Herbert Hoover National Historic Site: Geologic resources inventory report

Graham, John

This report synthesizes discussions from a scoping meeting held in 2011 (Appendix A). Sections of this report discuss the geologic setting, distinctive geologic features and processes within Herbert Hoover National Historic Site, highlight geologic issues facing resource managers, describe the geologic history leading to the present-day landscape, and provide information about the previously completed GRI map data. Posters (in pocket) illustrate these data. Established in 1965 to commemorate the life of the 31st President of the United States, Herbert Hoover National Historic Site encompasses 75.6 ha (187 ac) of surficial, unconsolidated sediments. The historic site lies near the northern boundary of the Southern Iowa Drift Plain, a landform region in Iowa consisting of rolling hills and some of the most productive agricultural land in the world. During the Pleistocene ice ages, continental glaciers advanced several times into the region, depositing unconsolidated mixtures of clay, silt, sand, gravel, and boulders. These heterogeneous deposits are known as till. The most recent major glacial advance produced the Des Moines Lobe. The ice lobe did not extend into the present-day historic site, but the glacier ground the underlying sediment into fine-grained silt, which the wind deposited across the landscape. During the past 11,000 years, the rivers and streams of Iowa’s drainage systems have modified the landscape. Hoover Creek, a tributary of the West Branch of Wapsinonoc Creek, flows through the historic site and deposits alluvial sediments. Herbert Hoover, the only geologist to have become president of the United States, was a prominent mining geologist prior to becoming a public servant. He had been fascinated with geology since childhood. Hoover’s administration had a profound impact on the National Park Service and the land designated for national parks and monuments increased by 40%. Geologic features and processes in Herbert Hoover National Historic Site include the following: ● Fluvial (river) deposits of unconsolidated loam and loamy sand exposed along the banks of Hoover Creek. ● Loess (glacially derived windblown silt) deposited on upland divides, ridge tops, and side slopes. ● Till deposited during the Pleistocene ice ages and which now forms the oldest unconsolidated unit in the historic site. Bank instability caused by incision and erosion of Hoover Creek’s channel was identified as the primary geologic resource management issue during the GRI scoping meeting. Concerns associated with bank instability include: ● Slumping and an increased sediment load in the stream. ● Incision of Hoover Creek’s channel. ● Runoff and seasonal flooding that accelerates bank erosion. ● Damage to known Euro American archeological sites from the late 19th and early 20th centuries and potential American Indian sites that have yet to be found. The preferred alternative in the Final Hoover Creek Stream Management Plan and Environmental Impact Statement includes the construction of a storm water detention pond that would protect the historic site from flooding, including exceptional flood events that might occur only once every 50 years. Although no bedrock is exposed at the surface of Herbert Hoover National Historic Site, a bedrock map accompanies the surficial geologic map with this report. The youngest bedrock is found approximately 30–60 m (100–200 ft) below the surface of the historic site and consists of 393.3 million to 382.7 million year old (Middle Devonian) dolomite. Older carbonate strata underlie the Devonian layers. The fossiliferous limestone and dolomite in these Silurian Period formations represent episodes of sea level rise when the region was submerged beneath shallow seas.

Herbert Hoover National Historic Site: Geologic resources inventory report

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