Sunday, April 18, 2010

Weathered stylolites in Silurian limestone

We discover a weathered stylolite zone along outcrop bluffs of sub-Lockport Silurian limestone.

Stylolites in three dimensions, upper, lower, and intermediate dissolution seams bounding interlocking columns between.
39 50'46" 084 19'36"
(Currently closed to visitation. A new natural area restoration is underway here. Formal opening will be scheduled during June 2011.)

Weathered stylolites in this Silurian age limestone outcrop offer a 3-dimensional view of dissolution seams and interlocking prismatic columnar structures exposed along the vertical face of limestone bluffs overlooking the Stillwater River, West Milton, Miami County, Ohio.

Stylolites form after lithification of sediments, a diagenetic process. The original volume of rock is reduced along the seams of stylolites by at least the amplitude of the stylolites. The stylolite zone seen at this Stillwater River location is recessed into the face of bluffs in many places due to loss of chunks of material separated along dissolution seems.

L. Bruce Railsback offers a comprehensive internet atlas of stylolite features and discussion of formation:

"At the macroscopic scale, pressure dissolution between bodies of rock larger than individual grains leads to the development of dissolution seams and stylollites. Dissolution seams are subplanar non-serrate surfaces between two such rock bodies, whereas stylolites are serrate surfaces resulting from mutual interpenetration of two rock bodies, commonly as interdigitate columns. Because formation of dissolution seams and stylolites requires dissolution of bodies of rocks, rather than of individual grains, and requires coherent motion of those bodies toward the seam or stylolite to destroy any resulting pore space to and sustain pressure, lithification of the rock prior to development of seams or stylolites is required. Such lithification may occur through cementation or intergranular compaction."

An Atlas of Pressure Dissolution Features by L. Bruce Railsback, University of Georgia, Athens.

Sunday, March 21, 2010

Roche moutonnee plucked, rafted to Ohio

We discover another interesting landscape boulder near roadside, Calumet Road, Miami County, Ohio. Location: N 39 59' 11" W084 19'23".

We found this shaped, grooved, and striated glacial erratic, a small example of a landform called a roche moutonnee, plucked from its landscape of origin and rafted far south of the Canadian Shield where these teardrop-shaped glacier erosion features sculpt the surface of the ground. Evening's low sun shadows shallow linear groves and pitted chatter-marks cutting texture into the smooth surface of the granite boulder.

The grooves and striations follow the smoothly arching abrasion-shaped surface of the boulder. The large grooves and the deep striae are parallel. Lighter striations cross primary texture at obtuse angles. All roughly follow the length of this long glacial erratic, a chunk of granite rafted by the Wisconsin Ice Sheet. The upstream end narrows, the downstream end is wide. Both ends are shaped by glacier erosion. Roche moutonnees terminate in blocky ice-plucked ends like the one pictured.

This boulder tells a rock-story.

A roche moutonnee (sheepback) is a landform found where the Wisconsin Ice Sheet eroded crystalline bedrock in situ (in place), shaping minor bedrock prominences into smooth whaleback forms with truncated downstream ends. Early descriptions named them for their resemblance to a sheep's back or to mutton-oiled wigs popular in the day.

This discovery of a roche moutonnee made into an erratic is unique, geographically. The boulder is a landscaping feature at the entry to a private nature preserve in Miami County, Ohio, far south of the Canadian Shield. Today, the boulder rests in position with ice-sculpted side up as it formed in nature. Following formation in place by flowing glacier ice carrying abrasive rocks and sand, the shaped boulder was plucked by the south-flowing ice and rafted to its present position--well, nearby. The boulder has been repositioned from a field nearby to its feature position in landscaping bordering the entry to River Ridge Nature Preserve, The Lange Estate, West Milton, Ohio.