Part 2: Stone Walls

By Ian Burrow (


My first blog was mostly concerned with trees, and how they can help tell the story of people on Sourland Mountain.  Now I want to introduce you to an artifact (= a thing made, modified or used by humans): the Sourland stone wall.

You will encounter stone walls on many of Sourland Mountain’s preserved areas.  Sometimes they can be quite impressive, like this massive and well-built wall beside the Rock Hopper Trail near Lambertville (Figure 1).

Fig 1 Walls Blog

Figure 1. Rock Hopper Trail. Well-built revetment wall on north side of Rock Road: Panoramic View looking north from the roadbed of Rock Road.  Photograph: Ian Burrow, 2019.

Some examples are just a line of low, tumbled stones winding through the woods (Figure 2).

Fig 2 Walls Blog

Figure 2.  A poorly constructed “wall” on the Mount Rose Ridge in Hopewell Township.  Photograph: Ian Burrow 2019.

Others look more like banks or ramparts (Figure 3).

Fig 3 Walls Blog

Figure 3.  Wall along western side of the 1850’s Rock Road to Lambertville-Rocktown Road connector, showing the hollow-way character of the road on the left.  View facing south towards intersection with Rock Road.  Near the Rock Hopper Trail. Photograph: Ian Burrow, 2019.

Some are very well made (Figure 4), others much less so (Figure 5).

Fig 4 Walls Blog

Figure 4.  Cross-Section of an argillite stone wall on the Cedar Ridge Preserve.  The wall has been constructed from wedge-shaped slabs, and the wall narrows with height.  The flat “capstone” indicates that this is the full intended construction height of the wall, about two and half feet.  Photograph: Ian Burrow, January 2018.

Fig 5 Walls Blog

Figure 5.  A massive but roughly constructed diabase wall on the east side of the Rock Road north of Swan Creek on the Rock Hopper Trail. Photograph: Ian Burrow, 2019.

What are they all about? Are they special to the Sourlands? When were they built and by whom?

The easy answer to the first question is that most of them are field walls, sometimes called stone fences, built to create a boundary around a field or a group of fields.  That seems straightforward enough, but when we delve a little deeper we find it leaves a lot of other questions unanswered.  Why, for example, do most agricultural fields in central New Jersey lack stone walls? Many fields have no boundary around them at all, others may have boundaries in the form of hedges, treelines, or wooden fences.

It turns out that Sourland Mountain is one of the few areas in New Jersey where stone field walls are present. Stone walls are actually a rare cultural resource in the state.  How do we know this? In 1871 the U.S. Department of Agriculture helpfully published statistics of field fencing types in the United States. In New Jersey only 5% of the recorded fences were stone walls, most of them in the northern counties. 62% were post-and-rail, 29% were wooden zig-zag or “worm” fences, and 4% were made of boards.  Compare this to Connecticut, where one third of all the fencing in the state, or 20,505 miles, was of stone.  In Massachusetts 50% of all fences were of stone. In Rhode Island most of the 14,030 miles of fencing in the state were stone walls. New York State had an astonishing 95,364 miles of stone walls!

As you might guess, the geographical distribution of stone walls has something to do with geology and topography. In sandy southern New Jersey stone useful for walls is very hard to find.  The New England states, by contrast, are very rocky.  So it seems to make sense that if you find yourself in a stony part of the world you will build stone walls around your fields.

In many parts of New England, stone walls were created as a result of clearing the fields of rocks for cultivation or for pasture.  The collected rocks were dumped around the edges of the fields, sometimes as well-constructed walls, but often as rough linear piles of stone.  The mechanics of this meant that the most efficient field was a square roughly 300 feet on a side, and covering about 2 acres.  Is this an explanation for the stone walls we see on Sourland Mountain? The answer is sometimes, but not always.


Walls of Argillite Stone

My study of the Cedar Ridge Preserve in Hopewell Township showed differences from this New England model.  Cedar Ridge is composed of argillite stone.  This rock type breaks quite easily into slabs useful for building.  It is sometimes exposed in outcrops (Figure 6), and can also be recovered from the beds of the streams and brooks along the southern slopes of Sourland Mountain.

