GEOLOGY

VOL. 32, NO. 3/4

CONTENTS

Secret information, public information ...................................................... 1Rockslide in Sunbury, Pennsylvania, on January 9, 2002........................ 2“Fossil” and recent landsliding in Allegheny County ........................................ 8The Geology of Pennsylvania—Back by popular demand........................ 15

ON THE COVER

Interbedded sandstone, shale, and limestone of the Reedsville Formationalong Interstate Route 80, Milesburg Gap, Centre County. The rocks havebeen tilted from their original horizontal orientation. Sandstones (both lightand dark) are lenticular, crossbedded, and hummocky bedded, have ero-sional upper surfaces, and in some places are size graded. The thick, light-colored bed at the left is fossiliferous limestone (from A. M. Thompson, chap-ter 5, Ordovician, in The Geology of Pennsylvania) (see p. 15 in this issue).

PENNSYLVANIA GEOLOGY

PENNSYLVANIA GEOLOGY is published quarterly by the Bureau of Topographic andGeologic Survey, Pennsylvania Department of Conservation and Natural Resources,3240 Schoolhouse Road, Middletown, PA 17057–3534.Editors: Anne B. Lutz and Jay B. Parrish.Contributed articles are welcome. Guidelines for manuscript preparation may be obtainedat www.dcnr.state.pa.us/topogeo/mag-ins.htm or by contacting the editors at the addresslisted above.Articles may be reprinted from this magazine if credit is given to the Bureau of Topographicand Geologic Survey.

VOL. 32, NO. 3/4 FALL/WINTER 2002

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COMMONWEALTH OF PENNSYLVANIAMark Schweiker, Governor

DEPARTMENT OF CONSERVATION AND NATURAL RESOURCESJohn C. Oliver, Secretary

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Pennsylvania home page: www.state.pa.us

STATE GEOLOGIST’S EDITORIAL

Secret Information, Public Information

While I was working on my doctorate, I was required to study aforeign language. I took Russian, and I translated articles on geobotany.It was a fascinating experience, but frustrating in one way: I neverknew the location of the study I was reading about, because the So-viet Union viewed all maps as secret. Thus, these articles were pub-lished with no geographic information—no scale, no city names, no rivernames, nothing.

What should be secret and what should be public information?I was once warned that it was a security danger to digitally locate firehydrants. However, the data are already available because the hy-drants are readily visible from the street. Topographic maps are likethat in some ways. You can’t hide a hill or a stream; they are there foranyone to see. The map just reflects what you can see on the groundand makes it easier to see everything at a glance. It also containsmuch more information. Maps have been around for decades with norestrictions.

In many ways, we are all safer if the basic information neededby first responders or a caring neighbor is freely available. The localfireman can find that fire hydrant. The environmental scientist will

know if a town’s water supply is threatened.Our country has a long history of freedomof information. Librarians have always sidedwith the concept that information should befreely available. It comes down to a matterof how easily accessible the data shouldbe to you, the citizen.

Jay B. ParrishState Geologist

Rockslide in Sunbury, Pennsylvania, on January 9, 2002

by Jennifer M. ElickSusquehanna University, Selinsgrove, PA

INTRODUCTION. At 5:00 p.m. on January 9, 2002, a rockslide oc-curred in Upper Augusta Township, Sunbury, Pa., along State Route4012, also known as Snydertown Road (Figure 1). The slide took placeon a hill north of Shamokin Creek and the Sunbury Community Hos-pital. Engineers on the scene estimated that up to 6,000 tons of rockslid downhill from the roadside exposure, crashed through the guard-rail, and crossed over the yellow dividing line of the road (Figure 2).The road was closed for three days while Pennsylvania Departmentof Transportation workers cleared the debris and attempted to de-termine whether additional rocks on the slope continued to pose athreat to the public.

Rockslides such as this one have occurred in the past and arecommon in the Ridge and Valley physiographic province of Pennsyl-

2

Figure 1. Location of the landslide area near Sunbury, Pa. (A). The heavy linesare geologic contacts between Devonian units (from Berg and Dodge, 1981).The slide occurred in the Trimmers Rock Formation (Dtr), which is overlain bythe Irish Valley Member of the Catskill Formation (Dciv) and underlain by theHamilton Group (Dh). Similar rockslides involving the Trimmers Rock Forma-tion have occurred in the vicinity of the Kmart department store (B).

