Natural Features

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~~~~~Introduction missing

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The depth to bedrock also helps determine the suitability of a site for on-lot sewage systems.

Montgomery County is located in the Triassic Lowland and Piedmont Upland section of the Piedmont Physiographic Province, and Lower Merion lies entirely within the Piedmont Upland portion of the formation. The Piedmont Upland area was formed during the Paleozoic era, when southeastern Pennsylvania was then at the edge of an eastward extending continental land mass. This land area was composed of Precambrian metamorphic and igneous rocks, which are now exposed in the schists and gneisses of the Piedmont Uplands.

Map B-l indicates the three basic geological formations found in Lower Merion based on ground water yields. Major rock formations underlying Lower Merion Township are Granite Gneiss and Granite; Wissahickon Schist; and Granite Gneiss. The Granite Gneiss and Granite formation consists of a combination of granite and allied rocks, whose properties vary widely because of the number of different rocks that are mixed together. This formation usually yields a large quantity of water, and is located in Lower Merion roughly from City Line Avenue to the northern end of Narberth.

The Wissahickon rock formations occur in an area between Narberth and Ithan-Lafayette Roads. This type of rock is characterized by laminations along which the rock may be easily broken (schists), and by coursely bonded metamorphic rock (gneisses). The bonding develops due to the light and dark mineral alterations. Water supply yields are small to moderate.

The Granite Gneiss formation is found from Ithan-Layfayette Roads to Upper Merion Township and is composed mostly of quartz, feldspar and hornblende. Water yields are small to large.

All information in this section has been derived from a report entitled National Environment 1971 by the Montgomery County Planning Commission.


In order to determine the development capabilities and limitations of the township's land area the various soils categories are identified in this section. Soils characteristics are important because they determine the ability of the soil to withstand development and to absorb and filter sewerage. Also extremely important is the ability of the soil to absorb additional water runoff, the degree that soils are damaged by erosion, and the amount of flooding that occurs in flood areas. The following analysis therefore describes the basic traits of the ten different types of soils found in the township.

Generalized Soil Associations

1. Made Land - Glenelg - Chester Association

As noted on map B-2 the entire township contains two of the eleven soil associations found throughout the county. Land which is already extensively covered with development is classified as Made Land because the original soil features have been obliterated by construction activities and grading. The remaining Glenelg and Chester soils are well drained are nearly level to gently sloping. On map B-2 this association covers the southern portion of the township which is completely developed, and the rural areas in the northern part of the township which are outside the stream bed and steep slope areas.

2. Manor - Glenelg - Made Land Association

These are moderately deep and well drained soils underlain by schist and gneiss, and located on hilly uplands. The hilly areas contain steep, wooded slopes, and residential developments on large lots. These soils are adjacent to streams that have deeply dissected the uplands.

In general these soils are not well suited for intensive agricultural uses, and have some limitations for extensive development. Primarily these soils are best suited for parks and open space because they are on steep slopes, contain stony areas, and have moderate to low available moisture capacity.

3. Summary of Generalized Soil Association Features


Figure B-l



Feature Made Land-Glenelg-Chester Manor-Glenelg-Made
Depth Moderate to deep Moderate to deep
Drainage Well drained Well drained
Underlay Schist and Gneiss Schist and Gneiss
Location Undulating uplands Hilly uplands
Slope Level, gently sloping Steep, wooded
Erosion Easily eroded Easily eroded
Agricultural Limitations Slight to moderate Severe
Development Limitations Slight to moderate Moderate to severe



1. Bouldery Alluvial (BO)

These are soils in flood plains and along streams which consist of level or gentle sloping land covered by boulders and stones. Often these soils occur along with the Hatboro and Codorus soils. Bouldery alluvial soils are subject to flooding several times a year. In late fall, winter and spring the water table is at or near the surface. The hazard of flooding prohibits the use of these soil areas for development.

2. Chester Silt Loam (CG)

This type of soil is deep and well drained, and is located on level and gently sloping land. The surface layer is about eight inches thick, and is dark brown. The subsoil, about thirty inches thick is dark yellowish brown, firm when in place but broken into small blocks when disturbed, Typically bedrock can be found at about six feet, although this can vary between five and nine feet. The Chester soils are medium to strongly acid, with moderate permeability and high moisture capacity. They have few limitations when used for residential development, and the hazard from erosion is slight to moderate where the slopes are under three percent. Precautions to prevent erosion should take place when the slope exceeds three percent.

3. Codorus Silt Loam (CH)

These soils, like the bouldery alluvial, occur on flood plains and are unsuitable for development. The surface layer is typically dark brown about eight inches thick, while the substratum contains flakes of mica. Depth to bedrock ranges from three to six feet. Codorus soils are medium acid, with a high water table in late fall, winter and early spring, and are therefore subject to flooding during such periods. Flooding can also occur in summer after storms. While the hazard of erosion is slight because of the gentle slopes, the high water table and flooding make these areas unsuitable for any type of development.

