A cross section of Pleasant Valley Conservancy, drawn for the Savanna Oak Foundation, Inc. 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. This formation is exposed on the wall of the old quarry. 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.

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. 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 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.

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Soils

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 of the U.S. Department of Agriculture. 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. 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 has been 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 and the Valley Prairie. Details on converting agricultural fields to prairie can be found at the following 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. 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 (see photo above). The dominant plant on the lower slope is little blue stem, which is very attractive in the fall of the year.

 

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