Neat places of geologic interest in and around Clinton County

GEO-NOTES

Why is there a "bald spot" on Bald Eagle Mountain?

INTRODUCTION

A prominent, tree-less portion of the northwest-facing slope of Bald Eagle Mountain between Castanea and McElhatten has been dubbed "the bald spot," "Snoopy on skis," and other names over the years.  This feature, formally know as a felsenmeer, has an interesting geologic history, actually owing its origin to the past Ice Age.

Looking eastward from Lock Haven in the summer, this spot on the hillside attracts the eye because of the stark contrast between the surrounding green vegetative cover and the gray, irregular break in that cover.  In winter, its snow blanket often sets this region apart from its surroundings.

So what is the nature of this feature?

SURFICIAL GEOLOGY

Its a Surficial Deposit!

This hillside bald spot is simply a concentration of sandstone boulders on the surface of the slope.  Here, there is an absence of soil and fine-grained Earth materials; openings exist between the boulders.  Any fine-grained material generated by weathering will move down and into the openings.  As a consequence, soil does not accumulate easily, and the boulder concentrations are devoid of vegetation, or, at best, support only a sparse scattering of stunted shrubs and trees.

Like those throughout the Ridge and Valley terrain of central PA, this felsenmeer is a surficial deposit, a mantle of boulders and blocks overlying the bedrock.  Also referred to by geologists as boulder colluvium or a blockfield, this deposit comprises boulders that typically exceed 256 mm (>10 inches) in diameter, and range from angular to sub-rounded in shape.  It differs from a talus deposit in that the rock debris did not "fall" and accumulate at the base of a steep cliff.

These boulders derive from the thick sandstone bedrock units that underlie the upper slope of Bald Eagle Mountain, and, over the millennia, have moved slowly down the slope to their present position.  The actual thickness of this deposit has not been reported, but in similar accumulations, a minimum of several feet has been measured.  The adjacent soils contain high concentrations of similar rock fragments.

The Bigger Picture--Earth materials on their way to the sea

Rock units exposed at or near the mountain's crest are subject to weathering.  The resulting solid rock debris moves down-slope under the influence of gravity.  Eventually, materials collect at the bases of slopes where they undergo erosion by streams and rivers.  These waters transport rock debris downstream.  Ultimately, some of the debris reaches the coast and it will be deposited in the ocean basin.  One outcome of this collective set of processes:  over time and distance from the source area, solid particles decrease in size.  The time frame for these natural processes can not be measured in human life spans, but requires tens of thousands of years or more.

Once rocks are unearthed, either through major tectonic processes (e.g., rifting, uplift, mountain-building) or the more mundane weathering processes, they are attacked by a variety of physical and chemical weathering processes.  Climate also plays a significant role because (a) it governs the amount of water available, and, (b) it controls the temperatures.  As a general rule of thumb, warmer temperatures and more water facilitate chemical weathering, whereas, cooler temperatures and less water favor physical processes.  Physical weathering ultimately generates greater surface area so that chemical processes proceed more rapidly.

THE ICE AGE SETS THE STAGE FOR LOCK HAVEN'S BALD SPOT

This surficial deposit formed during the Pleistocene Epoch, that segment of geologic time often referred to as the Ice Age.  During this epoch, massive ice sheets moved southward into PA from NY and Canada many times.  Both the northeastern and northwestern parts of the state were covered by thick ice sheets.  Field evidence indicates that glacial ice covered much of Lycoming and Potter counties, and at least one ice sheet extended as far west as the confluence of Bald Eagle Creek with the West Branch Susquehanna River just east of Lock Haven.

However, the deposits are not a direct result of glacial ice movement.  They derive from the indirect effects associated with the presence of nearby ice and its influence upon the climate affecting the region at the time.  When the ice sheets were nearby, tundra conditions were the order of the day, and the climate that accompanied the ice extended well beyond glacial margins.  Conditions were much colder and harsher than anything we presently experience at our latitude and elevation.   Not only did colder temperatures and less precipitation affect the biota, these factors impacted local weathering processes.  Thus, "periglacial" (around the glacier) features were generated by processes that were enhanced by the rigorous, cold climatic conditions.

