A Natural History of Millbrook Marsh,
A Wetland In An Urbanizing Setting

Chapter 4
Geology
and soils

Physiography

Centre County exists partly in the physiographic Ridge and Valley province and partly in the Allegheny Mountain section of the Appalachian Plateaus province. The dividing line between the two provinces is the Allegheny Front, along the north side of Bald Eagle Valley. Millbrook Marsh is located in the Appalachian Mountain section of the Ridge and Valley province, in Nittany Valley, not far from the Allegheny Front. The provinces are defined by the bedrock and formation processes and each varies with regard to wetland types and frequency (Westerfield 1959, Davis 1993). The Ridge and Valley province has valleys of limestone and ridges of more erosion resistant sandstone. The Nittany Valley, as the others of the province, is underlain with limestone bedrock.

The existence of wetlands depends upon a variety of contributing factors, which also determine the type of wetland and its corresponding plant and animal communities. K. Bushnell (1989) entertained the question "Are wetlands dependent on or independent of the underlying geology?" Impermeability of the underlying substrate is one instance that results in a wetland. Another is the impermeability of the wetland deposits themselves, in the case of some fens, for example, peat deposits (Novitzki, 1989.) The fen at Millbrook Marsh exists because of a combination of characteristics of the site, including hydrology and geology. The limestone bedrock beneath the soil leaches calcium as the spring water continuously passes through, resulting in calcareous or alkaline, permanently saturated conditions at the surface. Often there are high concentrations of magnesium as well as calcium in the soils of fens (Western Pennsylvania Conservancy 1995).

The bedrock underlying Millbrook Marsh is comprised of interbedded limestones and dolomites, and groundwater occurs along joints and fractures, bedding planes, and solution cavities. Joints and fractures result from the structural deformation while solution cavities are caused by the chemical actions of groundwater. Two well-developed fracture traces, important as controls of groundwater movement, were found in the Millbrook Marsh area and intense solutioning is suggested by the large Bathgate Spring near the Axemann-Nittany contact (Clark 1965). The solution and joint interconnections seem to be well developed, which probably indicates a groundwater level in Millbrook Marsh essentially at the level of the topography, rather than perched. Slab Cabin Run separates Bellefonte Dolomite and Axemann Limestone beds. Axemann Limestone occurs on the west side of Slab Cabin Run and is characterized by a dominance of either calcilutite or calcarenite. It is mottled and banded with dolomite. Both Thompson Spring and Bathgate Spring are located in this formation. The numerous springs at the fen also issue forth from the Axemann Limestone formation. East of Slab Cabin Run is Bellefonte Dolomite, with thin sandstone and limestone occurrences in some areas (Clark 1965).

There is a significant amount of relief, elevation ranging from 290 to 365 m (950 to 1200 ft), on the Nittany Valley floor. The elevation of Millbrook Marsh ranges from 288 m (946 ft) above sea level to 290 m (950 ft) in the marsh area but up to about 297 m (975 ft) where Millbrook land abuts the Mt. Nittany Expressway. There are deeper soils on fairly flat upland areas than on slopes. In addition, an integrated drainage network goes headward into the surrounding upland areas. This is descriptive of a fluvial landscape, although when combined with the limestone and sinkholes, it forms a karst topography. Karst topography on the fluvial landscape is known as a fluvio-karst area (Agronomy Series 64 1980).

Soils

The geological conditions provide a partial explanation for the existence of the calcareous fen. Fens typically are formed by organic matter accumulation in wetlands primarily fed by groundwater that is high in calcium and bicarbonate ions. Calcite (CaCO3) is often present in the histosols, creating an alkaline reaction and a very mineral rich condition of the calcareous histosols, unlike most organic soils which are acidic (Richardson 1987). The soils of the Millbrook Marsh calcareous fen were sampled in 3 areas (Western Pennsylvania Conservancy 1995). The very wet open fen area had a peat depth of 0.40 m ranging in decomposition from H1 to H9, where H1 is completely undecomposed and H10 is completely decomposed. An interesting point is that the decomposition levels of peat are not found sequentially from the surface to the transition zone before clay. H1 peat was found to 0.10 m, then H2 to 0.25 m, H8 to 0.30, H6 to 0.35 m, and before the transition zone, H9 to 0.40 m. There was silty clay from 0.5 m to 0.65 m and clay from 0.65 m to 0.85 m. The wet open fen area had 0.20 m of H3 peat over a transitional zone from 0.20 m to 0.60 m, then muddy clay from 0.60 m to 0.75 m. and silty clay to 1.00 m. The wet shrub fen area had peat to 0.65 m with H3 peat at the surface to 0.10 m, H4 to 0.20 m, H8 to 0.35 m, and H3 again to 0.50 m. Silty clay was found from 0.65 m to 0.85 m and then silty clay and gravel to 0.90 m.

