One of the members of my hiking club is a geologist, and as a change of pace she took us on tour of the rocks involved in the architecture of downtown near my office today. It's interesting to have a dull, regular part of my life transformed by someone's knowledge.

The base of a building needs to be made of very weather resistant material, so there is a lot of granite and gneiss at foundations. These are very hard, metamorphic rocks and are polished and shiny. Granite and gneiss are expensive. Some buildings don't have that lower protective layer, and you can see the signs of erosion. Blue stone shale is sometimes used, which is a softer stone, but for some reason it erodes more slowly.

You can also see it in the curbs - the ones constructed of concrete weather more quickly. Most of the curbs are granite, mined in New Hampshire and Vermont. In some of the granite there are streaks of large grained igneous rock, where small superheated streams of magma crept into the fine grained granite and then very slowly cooled.

Above the weather resistant granite and gneisses, there are a lot of sandstone and limestone walls. These are interesting because a lot of them contain fossils. We saw calicified brachipods and corrals in the walls of the court house, and worm tracks and cronoids in the shale bricks making up the walls of a church.

Inside a couple of the buildings are polished marble foyers. The marble comes from Italy and has long veins of a darker black material. Marble is highly calcified, and although it is a hard stone, it is very vulnerable to the slightly acidic rains we get here, so you wouldn't want it on the outside of the buildings. If you wander in a graveyard you may find some older marble tombstones; they are very quickly eroded. Older slate tombstones from the late 1700s and early 1800s can actually be easier to read than marble tombstones from the early 1900s.

Karen, our guide, is in her late 60s, and has been teaching geology at a local community college for a few decades. She's done this tour about a dozen times previously, usually with her students at the school. It's nice to see her enthusiam for her field, even after all these years. She was really getting into it.

So anyway, it was kind of fun, and now I have something else to amuse my mind with on the walk to the courthouse besides naming weeds and observing trees.