Fig 6 Walls Blog

Figure 6.  Argillite outcrop, Cedar Ridge Preserve.  A substantial field wall lies about 30 feet to the south. Photograph: Ian Burrow, March 2018.

However, unlike the glacial rocks and boulders of New England, argillite is not found lying on the surface.  In fact, you have to dig about four feet down into what soil scientists call the Chalfont Silt Loam in order to reach argillite bedrock.  This means that if you want to build a wall of argillite on Cedar Ridge you have to either dig down for the stone, or bring it from somewhere else.

On the Cedar Ridge Preserve I have noted about 4500 feet of argillite stone walls.  There were probably more at one time.  Studying these walls I noticed that almost 75% of them run along long-standing property boundaries which are often modern lot lines.  In addition, in most places these lot/property boundary walls are associated with a substantial ditch on one side.  The best way to see feature this is to start at the parking lot on Van Dyke Road and head west along the trail which runs just south of the stone wall.  This wall runs almost continuously for about 2000 feet, and for much of that distance there is a ditch on its southern side.

I have concluded that these ditches were used as the source for the stone for the walls.  They probably also served a drainage function on the poorly drained soils of the locality.  My guess is that the walls were built by the person on the ditch side of the wall.

There are two substantial lengths of stone walling which do not have associated ditches.  These lie close to the argillite outcrop shown on Figure 6, so perhaps the stone for these walls came from there.

One of these is not on a modern lot line, but does form the boundary between two of the largest fields in the area.  Like the others, however, it is very well built (Figure 4)

So what does all this tell us?  I predict that in argillite bedrock areas on Sourland Mountain, stone walls will only be found marking important, long-lasting boundaries.  This is because the effort of both quarrying/hauling and building the walls was very considerable.  In 1858 it was estimated that one man could erect three rods (about 50 feet) of stone walling in a day. This figure assumed that the stone had already been gathered and brought to the construction site, tasks that probably entailed an equal amount of labor to the construction itself.  If we halve the amount that could be built in a day to 25 feet (to allow for the quarrying), that means that it would have taken one man about six months to build all the stone walls observed at Cedar Ridge.

But, as one might expect, these walls were not all built at the same time.  With the help of a friend, I cleared away the fallen stones from the area around the T-shaped junction of two walls (Figure 7).

Fig 7 Walls Blog

Figure 7.  Investigation of the junction of two argillite walls on Cedar Ridge.  View looking north.  The west and east sections of Wall 8A survive to close to original height, while the area of the intersection with Wall 8B in the center is much lower.  Large slabs mark the east face of 8B, which was built later than 8A. Photograph: Ian Burrow, April 2018.

Here we could clearly see that the north-south wall forming the stem of the “T” was later than the east-west wall which it joined.  The north-south wall had been roughly keyed-in to the existing wall, and had actually caused the latter to bulge outwards on its northern side.

Walls of Diabase Rock

The main spine of Sourland Mountain is composed of diabase rock.  This is the geological formation which produces the interesting and sometimes spectacular rock outcroppings and boulder fields to be seen at the higher elevations.  It is a dark, dense, hard, fine grained volcanic rock.  The Rock Hopper Trail and the surrounding preserved area around the Swan Creek Reservoir just southeast of Lambertville lie almost completely on diabase.  Much of the preserve is very stony, but areas north of the reservoir are less so.  Some parts here are still farmed, and the present woodland north of the reservoir lies on former farm fields for the most part.

Much of the Rock Hopper Trail follows the route of the Rock Road, abandoned in the mid-20th century. It is chiefly along or close to this road that we find walls made of diabase. As you walk the trail from east to west you encounter a narrow stone bridge (Figure 8).

Fig 8 Walls Blog

Figure 8.  Stone-arched bridge carrying Rock Road across Swan Creek on the Rock Hopper Trail.  Photograph: Ian Burrow, 2019.

