Geology and Soils

Pleasant Valley Conservancy is in the Driftless (unglaciated) region of southwestern Wisconsin. In this region, the landscape is controlled to a great extent by the underlying rock formations. This is the "Ridge and Valley Province", a scenic landscape where steep hills are separated by narrow valleys.

Geology of Pleasant Valley Conservancy

A geological cross section of Pleasant Valley Conservancy, drawn for Pleasant Valley Conservancy by University of Wisconsin-Madison geologist Robert H. Dott, is shown below. This section was prepared from field surveys of rock outcrops (including those in an abandoned quarry), well logs, and the extensive geological research which Professor Dott has carried out in western Dane County. The rocks here are of Cambrian or Ordovician age, formed in early seas around a half-billion years ago. The rocks at the base of Pleasant Valley Conservancy are part of the Mazomanie Formation of the Tunnel City Group. This is a sandstone characterized by large numbers of dark crystals of glauconite, an iron aluminum silicate mineral. Glauconite forms in a marine environment, usually crystallizing around fecal pellets of marine animals.



Above the Tunnel City Group is a narrow band of a harder rock called the Black Earth Dolomite. The Black Earth Dolomite is a member of the St. Lawrence Formation. Dolomites are calcium magnesium carbonates and are more resistant to erosion than sandstones. Because of this, there is a narrow "bench" (an area of more level ground) about half way to the top of Pleasant Valley Conservancy where this rock formation is present. This bench is well seen along the gravel service road that provides access to the top. After the first steep rise of this road, there is a relatively flat area before the road starts to climb again. This flat area marks the area of the Black Earth Dolomite.

Above the Black Earth Dolomite is another thick layer of sandstone called the Jordan Sandstone. This is a fairly soft sandstone that erodes easily. Above the Jordan, at the top of the ridge, is another resistant dolomite, the Oneota Formation of the Prairie du Chien group. This formation, of Ordovician age, is the rock type in which many caves have formed in southwestern Wisconsin, and two small caves have been found on Conservancy land. The Oneota forms the cap rock at the top of Pleasant Valley Conservancy, especially in the center part of the ridge, where the elevation is the highest. There is also a small dome of Oneota formation at the far eastern end of the Conservancy (in Unit 13) (see map below).

There are a number of rock outcrops along the top of the bluff, and several on the south-facing slope provide dramatic views of the creek and provide a view of Blue Mounds State Park to the south. Some of these outcrops have characteristic structures called "stromatolites," which were formed by microscopic algae in the ancient Cambrian oceans.

The bedrock layers in this part of Wisconsin are virtually flat lying, so that these same rock formations will be found throughout southern Wisconsin at about the same altitude. Use of a topographic map will provide altitude values.

A new version of the map done using ArcGIS, is shown below. Note the narrow band of Black Earth Dolomite (purple color) that traverses the whole property and the small area of Oneota Dolomite at the far-eastern edge of the Conservancy. A higher resolution version of this map as a PDF can be downloaded at this link.


Plant Distribution in relation to Geology

The geological map of Pleasant Valley Conservancy has provided interesting evidence connecting the distribution of the savanna oaks to the underlying rocks. A database has been constructed of all of the trees above 10 inches diameter in the oak savannas. Using ArcGIS, a spatial analysis tool, it has been possible to show that the bur oaks are present primarily on the Oneota dolomite whereas the white oaks are found mostly on the Jordan sandstone. (There is also a series of bur oaks along the Black Earth Dolomite "bench".)

Further, certain plant species that live primarily in moister soils are also found associated with the Black Earth Dolomite, where they are surrounded by the two drier sandstone layers. It is assumed that the Black Earth Dolomite provides a zone where groundwater from the Jordan sandstone comes to the surface, thus providing the moister conditions that these plants need. Examples of two wet-mesic species found on the south slope only on the Black Earth Dolomite, and neither above or below, are ironweed and Culver's root.


Soils of Pleasant Valley Conservancy

The soils of Pleasant Valley Conservancy are mapped on Sheet Number 72 of the Dane County, Wisconsin soils map, created by the Soil Conservation Service (now Natural Resources and Conservation Service; NRCS) of the U.S. Department of Agriculture. These soil maps are also available via the Internet, from the NRCS. (Search for NRCS, county name, and keyword "soils". These soil maps are georeferenced, so that they can be incorporated as layers into ArcGIS.) Because there are no glacial deposits, our soils have to the most part formed directly upon bedrock.

The soils formed on the dolomite cap at the top of the ridge are of the Dunbarton series, consisting of thin, well-drained, gently sloping to steep soils. Fractured dolomite can be found at a depth of 10 to 20 inches. These soils have low fertility and cultivation is discouraged. Before settlement, most of our area on the Dunbarton series was bur oak savanna. After settlement, these soils were either in pasture or in woodland. At Pleasant Valley they have now been mostly restored to savanna again.

On the Jordan Sandstone on the upper slopes the soils are either of the Eleva series or the Elkmound series. Both series consist of thin, excessively drained soils on sloping to very steep uplands. These soils have formed by weathering of the sandstone bedrock. They are of very low fertility and low water capacity, making them unsuitable for crops. Before settlement, most of our area on these soils was probably oak savanna, and after settlement they were primarily in pasture.

There is an Eleva soil area on more level ground at the east end of Pleasant Valley Conservancy that was historically somewhat suitable for cultivation. In dry years crops probably did poorly, but during wet cycles, crops such as oats or alfalfa were raised. In 1987 this area was put in the Conservation Reserve Program (CRP) and in 1998 was planted to prairie (Toby's Prairie). It is a thriving prairie now, and may well have been so before settlement.

There is a large field at the east end of the Conservancy, on a lower slope, which is of the Seaton series. This soil developed on more level ground, and is of higher fertility and moderate to high water-holding capacity. The soil probably receives seepage water from the higher lying, bedrock-controlled upland. This soil was cropped for corn, oats, or alfalfa, but for most of the historical period it was probably in alfalfa or grass hay. It was put in the CRP in 1987 and planted to prairie in 1999. Most of this field is now a quite attractive mesic prairie (now called the "Pocket Prairie"), with dry-mesic prairie on the upper slopes. Other agricultural fields now in prairie include the Barn Prairie, the Valley Prairie, and the Ridge Prairie. Details on converting agricultural fields to prairie and links to each of these individual prairies can be found at this link.

The Soil Conservation Service classifies the soils on the steep south- and north-facing slopes as "Stony." These soils consist of a very thin layer of soil material with bedrock very close to the surface. There are also outcrops of bedrock, as well as fragments of rock lying on the surface. Many of these fragments are dolomitic, derived from the rock outcrops above. Stony soils are very low in fertility, with low water-holding capacity. The rooting zone is very shallow and runoff is rapid. On Pleasant Valley Conservancy, some of the south slope was probably grazed (fences were removed when restoration began), and grazing may have caused some erosion. However, the slope is so steep that animals would have had difficulty finding stable footing. After grazing ceased, the south slope may have reverted to open savanna and prairie, which were gradually replaced by red cedars, elms and walnuts, and woody brush. The rocky south slope has now been restored to near its native state: prairie on the lower slope and bur oak savanna on the upper slope. The dominant plant on the lower slope is little blue stem, which is very attractive in the fall of the year.

Map of soil pH in relation to bedrock distribution
Details of the origin of this map can be found on Tom's Blog
Download a high-resolution version (PDF).