by Sandy Eldredge

Have you ever looked at a stone wall and wondered where the different rocks came from and what story they may tell? Sitting at your computer, read the “Glad You Asked” column, What Kind of Rock Makes a Good Wall?, to get you started.

In the field, select a stone wall to look at with your students. Close observations will probably lead to numerous questions, many of which can be answered by logical thinking processes.

Try to answer the following questions:

1. What type of rock(s) make up the wall?

2. Where did the rock come from? Is it local or was it brought in from somewhere else? Are the rocks large or small? Are they from a river bed, a quarry, or ??

3. Does this area have a large supply of these rocks naturally?

4. Why is the wall here? Decorative? Retaining? Border? Other?

(For example, in New England there are numerous rock walls that appear to be out of place today – such as in the middle of a veritable forest. The reason? The answers lie in both history and in local geology. Centuries ago, New England farmers had to clear land to make fields. Not only were trees felled, but numerous small and large rocks, left behind by glaciers, were strewn all over the fields and had to be cleared. What better to do with them than to make walls. Now many of the walls are hidden where trees have reclaimed the landscape. Think of local examples, such as why would more rock walls exist in the foothills around Salt Lake Valley than in the valley? Hint, see question #6)

5. What’s the local geology?

6. How does geology affect what was built around us?

Reading this wall (see photos) in Midway, Utah reveals the following answers:

1.Tufa.

2. Local rocks, small enough to be carried by hand, from the nearby Midway hot pots (read “GeoSights” column about the Midway hot pots). Tufa forms around the hot pots when calcium carbonate precipitates out of the spring water due to a change in the water’s chemistry as it flows out of the ground.

3. Not a huge supply, but enough for several decorative uses in the past. The tufa cannot be collected today, as it is on private land. This could also segue into a history discussion, and include a question about the age of the wall.

4. This wall appears to be mostly decorative; the ground is relatively flat, so the wall is not a retaining wall. The rock wasn’t lying around the yard naturally, so the purpose was not to get rocks out of the yard.

5. See answer to #2 and the “GeoSights” article, What Kind of Rock Makes a Good Wall?.

6. Many possible answers, which are not listed here.

We welcome any queries about or results of your project. Please email them to the Sandy Eldredge (sandyeldredge@ utah.gov) at the Utah Geological Survey.