4. Glenelg Silt Loam (GN)

The Glenelg soils are moderately deep to deep soils, well drained, on gentle to moderately steep slopes. The dark brown silt loam is about eight inches thick, while the substratum is about two feet thick. Depth to bedrock ranges from two and one half to five feet. Water permeability is moderate in the subsoil, but becomes moderately rapid in the substratum. Glenelg soils also are moderately to very strongly acid, requiring corrective measures wherever crops and plants are grown. On three to eight percent slopes the closeness of bedrock may make septic tanks unfeasible. In addition, on eight to fifteen percent slopes the surface runoff is medium to rapid, and precautions should be taken to prevent erosion. On slopes from fifteen to twenty-five percent the surface runoff is rapid and the hazard from erosion is severe. Consequently the Glenelg soil is well suited to residential development in the moderate slope ranges, and can receive limited residential development in the steeper areas provided proper site control features are incorporated to protect the soil and prevent erosion.

5. Glenville Silt Loam (GS)

The Glenville soils occur on nearly level and gently sloping land, but permeability is slow which is a detriment for septic tank installations. The topsoil is about eight inches thick, and the substrata is about two feet thick. Depth to bedrock ranges from four to eight feet. These soils are also very strongly acid. The seasonal high water table and the lack of permeability and good drainage is a severe limitation for use in residential developments, especially where septic tanks must be used.

In Lower Merion there are very few areas where this poor development soil occurs to any great extent. Basically there are three pockets of Glenville soil, all in the northern part of the township. Two of the areas are not serviced by municipal sewer systems. The first area of Glenville soil is located at the headwater of Saw Mill Run, between Stoney Lane and Colton Road. It extends southward just over Conshohocken State Road. The second area is the largest pocket of Glenville soil, and is basically associated with Arrowmink Creek. It extends eastward from the Creek to Spring Mill Road, and is bounded on the south by Morris Avenue and on the north by Pine Wood Road. Portions of this pocket extend eastward across Spring Mill Road. The last area of Glenville soil is a narrow strip along the western side of the branch of Mill Creek near Great Spring Road. All of this area is serviced with municipal sewers.

6. Hatboro Silt Loam (HA)

The Hatboro soil is situated in flood plains, and consequently is unusable for development. The surface layer of soil is about ten inches thick, and bedrock can be found at depths ranging from four to eight feet. This soil has a high acid contest and moderately slow permeability. The water table is very high, creating a hazard for flood conditions and all types of development.

7. Made Land (MD)

Made land consists of soil that has been so altered through construction development that the original soil characteristics are no longer identifiable. This usually takes place during grading and leveling of development sites, highway construction, and the building of other structures such as parking lots and sediment basins. Made soils in Lower Merion occupy the southern half of the township where density of development is greatest, and scattered areas elsewhere throughout the township consisting almost entirely of residential developments. However, made land does not always cover an area entirely, and therefore certain conclusions can be made, derived from the original soil characteristics which still have an influence on these built-up areas.

In Lower Merion the made land consists almost entirely of schist and gneiss materials which are found on moderatley sloped land (3 to 8%) and steeply sloped land (15 to 25%). The moderately sloped land contains a large amount of mica, and has a variable depth to bedrock which sometimes crops out at the surface. In depressions and low areas, the water table comes close to the surface during winter and spring. The soil is acid, and erosion can be a problem unless adequate cover is provided.

The steeply sloped made land occurs on narrow areas on hillsides and along streams. Because water runoff is rapid, there is a hazard of erosion taking place. In addition, most of these areas are poorly suited for septic tank systems.

8. Manor Channery Silt Loam (MH)

The manor channery soils are moderately deep to deep soils with a surface layer that is about seven inches thick. Bedrock is about five feet from the surface, but can range from two to ten feet or more. This soil is acid, and has a low natural fertility.

In Lower Merion the soil is found on three slope ranges, 3 to 8 %; 8 to 15%; and 15 to 35%. On the 3 to 8% slopes the soil is moderately permeable and presents only a moderate hazard of erosion. Therefore there are few limitations for development. The 8 to 15% slope areas also have moderately rapid permeability, but since the slopes are greater the runoff is more rapid and the erosion hazard becomes moderate to severe. The varying bedrock also is a detrimental element for septic tanks. The last manor channery soil category, 15 to 35% slopes has rapid water runoff with a subsequently high hazard for erosion. Consequently there are severe limitations for residential development.

9. Manor Very Stony (MN)

Manor very stony lands are actually a sub-category of the manor series, and are characterized by the increased amount of sones in the soil. The stones are flat, about four to twelve inches thick and about fifteen to thirty inches in diameter. Where the slope is 0 to 8% the soil is moderately permeable with a slight erosion hazard. Therefore these areas are suitable for construction and development. In places where these soils occur on 8 to 25% slopes, the hazard of erosion is increased, and the limitations for development become moderate to severe.