A

B

Dciv

Dh

Dh

Dtr

Dtr

0.8 KM0

0.5 MI0SCALE

76°49´10´́ 76°46´03´́

40°51´08´́

40°52´10´́

vania. The factors that contribute to their occurrence are the humidand variable climate, the steep slopes and rugged topography, the ero-sion and weathering characteristics of the bedrock, and the modifi-cation of landscape features by humans (Hamel and Ferguson, 1999).Engineers have estimated that occurrences of slides of similar mag-nitude take place in this area about every 10 years. Other rockslidesin the region, such as the three slides along U.S. Route 15 in GreggTownship, Union County, above the West Branch Susquehanna River,also caused temporary highway closings (Delano and Wilshusen,1999). These slides were related to extensive highway widening, andthe types of failure were documented as wedge failure (where theslip surfaces are two or more intersecting surfaces), planar (wherethe slip surface is a single plane, commonly a bedding plane), androck topple (where the moving material rotates forward out of theslope) (Delano and Wilshusen, 1999). The rockslide that occurred in

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Figure 2. West-facing view of the rockslide along Snydertown Road in Sun-bury, Pa. The Trimmers Rock Formation has a strike of N74°E (shown by theline having arrows on both ends) and a dip of 48 degrees to the north, awayfrom the road (indicated by the short line). The dip directions of the threemajor joint sets are indicated by the numbered lines. Slabs of rock broke alongjoint sets 2 and 3 and slid along the surfaces of joint set 1 into the road.

1

2

3

Sunbury is considered planar and translational (the material did notrotate as it slid down the slope).

Rockslides and other mass movements are mechanisms by whichmuch of the landscape in Pennsylvania is shaped; they are also geo-logic hazards to the unsuspecting public, causing damage costingmillions of dollars.

THE GEOLOGY. The rocks along Snydertown Road belong to theTrimmers Rock Formation (Upper Devonian, Frasnian), a successionof interbedded dark-gray shale and thin beds of light-gray- to olive-colored fine-grained sandstone(Figure 1). These rocks formpart of the northern limb of theSelinsgrove anticlinorium, adoubly-plunging structure ori-ented northeast-southwest.The rocks at the slide sitestrike N74°E and dip 48 de-grees to the northwest. Theydip away from the east-west-oriented road (Figures 3 and4). There are at least threejoint sets in the slide exposure(Figures 2–4), which providethe weak planes along whichrock movement occurs. These joints are extensional, well developed,and moderately to closely spaced; they create a blocky pattern andsteep slopes (Geyer and Wilshusen, 1982).

During the landslide, large rock slabs up to 15 feet long slid alonga steeply dipping (51 degrees), south-facing slope (joint set 1 in Fig-ures 2–4). Although the strike of this plane is nearly identical to thatof the bedding plane, joint set 1 dips in the opposite direction of thebedding. This slope exceeds the angle of repose, the maximum angleat which loose material remains stable. Therefore, loose material tendsto slide or roll down the slope. Joint set 1 is oriented nearly parallelto Snydertown Road at the rockslide location. As rock material slidesdown this south-facing plane, it can move onto the road. Joint sets2 and 3 cut across the bedding plane and have mean orientations ofN3°E and N19°W (Figures 2–4); these joint sets provide fracture sur-faces that make the rock internally weak. Large slabs of rock breakloose where these fractures intersect. Under the influence of gravity,these slabs can move down the surfaces formed by joint set 1 andonto the road (Figures 3 and 4).

4

STEREOGRAMS. Geologists showthree-dimensional representationsof the trends of beds, joints, andother planar structural features ona circular diagram called a stereo-gram. Stereograms are made byplotting measurements of the bear-ing of the structure (a joint set, forexample) at several localities. Plot-ted points form clusters when theybelong to the same structural fea-ture. In Figure 3, the trends of thethree distinct joint sets and the rockbedding are shown.