A couple of hundred yards beyond that and you meet this dramatic wall on your right (Figure 1).

This wall extends for about 500 feet along the top of a slope on the north side of Rock Road The base of the wall is about five feet higher than the bed of Rock Road, and the wall itself is typically about five feet high.  Rather than being a free-standing wall, this is actually the retaining wall for a terrace, so the top of the wall is almost level with the ground surface at the rear (northern) side (Figure 9).

Fig 9 walls blog

Figure 9. Rock Hopper Trail. The eastern end of the Revetment Wall, looking west. Note that the ground surface on the right (north) side is about two-thirds of the way up the wall at this point. The roadbed of Rock road lies about 20 feet to the left (south) and five feet below the base of the wall. Photograph: Ian Burrow, 2019.

The wall was finished off with vertical “coping” stones. Where these survive, it can be seen that the overall height of the wall was intended to be about six and a half feet.  The wall itself is well built of massive stones, many of them far too large for one person to handle (Figure 10).

Fig 10 walls blog

Figure 10. Close-up view of a section of the Revetment Wall with vertical coping stones still in place. Note the large scale of many of the diabase stones used for the body of the wall. At this location the wall is close to six feet high. The ground surface behind the wall is at the level of top of the scale pole. Photograph: Ian Burrow, 2019.

As with all the Sourland stone walls, we ask the following questions of this one:

When was it built?

Who built it?

What was it for?

It turns out that we can partly answer the “when” question: This wall was probably constructed no earlier than 1851.  We reach this conclusion in the classic cultural landscape history way: by looking at the landscape itself, and by consulting historical documents.

The western end of the wall curves around a corner at the intersection of Rock Road with an unnamed and long-abandoned road which connects Rock Road with the Rocktown-Lambertville Road (Figure 11).

Fig 11 walls blog

Figure 11. Rock Hopper Trail. The western end of the revetment wall, view facing southeast from the road-bed of the 1850’s Rock Road-to-Lambertville-Rocktown Road connector. Note how the wall is continued around the corner from Rock Road to the east side of the connector, indicating that the revetment is of the 1850’s or later. Photograph: Ian Burrow, 2019.

This road is very similar in character to the Rock Road, especially in its southern portion, where it is a moderately deep hollow-way.

Now, it is clear that the wall could not have been built before the connecting road was in place.  Fortunately, historic map evidence strongly suggests that the road was built after 1851 and before 1860, because it is shown on a map of 1860, but not on a map of 1851. So all we can strictly say is that the wall is later than 1851, but that in itself is worth knowing.  Supporting this dating is the fact that one of the stones at the end of the wall has a drill hole in it of a type probably not in use in this area until after about 1830.

So, what about the who and the why?  Detailed historical research on the owners of the property that includes the wall might give us a clue to the who. As for the “why”, it seems clear that this is no ordinary field wall.  It would be quite useless for keeping animals in fields and off the road.  The wall is so low on its northern (terrace) side that nimble sheep would easily get over it and scramble down to the road.  Cattle or horses might simply fall over the edge and injure or kill themselves.  So the land behind the wall was used for something else.

Seen from the road, however, there can be no doubt about the message sent by this wall.  The message is “Keep Out”.  Anyone wanting to climb the wall from the road had first to scramble up a steep grassy slope and then find footholds and handholds in the face of the wall.  Then that individual would have to negotiate the upright capping stones in order to get across the wall.  It would be clear to that person, and to anyone else, that they had crossed a line that was not meant to be crossed.  This, and the resources that were put into to building this wall, indicate very strongly that this was the boundary of a person of means who intended to maintain an exclusive privacy, particularly from people travelling along the road.

These examples will, I hope, give you some idea of the interesting stories built into the stone walls of Sourland Mountain.


Next Time: Puzzles and Mysteries





  1. Lois Marie Harrod

    Hi Ian, Looks like your walking stick is the celebrity in these pictures. A reminder to take one when we go on these hikes. I really enjoyed the blog and learned a lot about stone fences. Thank you,



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