Thursday, January 10, 2013

Black Hand Sandstone terrains, scenery and refugia

Soft hemlock boughs swathe coarse sandstone walls throughout Black Hand Sandstone terrains in eastern Ohio. The Hocking Hills (Hocking County), the most extensive outcrop area of this sandstone, offers some of the best of this unique scenery.

Contorted hemlocks cling to Black Hand Sandstone walls.

The Black Hand Sandstone forms unique ravine and gorge topography, here and there, wherever it outcrops in central eastern Ohio counties trending north-south from the hills surrounding Mansfield, Ohio and the Mohican Gorge parks region in Richland and Ashland Counties, south to the Hocking Hills region in Hocking County and northern Vinton County. Ohio state parks and natural areas, Mohican State Park, Blackhand Gorge State Nature Preserve, and Hocking Hills State Park, offer rewarding access to beautiful scenery year 'round. Our favorite scenery is formed by the gigantic rock shelter at Ash Cave in Hocking Hills State Park. Nearby Rock House is a blast for kids and parents together.

Winter is a great time to find open vistas. Every winter, giant icicles gleam along seeps and delicate streamside ice crystal formations can be found near ravine heads. Uncommonly cold and wet winters in Ohio produce extraordinary ice formations wherever water flows along or seeps from Black Hand Sandstone formations. Rarely, giant frozen waterfalls connect cliffs to bottoms with giant columns of ice.

Black Hand Sandstone terrains offer much more than great scenery, unique qualities make Black Hand Sandstone terrains important refugia for plants and wildlife including disjunct populations of northern breeding bird species, Canada Warbler is a beautiful example.

All bedrock outcrops offer epilithic (stone surface) niche opportunities for unique living things from bacteria extremeophiles living among sand grains millimeters below the surface of sun-baked southern rock exposures, to abundant clinging lichens adapted to dry and moist rocky surfaces, to ancient liverworts' colonies plastering dripping moist recesses under rock overhangs, to mosses and ferns of ledges including fibrous-rooted spleenworts and cliffbrake ferns found clinging along the tiniest crevices. Beneath shady hemlock groves growing in the acidic sandstone soil of steep slopes or clinging to crags and rocky bluffs in Black Hand Sandstone terrains, moist ravine cliffs and headwalls mingle groundwater with atmosphere, significantly modifying the microclimates of ravines year 'round. Moist sandstone ravines are special places, they are local biodiversity hotspots.

Honeycomb weathering.
Geologists describe this type of sandstone deposit as "massive" due to its tendency to form large uniform blocks of homogeneous sandstone. Narrow bands of pebbly sand called conglomerate, and cross-bedding of sand layers, and absence of marine fossils suggest the deposits originally collected in a sediment-choked braided stream environment.

Quartz sand forms the backbone of the stone, more than 90 percent of grains are composed of sturdy quartz, iron oxide cement holds most of the sand grains together forming the erosion resistant rock of the steep ravine walls and formations.

Geological weathering and erosion produced the scenery and rock features including the fine scale surface patterns of Black Hand Sandstone outcrops. Differential erosion; softer, poorly cemented lower levels of sandstone eroding more deeply than firmly cemented upper levels, caused formation of outcrop features, the overhangs and rock shelters and other large scale features of Black Hand Sandstone terrains. Weathering and grain by grain erosion of high angle surfaces of the sandstone outcrops forms intriguing honeycomb surface patterns. Both the large scale features and the honeycomb surface patterns result from properties inherent in the sandstone bedrock.

The Black Hand Sandstone is porous and permeable, groundwater moves through it: groundwater is key. Groundwater dissolves and moves minerals in solution through the matrix of the permeable bedrock toward lower crevices, seeps, and rock surfaces. Near open water where humidity is high condensation moistens rock surfaces. Abundant moisture from rain, groundwater, and condensation dissolves and carries away the bedrock's mineral cement ions, drop by drop, and where evaporation outpaces flow, it redeposits oxides and sulfides among the sand grains of exposed surfaces.

