Monday, October 20, 2008

A groundhog's view of a muddy farm field...

Geology in a basement excavation in the Central Ohio Clayey Till Plain, Delaware County, Ohio.
Location: 40.24N 83.07W
(excavation now back-filled, construction site off-limits)

Geologist's love holes in the ground. Excavations for basements expose a brief glimpse beneath the surface to see soil profiles, sediments, and more. A muddy hole in the ground brings subsoil secrets to light.

I volunteered to gather photographs and specimens at the site of a new nature center under construction for eventual use in an, "Under Your Feet" educational program, following a morning walk discovering birds with the Ohio Young Birders Club, Delaware Chapter.

Basement excavation and poured walls of a new nature center under construction at Deer Haven Preserve, Preservation Parks of Delaware County, Ohio. This fourteen-feet deep excavation offered a rare glimpse under the surface of a typical muddy farm field in central Ohio slated for restoration to wetlands, forest and meadow.

Glacial till in central Ohio is composed of a lot of clay-size particles with some sand, gravel, and larger stones mixed-up in the clay and exhibiting little or no sorting normally seen when sediments are laid down by running water. The stones are matrix supported, floating in the mud.The till at this site is at least fourteen-feet deep. There is no sign of bedrock exposed in the bottom of the hole.

Stones in till often have many different lithologies (recipes) and origins (where the rock was made). Each grain, whether a tiny clay particle or a large granite boulder, was picked up somewhere to the northward by south-creeping glacier ice and transported to its resting place in the till by the motion and melting of glacier ice.

Views beneath the surface of a farm field in the Central Ohio Clayey Till Plain. Rounded cobbles of limestone and other rocks are locked in a matrix of heavy clay. Rounded rocks have spent time in running water (nature's rock-tumbler) before getting mixed into the clayey till.

A powdery-white residual silt and mineral precipitates coat a deep thin open till fracture side-wall in this excavated ground moraine. Clay has been flushed from the crevice by infiltrating water. The opposing side-wall fell away into the open pit during excavation.
Remains of old branching rootlets are seen in the lower image. These rootlets of long-gone trees penetrated along the open fracture between eight and ten feet below the surface, following the soil's natural plumbing.

One side-wall of the excavation separated and fell away along the uneven plane of the natural soil fracture pictured above, exposing the till's natural plumbing system. Water drainage through open interconnected fractures in till is an important natural process throughout glaciated areas of the Midwest, and worldwide. Physical and chemical changes in the till sediments due to surface rebound following unloading of glacier ice and weathering through time cause broad area surface sediment (till) stretching and local till shrinkage (drying out and thawing from freeze). Tension across the till layer opens natural soil fractures.

Interconnected deep fractures through the till layers function as a natural plumbing system aiding water drainage. Infiltration through secondary porosity (through the fractures) can operate several orders of magnitude faster than infiltration through primary porosity (between grains of till).

Fracture porosity, the existence of interconnected open fractures in glacial till, is an important modern focus of research for geotechnical scientists and engineers. Till used to be considered a good subsurface material for limiting downward movement of chemicals and leachates. Small amounts of clayey till tested in the laboratory are found to be very tight, greatly limiting seepage downward. Recent experiments with carefully collected large samples of soil test differently, they drain quickly through natural cracks in the clay--through natural fracture porosity. Field tests support the findings (Weatherington-Rice, J. et. al., 2000).

Weatherington-Rice, J. et al 2000. Ohio's Fractured Environment: Introduction to The Ohio Journal of Science's Special Issue on Fractures in Ohio's Glacial Tills. Ohio Journal of Science,100 (3/4):36-38, 2000.

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