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CALL Course Field Trip #3
July, 2006
Meandering Streams and Recent Uplift of the Region
Thanks to Terry Engelder for some of these notes

Stop 1. Soil pit near Beaver Stadium

     This pit, dug by the PSU Agronomy Department, exposes a soil developed on the Stonehenge Limestone formation, one of the carbonate units that principally comprise the Nittany valley. The soil consists of three horizons, the upper, black, organic-rich A horizon, a few inches thick, the clay-rich B horizon, which constitutes most of the exposed walls of the pit, and the C horizon. The C horizon is a mixture of pieces of bedrock and soil, so we see only the top of the C. Underneath the C would be limestone bedrock.

     A soil is the product of weathering of bedrock by acidic waters. As bedrock breaks down, some chemical components of bedrock are removed, and some are transformed into new minerals that remain in place. For limestone, most of the rock is removed in solution. An analysis of a typical limestone is shown (these are chemical constituents, not the actual minerals present in the rock).

Table 1. Typical limestone analysis

     All the Ca in the rock, present in calcite, dissolves as Ca²⁺ ions, as does the CO₂, present in calcite, as HCO₃^2- ions. Some of the SiO₂, present as quartz and feldspar, dissolves. These materials reach a local stream, then the Susquehanna River, and then the ocean. In fact, chemical weathering is one of the mot important factors in determining the chemistry of seawater.

     On the other hand, most of the iron is transformed into new minerals, goethite and hematite, that stay in place and color the clay red to brown. (You’ll remember these minerals from Field Trips #1 and #2.) Mn forms MnO₂ minerals that are black in color. Aluminum, present in feldspar, KAlSi₃O₈, is transformed into a new mineral, clay, and stays put for the most part.  Clay, Al₂Si₂O₅(OH)₄, contains 40% Al₂O₃, so if we assume that none of the Al was carried away in solution and that the entire soil consists of clay, simple math tells us that 5 vertical feet of soil represents the end product of weathering  of about 1000 vertical feet of limestone.

     Weathering requires acid, the acid being produced by solution of CO₂ in water.  Weathering thus uses up CO₂.  On a timescale of hundreds of years, it is an important mechanism for reducing levels of CO₂ in the atmosphere.  Scientists call this process a negative feedback.  It has little short-term effect, however.

     Radioactive decay of uranium produces radon gas.  Radon is dangerous precisely because it is a gas and can be inhaled.  While inside the lungs, radon radioactively decays into polonium, which does not leave the lungs.  Both radon and polonium produce alpha particles, which penetrate and damage lung tissue and can cause cancer.  Uranium is present in limestone in very small quantities.  Limestone does not produce enough radon to be a problem in houses built above limestone.  But the minerals that contain uranium will concentrate, by the same enrichment factor as aluminum, in soils.  Thus many houses in State College, even though built above limestone bedrock, have above-average radon levels.

 Stop 2.  The floodplain and meanders of Slab Cabin Run (Branch Road just west of S. Atherton)

      Slab Cabin Run is a small stream that, over time, has established a meandering pattern and a wide floodplain.  The floodplain is the result of the stream channel moving back and forth, ever widening the floodplain and cutting away surrounding hills. Maximum erosion takes place on the outside of the meander, with point bars forming on the inside of a meander where sediment is deposited.

 
Figure 6-3.  Topographic map of Slab Cabin Run.  The light gray stipple defines the area within the 1060' contour and the darker gray stipple defines the area within the 1040' contour.

Thus the meander extends itself  even further, cutting away surrounding hills.  Sometimes a stream will cut through its banks and break into a downstream reach, establishing a new course for the stream, and abandoning its former meander.  We can see such an abandoned streambed here that is now nearly filled with sediment.  But at one time it was filled with water like an oxbow lake.

      Why does a stream meander?  Suppose Slab Cabin did not meander, but went straight downhill.  It would then have higher velocity and more stream power and would erode its bottom and banks.  It would contain more sediment than it could transport, so it would dump the sediment and then would have to detour around that sediment.  A meander would begin.  A meandering stream lowers its gradient, or elevation drop per mile, to keep its stream power and sediment load in balance.       Given enough time, Slab Cabin and other streams in the valley would establish coalescing floodplains, reducing much of the topography of the valley.  It is thought that about 10 million years ago the topography in central Pennsylvania looked like that: the surface had much less topography than today but lay at an elevation coincident with the tops of today’s ridges.  (The bedrock then would have been mostly sandstones and shales, not limestone.)  The same surface today forms the Allegheny Plateau.

     Bankful discharge of a stream is, by definition, a flood.  A stream will on average flood every 2.3 years.  A discharge that exceeds a certain level every 50 years, on average, is a 50-year flood.  The actual time interval between 50-year floods can vary considerably, of course.

Stop 3:  The Lark Member of the Stonehenge Formation: a Dolomite (Rock Road 1.2 miles north of the Rock Road/Trout Road intersection)

     Geologists use many observed features of sedimentary rocks to deduce the sedimentary environment in which the sediment was deposited.  This exercise is critical in figuring out the geologic history of an area and is also a key element of oil and gas exploration. Many carbonates form in shallow-water, marine environments.  These can include tidal and intertidal deposits where sediments are disturbed by storms.  This outcrop of Larke Dolomite contains nice examples of sedimentary structures called rip-up clasts.  The dolomite was deposited in layers of fine carbonate that hardened and shrunk to form mud cracks by desiccation when a shallow tidal flat evaporated.  Subsequently, shrunken fragments were disturbed by a storm that tore the cracked layer apart and depositing the fragments in a rubble of rectangular fragments.  Thus the environment was marine, but the water was sufficiently shallow that it could be disturbed all the way to the bottom by storms.

Stop 4: Spring Creek meanders (Rock Road 1.5 miles north of the Rock Road/Trout Road intersection)

Here we can see that Spring Creek meanders, but there is little floodplain, and there are prominent outcrops on both sides of the stream.  Moreover, the stream is superimposed on the structure of the Nittany Valley, largely because it does not parallel the folds of the Valley and Ridge.  This portion of Spring Creek flows in roughly a north-south direction, whereas the structures of the Nittany Valley strike roughly ENE.  This type of stream is called rejuvenated and superposed.  The idea is that a meandering stream with a broad floodplain was established 10 million years ago on an erosion surface coincident with today’s ridgetops (see stop #1).  The region was then uplifted relatively rapidly.  The meandering stream rapidly cut down, keeping its meanders, but without establishing much of a floodplain.  Another characteristic of a rejuvenated stream is that its gradient is not in balance everywhere, so rapids alternate with quiet stretches.

     The Susquehanna River north of Harrisburg is spectacularly superposed on, and cuts through, a series of ridges.  The Susquehanna, although wide and shallow, really has very little floodplain at Harrisburg.

     The Spring Creek Canyon is best developed east of here, on Rockview SCI land.  It supports a microecology with lush plant and animal life, now seen only by a few.  This land is scheduled to be donated to a governmental or educational entity and made available to the public.

Topographic map of Spring Creek on Rockview SCI property, east of Stop #4.  Note the meanders and the very narrow floodplain.