricts groundwater movement between the permeable St. Peter Sandstone and the overlying carbonate rocks. Water commonly moves laterally along the sandstone–shale contact, producing seeps, springs, and wet zones where the contact intersects the land surface.
Coverage
Sogn Valley, Goodhue County
Creator
Bereket Haileab
Relation
Collection
Citation
Bereket Haileab, “BH250-128g,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/388.