A piece of the Ice Age puzzle, glacial striations...

We find a rare faceted and striated boulder...

Location: 40.24N 83.07W
(construction site currently off-limits)

See it at the new Nature Lodge opening during the summer of 2009 at Deer Haven Preserve, Preservation Parks of Delaware County, Ohio.

Ice sheets scoured the high spots and filled-in the low spots throughout the northwestern two-thirds of Ohio and much of the Midwest, North America's glaciated flat-lands. Glacial geology lessons abound throughout the landscapes of Ohio's glaciated counties, and beyond; but we rarely find well preserved examples of eroded bedrock like this striated boulder recently unearthed during back-fill operations for the new Nature Lodge under construction.

This rough boulder is a gem of a find, a 'diamond-in-the-rough' reflecting a sub-glacial erosional process that shaped Ohio and the Midwest.

A limestone boulder, plucked by glacier ice from the glacially scoured surface of the Middle Devonian age Columbus Limestone formation, shows angular fractures surrounding a striated facet. Much of the surface of Ohio's buried limestone bedrock is found flattened, striated, grooved, and chatter-marked when glacial deposits are removed, exposing the buried bedrock surface. This boulder offers a rare glimpse of the buried bedrock surface.

The flattish facet surfacing the boulder pictured above was chiseled by debris 'tools' once frozen in a basal ice-debris matrix, the 'dirty' ice at the base of the moving Wisconsin-age Laurentide Ice Sheet.

Resistant bedrock surfaces were scoured and gouged by debris held in glacier ice for tens of thousands of years underneath the Laurentide Ice Sheet that spread across northern North America. In central Ohio, the erosion process continued until at least twenty-thousand years ago.

The sharp fractures surrounding the boulder's facet suggest the boulder did not travel far after it was faceted, and then plucked from the bedrock surface. The striations would have been smoothed, and the sharp edges of the fractures would have been rounded by long dynamic transport within the ice or by transport in flowing melt water.

Detail of striations. Parallel striations were cut when the boulder was part of the bedrock surface the glacier slid over. Hard rocks, frozen in the basal ice-debris matrix, scraped the soft limestone bedrock like coarse sandpaper on wood. A second group of deep gouges, crossing the first, may have resulted when the bedrock block was rotated and plucked from its original position at the ice-rock interface (the bedrock surface) to begin its transport within the ice from its nearby origin to the site of the new nature lodge at Deer Haven.

The boulder was unearthed from clayey till, a ground moraine left in central Delaware County, Ohio by the most recent retreating ice sheet. Till is a mix of fine and coarse grained sediments kneaded together by pushing-grinding ice flow, and released from melting ice without the sorting action of flowing water.

During the 1.8 million years of the Pleistocene Epoch (beginning 1.806 million years ago*), much of the Midwest was buried by ice sheets for most of the time. We live in the Ice Age still, though we formally end the Pleistocene Epoch at 11,700 years ago.

*International Commission on Stratigraphy.

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).

Resources:
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.