CLIMATE. A few days before the rockslide occurred (January 6 and7), 4.4 inches of heavy, wet snow accumulated in the Sunbury area.On January 9, a warm air mass moved through the region, and bythe afternoon, much of the snow had melted. The weight of remain-ing snow and water on the unstable rock increased the probability ofshear failure on the steep slopes of joint set 1. Excessive amountsof water moving along the joints and softening the clay may haveprovided a trigger for the slide.

The maximum temperature reached 46°F in the afternoon, andthe minimum temperature that night was 33°F. Such a range in dailytemperatures can cause rocks to expand during the day and con-tract as the temperature falls just after sunset. Temperature changesalso produce freeze-thaw cycles that affect water that is present incracks in the rocks. These cycles result in changes in volume of thewater, which can gradually push the rocks apart. These minor move-ments of the rock, in combination with its extra weight and reduced

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Strike of rockunits

EXPLANATION

Strike of jointsin joint set 1

Strike of jointsin joint set 2

Strike of jointsin joint set 3

Mean orientationof joint set

N

Figure 3. Stereogram showing the orientation of the beds in the TrimmersRock Formation (shaded semicircle) and the mean orientation of the joint sets(bold lines). Joint sets 2 and 3 intersect the bedding planes of the TrimmersRock Formation, and joint set 1, oriented toward the south, provides the slopealong which the slabs slide onto the road.

strength due to the presenceof water, may have resultedin the occurrence of the rock-slide.

OTHER FACTORS. The re-cent development of theland on the top of hill wherethe slide occurred may havecontributed to the slide by

causing water from snowmelt to be diverted along certain paths,which resulted in a reduction of the shear strength of the rock. In ad-dition to this, periodic minor rockslides occur simply because of thehigh slope angle and the orientation of the slope toward the road.

DISCUSSION. There were many factors that contributed to the rock-slide in Sunbury, including the local geology, landscape, climate, anddrainage patterns. This slide is of particular interest because it showshow intersecting joint sets can affect and control sliding. In addition,the rapid change in weather just prior to the slide was a strong in-fluence on the behavior of the rocks.

Another example of the influence of rock structure on rockslides inthis area can be observed nearby at the Kmart department store, lo-cated where northbound U.S. Routes 15 and 11 split in Shamokin Dam(Figure 1). At this location, the rock units and their orientation are simi-lar to those at the location of the first slide (Figures 3 and 4). The oc-currence of periodic rockslides along the south-facing wall, along a jointset that has a steep south-dipping orientation, has forced the landownersof the property to eliminate parking spaces in the vicinity (Figure 5). Aretaining wall has been installed to protect stored items in the parking lot.

Sites of potential rockslides similar to the one described in thisarticle can be identified reasonably well through geologic field map-ping. In regions of Pennsylvania where the bedrock is steeply dipping,the location of unstable sites can be identified by evaluating the abun-dance of joints in an exposure, determining the orientation of inter-secting joint sets, and considering man-made structures and excava-tions near jointed rock units (Delano and Wilshusen, 2001). It is alsoimportant to understand the orientation of other planar structures such

6

1

2

3

Slidedebris

Snyderto

wn road

N

Figure 4. Illustration of how jointsand bedding-plane orientationsaffect rocksliding along Snyder-town Road. Numbers refer to jointsets; the heavy arrow indicatesthe slide surface.

as cleavage planes. East-west-oriented roads in this area are par-ticularly susceptible to rockslide due to the occurrence of steep north-and south-facing slopes that exist along east-west-oriented joint sets.

Before structures are constructed, it is imperative that geologistsand engineers understand the landslide history of a region so thatdamage from potential rockslides can be prevented. It is also im-portant to recognize that sudden changes in the local weather mayinduce landsliding.

REFERENCES

Berg, T. M., and Dodge, C. M., compilers and eds., 1981, Atlas of preliminary geo-logic quadrangle maps of Pennsylvania: Pennsylvania Geological Survey, 4th ser.,Map 61, 636 p.

Delano, H. L., and Wilshusen, J. P., 1999, Landslide susceptibility in the Williams-port 1- by 2-degree quadrangle, Pennsylvania: Pennsylvania Geological Survey,4th ser., Environmental Geology Report 9, 192 p.

