Traditionally, the choice of the stone used in stone bridges was primarily a reflection on what was locally available.
In New England, there are many stunning stone arch bridges made out of the locally available granite boulders. These structures attest to the ingenuity of the masons who were able to create lasting structures using such irregularly shaped material.
In the Midwest, the stone arch bridges are typically made of sedimentary rocks — usually limestone — and as such are made of much more regularly shaped blocks. Not only are sedimentary rocks generally easier to shape, but they typically come out of the ground in a fairly well-shaped state. This is because they are typically laid on an even strata and tend to separate on these flat planes.
In cases where expense was less of a concern, outside material was imported for bridge projects.
In the case of Minneapolis’s Stone Arch Bridge, several types of stone were employed to achieve an excellent quality/cost compromise. The lower parts of the piers are made of granite, the outside facing is made of dolomite, the interior stonework made of local limestone, and the trimmings marble.
Why Use Different Materials
Using granite for the lower parts of the piers and anywhere where the stone is exposed to the regular action of water is a superb idea. Not only is granite less vulnerable to erosion than almost any material available, but it also resists “decay” due to freezing and thawing cycles and any pollution in the water.
Incidentally, in the Kansas stone arch bridges, which are usually made entirely of limestone, even in bridges otherwise in superb condition, at the waterline the stonework is often cracked and broken, and even, in extreme cases, discolored and channeled into.
The use of dolomite for the facing allows for easy cutting and superb durability. Dolomite is much less vulnerable to pollution and, furthermore, is frequently a nice, hard, durable stone — although not so hard as to be an exercise in aggravation to cut. As the facing is exposed to the action of the elements, it makes sense to make it a little more rugged.
The interior stone work of the bridge is largely protected by the facing and need not be anything fancy, hence using inexpensive local stone, even if not the most robust available, makes sense.
The Stone Arch Bridge in Minneapolis also features marble for the trimmings, as mentioned above. While perhaps excessive for most rural bridges, nice marble trimmings adds a fine finishing touch for such a monumental structure.
Stone Choices for the Amateur
But what about a beginning mason? Or perhaps the person who wants to build a stone arch bridge as a DIY project, but is not by any means an expert stone cutter? What type of stone should such an individual use?
Probably the best stone for beginning masons and stone cutters is sandstone — not the cherry red crumbly type, nor hard quartzite, but the so-called “brownstone.” Not only does sandstone frequently appear in nature as nice, thin, flat slabs (which make turning an arch easy), but sandstone is phenomenally easy to cut.
You can often get excellent results by simply chiseling along the line you want the stone to split at. A light stonemason’s hammer can also apply the finishing touches, allowing for surprisingly good results, even for a beginner.
Limestone is all right, however, it takes quite a bit more practice than sandstone does to get good at cutting it. Happily, in some parts of the country it comes as thin, manageable slabs that are still very easy to build with.
Anything volcanic in nature is going to be a challenge to cut, which is why, for the beginner, sedimentary rocks are best to build with. The main exception to this is the metamorphic rock schist, which reportedly comes in thin slabs naturally as well and is reasonably easy to cut.
A final tip for the amateur mason is to avoid using shale for anything intended to be lasting — it usually crumbles to nothing in a rather shockingly short time.