The residual soils cover the carbonate rocks, the interbedded limestones, dolomites, and sandy dolomites of the Nittany Valley. Chemical decomposition lowers the surface of the rocks and the residuum ranges in thickness up to 30 m (100 ft) on top of them. The soil series found at Millbrook are Hagerstown/Opequon, Dunning, Lindside and Melvin soils (Map 8). Hagerstown and Opequon soils are residual soils developed from limestone and dolomite (Ciolkosz et al. 1980). They have a high capacity for holding moisture and moderately rapid permeability. The other soils on site are alluvial. Dunning soils developed in alluvial materials from uplands of limestone and have the water table at the surface or the soils are ponded for most of the year. They are fine textured with slow permeability and hence, very poorly drained. The Lindside and Melvin soils are probably only 200 to 300 years old and due, most likely, to the erosion caused by logging for charcoal in the early 1800’s (Ciolkosz et al. 1980). They were formed in recent silty alluvium also from limestone upland areas. Both the poorly drained Melvin Series and moderately poorly drained Lindside Series are considered by the Soil Survey Division to be hydric soils (USDA, NRCS 1995).

The soil types occurring on site are depicted in Map 8. Along parts of Thompson Run is a swath of Opequon-Hagerstown, a steeply sloping section with rapid runoff rates due to a grade of 15 to 25%. There is high to moderate erosion potential for this area. Depth to bedrock is shallow and there are limestone outcrops throughout.

At the junction of Thompson Run and East College Avenue, as well as most of the land surrounding the corridor of Slab Cabin Run and Bathgate Spring Run, soils consist of Melvin Silt Loam. Since the slope is merely 0 to 2%, runoff rates are very slow, as are rates of erosion. There is frequent flooding and ponding here, and the water table is seasonally high. This type of soil is commonly found in the flat floodplain of limestone valleys.

To the west of Thompson Run, and on parts of the area previously known as Farm 12, is Hagerstown Silt Loam with a gentle 3 to 8% slope. This soil type and slope is moderately prone to erosion and runoff rates are moderate as well. Sinkholes and clay pans are not unusual for this classification found in valley floors of limestone uplands. Sections of 3 to 8% sloping Hagerstown Silt Loam are found near the Mt. Nittany Expressway as well as along the south edge of the site adjacent to East College Avenue. Some of this section, especially that area that contains the calcareous fen, is probably fill of an as yet unidentified type (PennDOT 1981). It appears that groundwater is filtering through a good portion of the fill, yet continues to support the fen vegetation of Millbrook Marsh. The fen and surrounding vegetation may benefit from removal of this cover of fill and rubble.

Directly adjacent to the Mt. Nittany Expressway is a portion of Hagerstown Silt Loam with a slope of 8 to 15%, yielding moderate to rapid runoff rates and erosion levels. Clay pans and sinkholes are possible here as well. Beside it, also adjacent to the bypass, is Opequon-Hagerstown with a slope of 8 to 15%. The limitations are the same as those for Hagerstown Silt Loam, though in addition, depth to bedrock is shallow.

There is a portion of Opequon-Hagerstown with a severe slope of 25 to 90% along the same edge of the site in which runoff is rapid and erosion high. This area has a very shallow depth to bedrock, and here too, clay pans and sinkholes may occur.

In the triangular area between Thompson and Slab Cabin Runs is found Dunning Silty Clay Loam with a slope of just 0 to 2%. Runoff is very slow and erosion probability is slight except in flood situations. This is, though, a floodplain soil, with potential ponding, frequent flooding, and high water table. Also in this triangular section, on both sides of Slab Cabin Run, are Lindside Soils with a 0 to 2% slope. Lindside Soils are found in the flat areas of floodplains in limestone valleys. Runoff is slow due to minimal grades and erosion occurs only in flood situations. As with the Melvin Silt Loam, Lindside has a seasonally high water table.

One section along the southern edge of Millbrook Marsh is classified as Urban Land. Soil identification was not possible. Runoff is rapid and sinkhole formation a possibility.

Discussion

In general, due to a combination of overland water flow, numerous springs, seasonally high water tables, and flooding regimes of varying duration, much of the site consists of hydric, anaerobic soils. As stated earlier, portions of Millbrook Marsh have maintained relatively intact wetland functions and characteristics despite being farmed, possibly tile drained, or covered with fill.

It is expected that the soils in the fen differ from the other emergent wetland soils by exhibiting lower bulk density, higher water holding capacity, lower hydraulic conductivity, lower percent ash and high organic matter content, higher organic nutrient content and high cation exchange in the calcareous fen soils. There should be very high Ca, Mg, and Na content (Richardson 1978). Ion concentrations of water samples will reflect bedrock and soil composition of the fen (Table 4-1). Because of the soil differences, the conductivity, and the ion concentrations in fen water due to the soils and bedrock, previous extent of the fen, before the fill was added, may be delineated. It is possible that the full potential of Millbrook Marsh including the calcareous fen component is not being realized now, but it is not irreparably damaged at this time.

Table 4-1
Water Analysis Data for 3 Locations in Millbrook Marsh Calcareous Fen
(Western Pennsylvania Conservancy 1995)

     

Ions

Fen Type

pH

Conductivity

Ca

Si

Fe

Sr

Mg

K

Na

Very Wet Open

7.6

800

60.2

8.7

0.1

0.5

24.5

31.7

18.2

Wet Open

7.5

860

94.9

6.1

0.0

0.7

11.9

28.7

28.9

Wet Shrub

7.2

710

105.7

5.0

0.0

0.7

10.0

40.5

27.2

  On to Chapter 5