________ 2001, Landslides in Pennsylvania: Pennsylvania Geological Survey, 4thser., Educational Series 9, 34 p.

Geyer, A. R., and Wilshusen, J. P., 1982, Engineering characteristics of the rocks ofPennsylvania: Pennsylvania Geological Survey, 4th ser., Environmental GeologyReport 1, 300 p.

Hamel, J. V., and Ferguson, H. F., 1999, Landsliding, chap. 48 of Shultz, C. H., ed.,The geology of Pennsylvania: Pennsylvania Geological Survey, 4th ser., SpecialPublication 1, p. 704–711. [Co-published with Pittsburgh Geological Society.]

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Figure 5. The landslide that occurred near the Kmart. To the right of center isa light-colored surface along which the slide occurred; a talus pile is at thebase. The tree growing there may be younger than the slide event. The retain-ing wall on the left is located opposite the store. Before the slide, there weredesignated parking spaces along the outcrop to the right. Now, the yellow lineshave been removed, and parking is discouraged.

“Fossil” and Recent Landsliding in Allegheny County

by Charles H. ShultzSlippery Rock University of Pennsylvania

John A. HarperPennsylvania Geological Survey

A “fossil” landslide in western Pennsylvania, well preserved inLate Pennsylvanian rocks of Allegheny County (Shultz and Harper,1996), is, according to landslide experts from around the world, oneof the world’s oldest preserved major landslides (based on commentsreceived at the 8th International Conference and Field Trip on Land-slides in Granada, Spain, in 1996). Only a quarter of a mile away, aseries of recent (1980s–90s) landslides affecting the same landslide-prone rocks damaged a warehouse and a lot at an industrial park.The juxtaposition of the two landslides, separated in time by approxi-mately 310 million years, is fascinating.

SLIP SLIDING AWAY. Landsliding due to slope failure is a commonenvironmental problem throughout western Pennsylvania. Pomeroy(1982), for example, recognized more than 3,000 recent landslides and12,000 prehistoric (but less than 10,000 years old) landslides in Al-legheny and Washington Counties alone. Many recent landslides arereactivated prehistoric landslides, usually a result of human distur-bances such as construction, excavation, and dumping of fill materials.

Western Pennsylvania landslides occur primarily in soils thatform unstable slopes and that are derived from equally unstable bed-rock. Such landslides typically are small, less than 200 feet in thegreatest dimension, only a few feet thick, and less than 7,000 cubic feetin volume. However, some very large landslides occur almost everyyear, affecting private and public property, highways, and buildings.Loss of life is a very rare event, most often associated with large rock-falls, but property damage can be enormous.

Slopes underlain by the notorious Pittsburgh red beds (Figure 1)are particularly vulnerable to failure. Readers familiar with the Pitts-burgh area will readily recognize these brick-red rocks and the soilsderived from them from numerous exposures in southwestern Penn-sylvania.

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RECIPE FOR DISASTER. The Pittsburgh red beds consist of approxi-mately 30 to 50 feet of red, green, and gray claystones and minoramounts of sandstones, siltstones, and shales. Good exposures alsodisplay occasional land-plant root casts, calcite-rich nodules, andother evidence of ancient soil development. Like shales, claystonesare composed of clay minerals. But claystones lack fissility, that char-acteristic of shales that allows you to split the rock into thin, flat frag-ments. The microscopic clay minerals in claystone are oriented ran-domly, rather than in a single plane as in shale. Claystones have con-siderable pore space that can hold water, but the pores are not wellconnected (the rocks have low permeability), so any water in the rockbecomes trapped. This can cause excessive pore-water pressure thatleads to reduced shear strength internally. In addition, many claystonescontain minerals that expand in the presence of water (Pomeroy,1982). Add water and these claystones literally push themselvesapart from the inside out. You can try it. Place a piece of Pittsburghred claystone in a glass of water and watch it fall apart into a massof red clay on the bottom of the glass. Because of this tendency, thered beds tend to lose strength during seasonal wet-dry and freeze-

