Three kinds of ground: digging fence posts from Pottstown shale to Boyertown rock to Oley limestone

Most fence advice assumes every yard has the same soil. That is not true between the Schuylkill and the Lehigh County line. A post hole in a Sanatoga backyard, one on a Barto hillside and one in an Oley pasture can each need a different tool and a different plan. Knowing which ground you are on before the crew shows up is the difference between an install that goes to plan and a day of surprises.

Which ground is under my yard?

Here is the rough split. It follows the geology, not the ZIP codes, so treat it as a starting point. Some lots sit right on a boundary.

Simplified from the Berks County regional plans and the USDA soil survey. A site visit tells us what your lot actually has.

Pottstown and the Perkiomen valley: why red shale stops the auger

If your garden soil is brick-red, you are on the Newark basin rocks that run through the Pottstown area. Geologists even named a rock unit after the town. The "Pottstown shales" are Late Triassic beds described from exposures near Pottstown. The soil that forms on these beds is the Penn series, and its type location is in Montgomery County. The USDA describes Penn soils as reaching bedrock 20 to 40 in down.

That depth matters because a fence post usually wants to go deeper than that. When the auger hits shale at 26 in, the crew has a few choices, and the right one depends on how the rock behaves:

• Weathered, platy shale often breaks up under a rock bit or a digging bar. We keep going to full depth and clean out the hole.
• Hard, unweathered rock may call for a core drill. The post then sits in a socket cut into the rock, and the concrete bonds rock to post.
• Shale that crumbles and holds water needs gravel at the bottom of the hole, so water can drain away from the post's end.

The good news: shale that is firm enough to stop an auger is also firm enough to hold a post. A shorter post set into solid rock can be steadier than a deeper one in loose fill. What we won't do is cut posts short just to make the hole easier. If a post can't get the bury it needs, we tell you and change the plan, for example by moving the line a foot or using a different post spacing.

Boyertown, Bally and Hereford: Reading Prong rock and rubble

North and east of Boyertown, the ground climbs onto the Reading Prong. In the eastern Berks region these ridges rise more than 800 ft above the Great Valley. The county's regional plan says the gneiss weathers into gravelly Gladstone soils, and that some mapped soil units there are 25 to 60 percent rubble slopes. The same plan counts more than 100 iron mines in the region at their mid-1800s peak. That is a reminder that these hills were dug into long before anyone set a fence post.

Rubble ground causes a different problem from shale. The auger doesn't hit a flat floor. It catches a cobble, kicks sideways and leaves a hole that wanders off line. Typical fixes:

1. Start the hole with a digging bar to find and pry out the first big stones.
2. Use a smaller auger or dig by hand around boulders too big to lift, then shift the post a few inches along the line if the stone won't come out.
3. Expect irregular holes. They take more concrete than a clean round hole, which we plan for in the materials list.
4. Set and brace corner and gate posts first, then run line posts between them, so the stony middle sections can flex a little without throwing the line off.

On the steep lots common on these ridges, we also decide early whether panels will be racked to follow the slope or stepped. Over half of Alsace Township's land is steeper than 15 percent, according to the Oley-area regional plan. Open styles such as aluminum, chain link and split rail rack well. Solid privacy panels usually have to step, which leaves triangle gaps under each panel that may need trim boards.

Oley and Exeter: limestone, karst and sinkholes

The Oley valley is famous for its farmland. Oley Township has more Class I soils than any other municipality in Berks County. Under that soil is carbonate rock. The Oley, Alsace and Ruscombmanor plan describes Ordovician dolomite and limestone through central Oley and Cambrian limestone along its eastern and northeastern edges. It says that Cambrian limestone "has many surface depressions and sinkholes."

Exeter Township shares the same kind of ground. The Exeter Commons shopping center has had repeated sinkholes. One opened in front of the Lowe's in summer 2010, a year after the center opened, and a later one swallowed part of the parking lot. Statewide, about 7 percent of Pennsylvania sits on karst, and state surveys have mapped swaths of it in Berks, Montgomery, Chester and Lehigh counties.

Fence posts don't cause sinkholes. Water does. DCNR's Bureau of Geological Survey says sinkholes form in limestone and dolostone and that stormwater and poorly maintained water lines speed them up. So on karst lots our job is mostly about not concentrating water near the fence:

• We walk the line looking for low spots, bowl-shaped dips and ground that sounds or feels hollow, and we move the line away from them.
• We don't aim downspouts, sump outlets or a driveway's runoff at the fence line, and we tell you if one already drains there.
• If a post hole suddenly drops away or swallows concrete, we stop. That hole gets flagged, not filled.
• You can look up mapped karst features for your parcel in DCNR's PaGEODE viewer before the estimate.

And the odd boulder field in Lower Pottsgrove

Not every rock fits the three-way split. Ringing Rocks Park in Lower Pottsgrove holds a boulder field of about half an acre that was written about in the 1740s. Ringing rocks are usually diabase, a hard igneous rock that freeze-thaw cycles cracked into blocks. If your lot sits near that kind of outcrop, the ground can switch from easy red shale to solid stone within a few feet.

Before anyone digs: utilities, then rock

Whatever the rock, buried lines come first. Pennsylvania's One Call law requires three business days' notice before excavation, and the PUC lists fence posts as a project that may need a ticket. When you hire a contractor, the contractor must place the call. We do that on every job, then plan the dig around the marks. Rocky ground makes it tempting to shift a post a foot sideways. We only do that after checking the new spot against the utility marks too.

What this means for your quote

Ground type is one of the reasons two fences of the same length can price differently. Rock coring, hand digging around boulders and extra concrete all take time. We look at the ground at the estimate, probe a few spots on longer runs, and write the plan into the quote rather than surprising you mid-job. Ask us to bring a probe bar to your free estimate and we'll show you what's under your fence line before you commit to a style.

Common questions