Both the Skunk River Valley and central Des Moines River Valley formed between 12,000 and 14,000 years ago as meltwater outlets of the Des Moines Lobe of the Wisconsin Glacier. As the glacier advanced and subsided, meltwater was released into outwash streams that flowed from the ice margins. The upper Skunk River valley was formed along the margin of the Altamont advance, which produced the moraines , kames , eskers and kettle holes such as those in the Squaw Creek Watershed near Bjorkboda Marsh in Hamilton County and the prairie potholes at Doolittle Prairie near Story City. The Altamont advance extended to the area of Peterson and McFarland Parks on the Skunk, to just beyond E18 on Squaw Creek, and to the Fraser area on the Des Moines. (map illustrating extent of Altamont advance)

The Des Moines Lobe region, having been impacted by the most recent glacial advance into the state, is among the youngest and "least developed" landscapes in Iowa, meaning that it is not as well drained (illustration of lesser drainage within Des Moines Lobe). And though the Skunk River Valley and tributaries are not as deeply incised into the landscape as the Des Moines, the Skunk is actually thought to be an older drainage, and the wide valley, terraces, and sand and gravel deposits reflect the size and power of an older glacial river.

Landform Regions of Iowa   Glacial Landmarks Trail   Sand and Gravel Resources of Iowa   Glacial Age Floods   Geology of Iowa

Historic Alteration of Surface Hydrology on the Des Moines Lobe, Iowa Geological Survey

Although estimates vary, it is generally agreed that approximately 99 percent of the original wetlands, marshes, and small streams of north-central Iowa (the Des Moines Lobe) were drained and plowed, while the larger streams and rivers were dredged and straightened to facilitate removal of surface water. The South Skunk was straightened from Ames to Polk County in 1893. (See 1898, 1899, and 1902 maps at Additional Links (History).)

Today, artificial stream systems replace the more absorbent wetlands and marshes, and these streams flow in direct contact with Iowa's vast agricultural landscape. Further, we have eliminated many natural stream meanders through straightening and channelization. Drainage of wetlands and channelization of streams and rivers have promoted a hydrological imbalance. Today, in the upstream or headwater portion of small streams, water moves off the land much faster, allowing greater stream bank and bed erosion, creating increased transport and deposition of materials (including soil and agricultural chemicals), along with more severe flooding downstream. Draining of wetlands has lowered the water table, causing natural underground springs and small streams to cease flowing and shallow wells to be deepened. Most of these changes in surface and subsurface hydrology have occurred within a human lifetime.

Historic Alteration of Surface Hydrology on the Des Moines Lobe   Surficial Geology of the Des Moines Lobe of Iowa, Boone and Story Counties (map, 48X36, 1.7 meg)

The Skunk doesn't boast scenic 300-million-year old Pennsylvanian sandstone bluffs and canyons such as those along the Des Moines at Red Rock, Ledges, Dolliver and Woodman Hollow. An uplift of older Mississippian limestone protrudes into Northwest Story County (Bedrock Geology, IGS) and these bedrock outcroppings are visible along the river at and above the US Filter dam.

Martin-Marietta's Ames Mine is said to be the largest single producer of limestone in the state, and is one of only eight underground operations. The Ames Mine takes limestone from the Gilmore City Formation and from the Eagle City & Maynes Creek members of the Hampton Formation , named for their outcroppings to the northeast. These formations are good water-producing aquifers, though Ames municipal wells draw from a shallow alluvial aquifer which overlays a gravel-filled pre-glacial valley.

Ancient River Channels

The Midcontinent Rift, Iowa's Almost Ocean

One billion years ago, the Earth's crust split across part of the North American continent. This tear, known as the Midcontinent Rift System, extended for 950 miles from what is now Lake Superior Southwest to Oklahoma. Rocks produced during the rift's formation can be seen today surrounding Lake Superior, including basalts along Minnesota's North Shore and sandstones along Wisconsin's Bayfield Peninsula.

Midcontinent Rift System in Iowa

In Iowa, the MRS are dominated by a huge, uplifted block (called a horst) of basalt that follows the axis of the rift across Iowa from south-central Minnesota to southeastern Nebraska. A couple major structural features along the eastern edge of the MRS, the Perry-Hampton Fault Zone and the Thurman-Redfield Structural Zone, pass through Story County just North and West of Ames.

(The rocks of the MRS are buried to a depth of 1,200 to 5,500 feet, but must somehow account for the deep connection some of us have to the waters of Lake Superior!)

The Midcontinent Rift, Iowa's Almost Ocean   MRS in Iowa   Location of the MRS   Exploring the Midcontinent Rift