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GROUP FORMATION

GENERALIZEDGEOLOGICSECTION

INDIVIDUAL BEDSOR MEMBERS

Conemaugh

Casselman

Glenshaw

Monongahela Pittsburgh

Allegheny

Pittsburgh coal

EXPLANATION

Red beds

Limestone

Sandstone

Coal

Shale andsiltstone

Upper Freeport coal

Upper Pittsburgh limestone

Lower Pittsburgh limestone

Connellsville sandstone

Clarksburg coal

Bakerstown coalUpper Saltsburg sandstoneWoods Run limestoneNadine limestoneLower Saltsburg sandstone

Pine Creek limestone

Buffalo sandstone

Brush Creek limestone

Upper Mahoning sandstone

Mahoning coalLower Mahoning sandstone

Pittsburgh red beds

Rocks exposed atBakerstown Stationrailroad cut

Ames Limestone

Duquesne coalBirmingham shale

Wellersburg coalMorgantown sandstone

Figure 1. A simplified stratigraphic section illustrating the Conemaugh Group,which includes the Pittsburgh red beds and other bedrock, at the BakerstownStation fossil landslide site (from Shultz and Harper, 2000).

thaw cycles. As aresult, most locallandsliding occursin the spring andearly summer, es-pecially after a par-ticularly wet winter.

A LANDSLIDE ATFAULT—OR AFAULT AT LAND-SLIDE! A large fos-sil landslide can beseen along the CSXrailroad tracks atBakerstown Station,just off BakerstownRoad (Figure 2) ap-proximately 1.8 mileswest of Pa. Route 8at Bakerstown innorthernmost Alle-gheny County (Fig-ure 2). The 90-footdeep, north-southrailroad cut was ex-

cavated around 1915 to bypass a single-track tunnel. The tunnel cutsthrough the ridge that forms a drainage divide between the Pine Creekwatershed in Allegheny County and the Connoquenessing Creekwatershed in Butler County. The bedrock at this locality consists ofstrata near the middle of the Conemaugh Group, from the fossilifer-ous Ames Limestone upward into the Morgantown sandstone (Fig-ure 1). The Ames Limestone and underlying Pittsburgh red beds arenow buried under talus, but soil and rock debris could be seen whenthe cut was relatively fresh (Ross, 1933). The authors found a fewpieces of Ames at the north end of the cut, and the red beds areclearly present a quarter of a mile north of the railroad cut. The thick-est and/or most prominent units exposed in the cut are the Duquesnecoal, Birmingham shale complex, and Morgantown sandstone of theCasselman Formation.

The fossil landslide consists of a series of listric (curved) normalfaults bounding about eight fault blocks containing upper Glenshaw

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ALLEGHENYCOUNTY

Pitts-burgh

Location of area

BAKERSTOWN

ROAD

Bakerstown

0 10 MI

0 15 KM

SCALE

8

,79

,79

,376

,279

,279

x

Ohio RiverAllegheny

River

MonongahelaRiver

Youghio-ghenyRiver

Figure 2. Location (indicated by the X) of the Bakers-town Station fossil landslide in northern AlleghenyCounty.

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Figure 3. Fault blocks of Birmingham shale and sandstone (B). Birmingham bedsare dipping south (right) on the east wall at the deepest part of the railroad cut.The arrows indicate the upper part of the steeply dipping fault. Note the infillsediment (I) in the swale between the blocks, and the angular unconformity (U)below the horizontal Morgantown sandstone (M) at the top of the photograph.Scale is 1 meter (3.28 feet).

and lower Casselman rocks (Pittsburgh red beds, Ames Limestone,Duquesne coal, and the Birmingham shale complex). Small amounts ofthe lower Morgantown sandstone also occur on the blocks. The con-tact of the fault blocks with the Morgantown is a prominent angularunconformity.