The effect of this harsh, "ice-age" climate on rocks, soil, and older surficial deposits was to increase the rate of physical disintegration and to facilitate the down-slope movement of debris.   Under these severe  conditions, the subsoil was permanently frozen.  During the "warmer" months, temperatures rose above and fell below freezing (0˚C/32˚F) many times affecting the surface, but not the subsoil.  As a result, the freeze-thaw cycle operated then as now, but with greater rigor.  Water, in the form of rain, snow-melt, and condensation, penetrated even the smallest cracks in exposed rocks.  As this water froze, it expanded, forcing cracks to widen, and eventually the rocks broke apart.  This process has been variously referred to as frost riving, wedging, shattering, and splitting.

Over thousands, perhaps tens of thousands of years, these and other weathering processes broke down the rock exposures into the boulders we see today.  As the extremes of the Ice Age waned, rates of physical weathering and movement decreased, and fine particles and soil filled in the spaces between the larger boulders.  Throughout central PA, nearly all the steeper mountain slopes are covered with vegetation growing in boulder-rich soils generated by these processes.  In some places, however, finer materials have not been able to gain a purchase between the boulders.  As a result, patches like Lock Haven's "bald spot" developed.  In time, voids will fill in, vegetation with take over, and even the larger boulder surfaces like ours will gradually disappear.

The edges of some of the boulders in the exposure vary as to their degree of rounding.  Abrasion through block-to-block contact during downslope movement smoothes some of the rough edges.  But it is spheroidal weathering, a combination of physical, chemical, and biologic weathering, that day-in and day-out acts to remove sharp edges and points from larger rock fragments.  The perception that this rounding requires long distances of transport does not apply, nor does it require severe climatic conditions to proceed.  Spheroidal weathering has been operating since the fragments were first separated from the rock outcrop.

Congelifluction, another periglacial process, involves the down-slope movement of debris.  The extensive, mountain-top exposures of these sandstone units underwent severe weathering and generated large volumes of rock debris.  This rock debris along with the soil on the mountain slopes was frozen to great depths during periods of ice advance.  During the warmer months, upper portions of the ground thawed and these lubricated surficial materials moved down the mountain slopes.

In some small, steep gullies, down-slope movements periodically turned into rapid debris flows which spread out onto the valley floor.  Farther to the east, at the McElhatten interchange of U.S. Route 220, debris flows generated a large alluvial fan, with its apex pointed into the water gap of McElhatten Creek.  Similar fans emerge from many of the water and wind gaps along the ridges throughout central PA.

Bedrock Source Geology

These rocks, derived from the Tuscarora Formation and its associated Castanea Member, comprise very resistant, quartz-rich sandstone.   This rock weathers very slowly and produces fragments ranging from large locks and boulders to fine sand and silt.  The Castanea Member, originally named for exposures in this area, is red-brown in color and generates reddish and brownish rock fragments.  The Tuscarora Formation weathers to produce white to grey colored rock debris.  An excellent artificial exposure of these rock units occurs just upslope from U.S. Route 220 as it goes through the Mill Hall water gap on the northwest-facing side of the ridge.  Geologists often bring students there to examine this exposure.

Lock Haven's boulder accumulation is not unique.  Similar tree-less patches can be seen on the steep north- and northwest-facing slopes of Bald Eagle Mountain and the west-facing slopes of Brush Mountain farther to the southwest.  This ridge stretches from Williamsport to Altoona.  Rock debris in these felsenmeer derive from the Tuscarora Formation.  However, on the slopes facing Nittany Valley, the  rock comprising these blockfields comes from upslope exposures of an older rock unit called the Bald Eagle Formation.  Bare patches occur on many other mountain slopes throughout the Appalachians of central PA and these derive from similar, quartz-dominated resistant rock formation.