Standing in rugged contrast to the planted aspect of a garden, dry-stack walls serve as an outstanding background or defining element in the landscape. And while it requires patience and a willingness to ‘work the stone’ to make the wall structurally sound, it is equally important to use your artist’s eye to ensure that it is aesthetically pleasing. Stand back often to see that the stone placement is very natural, not contrived.
The local lichen sandstone (more commonly called ‘moss rock’), with its characteristically flat or angular nature and widely varying color, is popular for use in the construction of these walls, although there are many other good choices. Block rock (squarish, predominantly tan/brown sandstone with little to no lichen) and flagstone (many colors available) are both easy to stack with a minimal amount of chiseling work. Typically resulting in more uniformly flat faces with tight joints, both make for very solid walls – especially for retaining; however, in lacking the variety of size, shape, color and texture that give a moss rock wall its more natural, random character, they do not compliment the garden’s floral display nearly as well. Another choice, river rock (mostly shades of gray), because of its rounded nature, is difficult to use for dry-stacking, and is not recommended for large walls. Yet, in situations of low retaining, such as raised beds, with groundcovers allowed to take root in the larger joints, they are very attractive.
Getting Started: If a rock retaining wall is to be built, first must be excavated a footer – a shallow trench - slightly larger than the base width of the wall. This will accommodate the partial burial of the large foundation stones that the rest of the wall is to be raised on. Use a pick if necessary, and then remove the loosened dirt with a flat shovel. Also, an over-excavation of the bank to be retained is necessary both to provide working room and for the placement of clean, compactable backfill as the body of the wall is installed. All retaining walls of 4’ or taller should be built with a ‘batter’, meaning slightly angled back into the hill, resulting in the finished wall being wider at the bottom than the top. The batter angle should be 2” per vertical foot of wall. (i.e.: a 4’ tall wall would have an 8” batter).
Tip: Use some of the larger, least stackable rocks in the base course. They needn’t be the prettiest rocks, either, as they will be partially buried. One good outward face is all that’s needed.
Fitting the Rocks: Use a critical eye when selecting rocks for placement. Spread out the pile with the best faces up for easy review. Take note of angles and curves, rounded faces, flat faces, and points that either could work to your advantage or, quite often, disadvantage, when trying to fit the rock into a certain spot. This will cut down on the frustration of selecting rocks that either require a lot of chisel work to fit or create a “bad leave”, a situation in which the rock to be installed adjacent to it will be very difficult without a similar amount of time-consuming work. Constantly picking up rocks, trying them here, there, and everywhere, finding out they don’t really work, then pitching them back into the pile only to start looking for another gets depressing.
Try to select rocks according to their size and shape, or something that indicates it is a good fit for a specific location in the wall. That said, leaving gaps here and there, and not fitting everything perfectly adds to the character of the wall. Some of the larger gaps may require “chink” stones, either to make the wall more structurally sound, or more consistent with the overall look of the wall. If, standing back from the wall, your eye keeps stopping on the same large gap or ill-fitted stone, it is an indication that it is not consistent with the rest of the work. It is much better to put a chink in it or change it out for a better fitting rock, because it’s only going to keep irritating you.
Tip: When you’ve chiseled off the point off a rock, split off a good-sized piece or it is just broken in the pile, save the best of these off to the side for chink stones. Consider them according to shape (long flat slivers and narrow points are the most usable), and as to whether they have a good flat face. Never, however, install the broken face of a rock outward, whether a larger rock in the body of the wall or a chink stone. Remember, any rock that makes your eye stop, whether because it is badly fitted, broken, or out of scale, hurts the finished product.
Making Sure It’s Not Going Anywhere: Many rocks fitted to the wall will lock in nicely with little work. There will be others, however, that will be generally uncooperative – rocking, rolling, unsupportive of their fellow rocks, or falling down onto your foot while you’re not looking.
These rocks don’t necessarily need to be jettisoned into the pile, but better persuaded to stay in place. We have ways of dealing with them. The aforementioned chink stones, well-fitted, and in combination with packing in clean backfill, is the best solution. Once the offending rock is stabilized, adjacent rocks can then be put in place to lock it solidly into the field. However, chinking a rock with many small stones pushed in side by side should be avoided. It does not look good, and, inevitably, will fail. A larger, better fitting single chink stone works much better. Also, the constant compaction of the backfill dirt as the wall is being raised is critical. Dirt should be forced into all crevices under and between rocks to stabilize the work as you go. Additionally, compacting the dirt between the excavated hill and the back of the wall with your hand sledge is the best defense against any future washouts. If there are any gaps open, or poor compaction, rainwater will eventually find its way in and jeopardize the structural integrity of the wall.
Tip: Work across the entire length of the wall as you go. Do not build to full height in some sections with large, unfinished gaps between. For all your hard work, those full-height sections could come tumbling down at any moment. Also, cover your incomplete work with a tarp when not working; for while those nighttime rains are great for your garden, it’s not so great for your normally cheery disposition as you’re picking through that big heap of rock trying to figure out which one was where the day before.
Finishing It Off: Depending on the style of construction, cap stones can be used to complete the wall. Often bedded in concrete on the back half of the rock, capstones give a more formal top to the wall, and, as such, are rarely installed as a single uniform course in random stone installations. Both for stabilizing strength and protection against gutting rains, capstones are usually more massive than the rock in the body of the wall, and wide enough to cover over the backfilled cut. They shouldn’t move if you walk on them. Not even after Thanksgiving dinner.
Tools of the Trade: A hand sledge, a smaller rock hammer, and two cold chisels – one 1” and one ¾”; a pick, a flat (square) shovel, a wheelbarrow, a small whisk broom, and work gloves. I-Pod optional.
Final Word: Consistency of style from the first rock to the last is essential to the aesthetic appeal of dry-stacked walls. Be it blocky and tightly fit, random with looser joints, thin rocks with large accents, or mixed rock with chink stones throughout (or no chinks at all), have a plan and stick to it. Take your time. Step back often. And don’t smash your fingers with the sledge; it hurts.
Find out more about how Santa Fe Botanical Garden celebrates, cultivates and preserves the rich botanical heritage and biodiversity of the region. Visit www.santafebotanicalgarden.org