The faults dip to the north, curving upward until they are nearly ver-tical at the tops of the blocks (Figure 3), and producing concave-upwardsurfaces (Figure 4). The fault planes strike generally east-west in thebest-exposed faults. The dip of the tilted fault blocks ranges from a fewdegrees to as much as 45 degrees (Figure 5). Numerous small, south-dipping normal faults (and even small reverse faults) occur in the overly-ing Morgantown sandstone, representing local adjustments in the land-slide complex (see below). Offset along the main faults is approximately15 to 30 feet in the Birmingham shale and 3 to 6 feet in the basal partof the Morgantown sandstone. The main body of the Morgantown ex-hibits normal, relatively horizontal channel-sandstone bedding, indicatingthat it was unaffected by faulting. Concave, lens-shaped bodies of sandy

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Figure 4. The lower part of a listric normal fault (at arrows), showing the con-cave-upward curve on the west wall at the deepest part of the railroad cut. Thefault apparently becomes parallel to bedding to the right of the photo, but it isobscured. The blocky sandstone in the upper third of the photo is basal Morgan-town sandstone (M), which has been offset by about 6 feet. The scale (at thelower right) is 1 meter (3.28 feet).

Figure 5. A fault block composed of Birmingham shale (B) in the center of thephotograph. Dip is about 45 degrees. Note that the horizontal Morgantownsandstone (M) above is in angular unconformity with the slide complex below.Also note the lens-shaped infilling of sediment (I) on the down-dropped back-side of the fault block to the left. The view is to the west, and the scale (nearthe center bottom) is 1 meter (3.28 feet).

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sediment, commonly having a breccia-like aspect and containing thinlenses of coal, have filled in the swales between fault blocks (Figure 3).

A FUNNY THING HAPPENED ON THE WAY TO THE PERMIAN . . .Figure 6 illustrates a hypothetical sequence of events explaining the fea-tures observed at the Bakerstown Station cut. In late Birmingham time,a great meandering river lay north of the site. It had eroded through theAmes Limestone and into the Pittsburgh red beds (Figure 6A). The cutbank was on the south side of the meander, so there would have beenno support for the thick accumulation of semi-consolidated sediment ad-jacent to the river. Then, some kind of catastrophic failure occurred. We

EXPLANATION

Morgantown sand,having basal pebbleand peat deposits

Morgantown clay

Laminated Birminghamsilt and clay

Birmingham sand

Birmingham grayclays

Birmingham dark,nodular clays

Duquesne peatbed

Silt, clay, andGrafton sand beds

Ames lime mud

Pittsburgh red clays

Graben

Listric normal fault

Décollement

S N

A

B

C

D

River

Figure 6. Diagrammatic sections of a hypothetical sequence of events in thedevelopment and preservation of the tilted fault blocks at Bakerstown Station(from Shultz and Harper, 2000). See the text for an explanation.

don’t know for certain what caused the grand-scale failure at BakerstownStation, but since the Alleghanian orogeny was under way 250 to 300miles to the east, seismic activity is a possible explanation. As a result,a large slab of land moved along a slip plane (décollement) on or withinthe Pittsburgh red beds. The slab rapidly segmented along faults into aseries of blocks (Figure 6B). At that time, the area may have resembledthe Turnagain Heights slide that occurred following the Good Fridayearthquake in Alaska in 1964 (see Grantz and others, 1964, Figure 17).

After the landslide, the river partially reestablished itself and cov-ered much of the area. The high ridges on the fault blocks eroded,and the river filled the depressions with deposits of gravel, sand,mud, and peat (Figure 6C). The weight of sediments deposited onthis inherently unstable mass caused some reactivation of the faults,perhaps again triggered by earthquake activity. The faults propagatedupward into the sand and mud that was to become the basal part ofthe Morgantown (Figure 6D). Finally, the great river covered the en-tire slide complex in later Morgantown time, depositing a thick massof undeformed channel sand over the whole area.

IRONY IN THE STEEL CITY. Situated about a quarter of a mile northof the fossil landslide exposure is an industrial complex having abuilding and a parking lot that experienced modern landsliding afterthey were constructed in 1980. This site originally consisted of swampyground below a 130-foot-high slope. The toe of this slope, which iswithin the Pittsburgh red beds, was excavated for about 600 feet alongits length. After excavation, the slope was seeded with crown vetchto retard erosion.