10. Stony Land, Steep (ST)

As the name implies these are stony soils that occur on slopes over 25%. Because of the large amount of stone in the soil and the steep slopes, these areas are only suitable for open space and recreational uses.


The ten soils described above have been grouped together by suitability factors into three major categories and plotted on map B-3. The Hatboro, Codorus, Stoney and Bouldery Alluvial soils form one group because these soils are associated with flood plains and are not suitable for development. Map B-3 indicates that all of these poor soils are situated in and immediately adjacent to most of the streams in the township. Development in these areas should be prohibited.

The second mapped category comprises the Chester, Glenelg, Glenville, Manor Channery and Manor Very Stony soils, These soils are most suited for development, and are basically located in the land areas between the stream locations in the northern part of the township. The degree of development potential varies, with the more steeply situated land being less capable of receiving intensive development.

The last mapped category is Made Land. The bulk of this land is situated in the southern portion of the township which contains the largest percentage of the township's population. As noted above, although these soils contain development, they can still be severely eroded where not covered by man made activities.


All of the soils contained in the township have been mapped as to specific locations. These maps are contained in a publication called Soil Survey, Montgomery County, Pennsylvania, 1967, by the U.S. Department of Agriculture. The maps are too extensive to contain in this plan, but an example is included in map B-4 to illustrate the level of detail and information available. The reader should refer to this source for specific data on subject sites.


The characteristics of all the soils found in the township have been summarized in Figure B-2 to illustrate limitations to development. The major limitations consist of steep slopes, erosion, flooding and lack of on-site sewerage capability.


Figure B-2




BO Bouldery Alluvial Severe, flooding, boulders Severe, flooding, high water table
CG Chester Silt Loam Slight Slight
CH Codorus Severe, flooding Severe, flooding, high water table
GN Glenelg 3-8% Slight Moderate to Severe
GN Glenelg 8-15% Moderate Moderate to Severe
GN Glenelg 15-25% Moderate Severe
GS Glenville Moderate, season high water table Severe
HA Hatboro Severe, flooding, high water table Severe, flooding, high water table
MD Made Land Slight to Moderate Moderate to Severe
MH Manor Channery 3-8% Slight Slight to Moderate
MH Manor Channery 8-15% Moderate Moderate to Severe
MH Manor Channery 15-35% Moderate to Severe Severe
MN Manor Very Stony 0-8% Moderate Severe
MN Manor Very Stony 8-25% Severe Severe



A flood plain can be defined as an area subject to frequent, periodic flooding, and delineated as alluvial soils by the Soil Conservation Service, and as to elevations in the township flood plain ordinance. Map B-5 indicates the flood plain areas of the township, which are a combination of soil data and flood plain locations as noted on the township's existing flood plain maps. The latter map has been compiled to implement the flood plain ordinance, and designates a set distance on both sides of all streams as flood plains. The flood plain ordinance essentially prohibits all major development on these flood plain locations, and provides for administrative remedies for redefining the locations on a lot by lot basis. Thus the flood plain ordinance and map insures the protection of these environmental areas, and decreases the potential loss to man from damage by flooding.

Map B-5 also indicates how these flood streams collect water from minor and major drainage basins. The township is divided into two sections which flow into different drainage basins. The line separating these two areas runs roughly along Montgomery Avenue from City Line Avenue to the northern end of Narberth. From there it crosses over to Lancaster Avenue and continues through Bryn Mawr to Delaware County. All of the lands on the south side of this line drain into the Darby Creek Drainage Basin, and the lands on the north side flow into the Schuylkill River Basin. In addition, the Schuylkill River drainage portion is further divided into minor drainage basins. The map indicates four separate minor basins, with the Mill Creek basin being the most extensive system.

The national protection area noted on map B-5 is derived from the Montgomery County Planning Commission report entitled National Environment Protection Area Study, 1973. These areas are listed as unprotected concentrated amenity areas, and are recommended for future preservation. Basically this protection area consists of the steep slopes around the northern edge of the township and three of the major stream valley areas.

The last item shown on Map B-5 is a ground water recharge area that is being overpumped. That is, more water is being drawn from this area than is being fed back into the ground from natural sources. The area is in the northern corner of the township, and is just one part of an extensive recharge area that continues into other parts of the county. The water supply is being drawn from a point outside the township, but this corner of Lower Merion helps feed this particular recharge system. The data source for this information is the Comprehensive Water Quality Management Plan for Lower Delaware and Schuylkill River Basins, prepared in February 1975 by Chester-Betz Engineers for the Pennsylvania Department of Environmental Resources. Since this corner of the township is sparsely developed, and no major water runoff systems have been designed to channel water runoff into other areas, it appears that Lower Merion is not.contributing to this water supply problem. Rather it appears that more water is being pumped out than is normally contributed from ground recharge systems in Lower Merion Township.

Overall map B-5 illustrates that most of Lower Merlon's water systems are located in areas of least development. This affords the opportunity to institute or continue measures to protect these resources along with compatible land use development in the future.