Evaporating water delivers much of the magic. Evaporation of mineral rich ground water, moved through the bedrock matrix by gravity and by capillarity, leaves behind surface precipitated mineral coatings on exposed surface grains. Ions leave solution as surface moisture evaporates and must recrystallize as precipitate between surface sand grains. The push of millions of ions drawn into crystal lattice positions around grains of sand is powerful. Collectively, this pressure of crystallization wedges sand grains outward loosening them from the rock surface, water and wind under gravity carry the dislodged grains away, one by one. Freezing water between sand grains obtains the same result during winter freeze-thaw events, though freeze-thaw processes are far less important at the small scales of sand grains. This grain by grain erosion process deflates stone faces and forms the intriguing honeycomb weathering patterns* common on steep rock surfaces in Black Hand Sandstone terrains. This unique grain by grain erosion process is called granular sapping. Constant sapping, aided by seasonal freeze-thaw loosening rock layers (called spalling) and blocks of stone (slump blocks), under gravity, carves the topography and unique features of Black Hand Sandstone terrains.

The characteristic U-shape, high walls and flat bottoms, of deep ravines of Black Hand Sandstone terrains is a result of differential erosion processes: the downward cutting by ravine streams is outpaced by the sapping and slumping of ravine walls once the ravine bottom becomes choked with sandy sediment. This pattern repeats in numerous ravines, a unique and interesting landscape phenomenon.

Evaporation does much more, evaporation at seeps and along surfaces of steep walls cools air and raises humidity in ravines during summertime. During winter, groundwater warmth ameliorates the chill in ravines. This microclimate influence supports uncommon biological diversity. More on the Black Hand Sandstone refugia in future posts...

More on sandstone terrains in GeoEcology
More on sandstone terrains at our blog Earth InsightCache

*Honeycomb weathering is commonly called tafoni. Tafoni results from weathering processes on steep to vertical surfaces of granular stone where humidity is sustained in splash zones or where surface and groundwater mingle. Tafoni is common in coastal areas where it forms as a result of salt weathering, recrystalization of salt from sea spray on surfaces of granular rock particles. Inland sandstone provinces form tafoni due to recrystalization processes supported by groundwater movement and by humidity-driven mobilization of mineral solutions on granular stone surfaces in microclimates of some outcrops.


Bob said...

I came across your blog searching for information on the rate of erosion of Black Hand sandstone. I'm a docent for the Fairfield (OH) County Historical Park District at Rock Mill, on the upper falls of the Hocking River, about 7 miles NW of Lancaster. Rock Mill overlooks a 40' deep gorge cut into the Black Hand sandstone by the Hocking.

I've been trying to get some sense of the time scale it took for the river to carve out the gorge. My guess is that it happened since the last glacier retreated about 13 kyears ago, but that's just a guess.

Can you provide a source I can cite as to the rate of erosion in Black Hand sandstone, i.e., how long has it taken to cut out the gorge, and how fast has the waterfall moved upstream?

I can be contacted at rlwcons (at) g Mail dot com.



Tom Bain said...


Thanks for the question. That’s a really interesting, but tough question. In short, I think the Black Hand terrains are much older than recent glaciations, generally, but a few gorges were cut recently by draining glaciers as the Hocking gorge might have been. Generally, I believe most scenic terrain are remnants of pre-glacial drainage topography. One reason I interpret the terrains as very old is that habitat refugia must have existed therein at the time the lost Teays River drained north and NW through Ohio’s deeply divided preglacial terrain. North flowing tributaries brought relic species from southern mountains into Ohio which remain to this day, living in isolated sandstone gorges. One standout example is bigleaf magnolia (Sharon Sandstone, Jackson County). Other examples may include rhododendrons and some magnolias in Hocking county.

Ohio's glacial boundary is perforated by breakthrough drainage gorges where dammed and ponded glacial melt spilled over cols and carved steeply and deeply eroded gorges very rapidly due to immense volumes of drainage water. Streams occupying these spillway gorges are inadequate, today. Their gorges obviously a product of a much greater volume of drainage.