Slope failure began one or two years after completion of con-struction. The landslide was about 200 feet wide and had a lobeliketoe and a crescent-shaped crown scarp about 20 feet high. By 1996,the toe of the slide was about 3 feet high and had moved about 30feet out onto, and beneath, the asphalt paving of the parking lot. Ithad high moisture content, hummocky topography, and sag pondsthat contained cattails. An asphalt driveway between the slope andthe building had closed to less than 7 feet, which prohibited vehicleuse. The driveway asphalt was domed, folded, and lofted severalinches up against the building. A reinforced concrete foundation andpaving slabs showed indications of brittle failure, including tensioncracks and compressional faults having small displacement. Somerivets had popped in the corrugated steel wall of the building, leav-ing open gaps that exposed the building interior. Doors were bentand jammed. The interior concrete floor had buckled and cracked, and

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some of the concrete slabs were slightly offset vertically, suggestingthe presence of a slip plane (décollement) beneath the building. Dam-age to the site finally was repaired in late 1996 and 1997.

Even after a 310-million-year hiatus, the Pittsburgh red beds stillare mechanically very unstable and will move whenever an unsup-ported face is created. This is especially true where a load has beenplaced on it or a relatively stable toe has been cut out, whether by aPaleozoic river meandering through the countryside or by Holocenehumans building roads, buildings, and parking lots.

REFERENCES

Grantz, Arthur, Plafker, George, and Kachadoorian, Reuben, 1964, Alaska’s GoodFriday earthquake, March 27, 1964—A preliminary geologic evaluation: U.S. Geo-logical Survey Circular 491, 35 p.

Pomeroy, J. S., 1982, Landslides in the Greater Pittsburgh Region, Pennsylvania:U.S. Geological Survey Professional Paper 1229, 48 p.

Ross, R. B., 1933, Structures in the Conemaugh Formation near Bakerstown Station,Pennsylvania: Pittsburgh, Pa., University of Pittsburgh, M.S. thesis, 13 p.

Shultz, C. H., and Harper, J. A., 1996, Pittsburgh red beds cause renewed land-sliding after a ca. 310 Ma pause, Allegheny County, Pennsylvania, USA, in Chacón,José, Irigaray, Clemente, and Fernández, Tomás, eds., Proceedings of the 8th In-ternational Conference and Field Trip on Landslides: Granada, Spain, 27–28 Sep-tember, 1996, Rotterdam, A. A. Balkema, p. 189–196.

________ 2000, Stop 8, Bakerstown Station railroad cut, in Harper, J. A., ed., Pitts-burgh at the millennium—the impact of geoscience on a changing metropolitanarea: Annual Field Conference of Pennsylvania Geologists, 65th, Pittsburgh, Pa.,Guidebook, p. 125–130.

15

SPECIAL PUBLICATION 1

The Geology of Pennsylvania—Back by Popular Demand

The Bureau of Topographicand Geologic Survey is pleasedto announce the reprinting of Spe-cial Publication 1, The Geology ofPennsylvania, copublished by theSurvey and the Pittsburgh Geo-logical Society. When it was firstpublished in 1999, the book be-

came one of the Survey’s all-timebest-sellers, and the edition soldout in less than one year. Continu-ing demand has resulted in an-other collaboration between theSurvey and the Society for a sec-ond printing. Financial support wasagain provided by both partners.

16

The 888-page two-color bookis divided into 10 parts (Introduc-tion, Stratigraphy and Sedimen-tary Tectonics, Structural Geolo-gy and Tectonics, Regional Geo-physics, Physiography, GeologicHistory, Mineral Resources, Wa-ter Resources, Environmental andEngineering Applications, and TheGeologic Tourist), which cover allmajor aspects of the state’s ge-ology. Within the 10 parts are 57chapters, authored by 90 indivi-duals from academia, govern-ment, and industry. The book con-tains more than 800 illustrations,an extensive bibliography, a de-tailed index, and a 16-page sec-tion printed in full color.

The Geology of Pennsylvaniawas edited by Charles H. Shultz,a professor at Slippery Rock Uni-versity. The managing editor wasReginald P. Briggs of Geomega,Inc. Pittsburgh Geological Societymembers were authors and/orreviewers of several chapters ofthe book.

The Survey also made signifi-cant contributions to the publica-tion. Staff geologists were authorsor coauthors of 19 chapters, andother staff were involved in thetechnical review and productionof the book for printing.

Special Publication 1 is avail-able from the State Bookstore,Commonwealth Keystone Build-ing, 400 North Street, Harrisburg,PA 17120–0053, telephone 717–787–5109. Books may be pur-chased over the counter for$23.50 plus $1.41 state sales

tax. There are two options for pur-chasing the book by mail. The op-tion 1 price is $28.25 (which in-cludes shipping by the U.S. PostalService Media rate, generally 3 to10 days delivery time) plus $1.70sales tax if mailed to a Penn-sylvania address. The option 2price is $32.00 (which includesshipping by either U.S. Postal Ser-vice Priority rate or United ParcelService, generally 1 to 5 days de-livery time) plus $1.92 sales taxif mailed to a Pennsylvania ad-dress. Make checks payable toCommonwealth of Pennsylvania.Please contact the Bookstore forinformation on ordering by creditcard or the cost of the book if itis to be mailed to a foreign ad-dress. An order form for SpecialPublication 1 is available on theBureau’s web site at www.dcnr.state.pa.us/topogeo/.

THE GEOLOGYOF

PENNSYLVANIA

CONTENTS

Secret information, public information ...................................................... 1Rockslide in Sunbury, Pennsylvania, on January 9, 2002........................ 2“Fossil” and recent landsliding in Allegheny County ........................................ 8The Geology of Pennsylvania—Back by popular demand........................ 15

ON THE COVER

Interbedded sandstone, shale, and limestone of the Reedsville Formationalong Interstate Route 80, Milesburg Gap, Centre County. The rocks havebeen tilted from their original horizontal orientation. Sandstones (both lightand dark) are lenticular, crossbedded, and hummocky bedded, have ero-sional upper surfaces, and in some places are size graded. The thick, light-colored bed at the left is fossiliferous limestone (from A. M. Thompson, chap-ter 5, Ordovician, in The Geology of Pennsylvania) (see p. 15 in this issue).

PENNSYLVANIA GEOLOGY

PENNSYLVANIA GEOLOGY is published quarterly by the Bureau of Topographic andGeologic Survey, Pennsylvania Department of Conservation and Natural Resources,3240 Schoolhouse Road, Middletown, PA 17057–3534.Editors: Anne B. Lutz and Jay B. Parrish.Contributed articles are welcome. Guidelines for manuscript preparation may be obtainedat www.dcnr.state.pa.us/topogeo/mag-ins.htm or by contacting the editors at the addresslisted above.Articles may be reprinted from this magazine if credit is given to the Bureau of Topographicand Geologic Survey.

VOL. 32, NO. 3/4 FALL/WINTER 2002

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(General Inquiries) 717–702–2073Christine E. Miles, P.G. 717–702–2044Caron E. O’Neil, P.G. 717–702–2042Anne B. Lutz 717–702–2043

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(East) 717–702–2047Jaime Kostelnik (West) 412–442–5828

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COMMONWEALTH OF PENNSYLVANIAMark Schweiker, Governor

DEPARTMENT OF CONSERVATION AND NATURAL RESOURCESJohn C. Oliver, Secretary

OFFICE OF CONSERVATION AND ENGINEERING SERVICESRichard G. Sprenkle, Deputy Secretary

BUREAU OF TOPOGRAPHIC AND GEOLOGIC SURVEYJay B. Parrish, Director

Bureau web site: www.dcnr.state.pa.us/topogeoDCNR web site: www.dcnr.state.pa.us

Pennsylvania home page: www.state.pa.us

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If you would like to receive up-to-date Bureau announcements,please send your e-mail address to pageology@state.pa.us.

LANDSLIDE SUSCEPTIBILITY IN PENNSYLVANIA(Points 1 and 2 on the map below show locations of landslides described in articles on pages 2and 8. Map from Delano, H. L., and Wilshusen, J. P., 2001, Landslides in Pennsylvania:Pennsylvania Geological Survey, 4th ser., Educational Series 9, 34 p.)