The St. Peter, Prairie du Chien and Jordan rock units are widely distributed across the Upper Midwest. The following diagram shows the distribution of the rock units in Minnesota, Wisconsin, Iowa and Michigan. These rock units also extend in the subsurface into Illinois, Kansas and other states. In some places these units are the uppermost bedrock; in other places they are deeply buried beneath other rock units.
In some publications (like the Groundwater Atlas of the United States, from which many of the following diagrams were taken), the St. Peter Sandstone, the Prairie du Chien Group and the Jordan Sandstone are considered a single aquifer, although water flows very differently through the different units. In many places in southeastern Minnesota, layers of low permeability (and very slow horizontal water flow) exist at the base of the St. Peter Sandstone, in the lower part of the Prairie du Chien Group (the Oneota Dolomite) and within the Jordan Sandstone. Under natural conditions, these low permeability layers act as leaky confining units. (For more information, see this recent Minnesota Geological Survey report), Thus, pumping a well that draws water only from the Jordan sandstone may not affect the water in the Prairie du Chien rocks. And a pollutant plume that enters the St. Peter Sandstone may take hundreds of years to reach the Jordan Sandstone in some regions - and a much shorter time in others.
Figure 1. This figure shows the size of the St. Peter - Prairie du Chien - Jordan aquifer, which underlies much of Iowa, southeastern Minnesota, southwestern, southern, eastern Wisconsin, along with part of the Upper Peninsula of Michigan.
The St. Peter Sandstone and the Jordan Sandstone are examples of aquifers where water flows between grains of sand and also through fractures in the rock. The hydrologic properties of the Jordan Sandstone are similar to those of the St. Peter Sandstone.
Figure 2. An example of St. Peter Sandstone 
In contrast to the St. Peter Sandstone and the Jordan Sandstone, the Prairie du Chien Group consists mostly of carbonate rocks like limestones and dolomites. Most of the water is stored and flows along fractures in the rocks, seen clearly in the following photo and as described by the Minnesota Geological Survey.
Figure 3. An example of the dolomite from the Prairie du Chien Group
Rock units vary from place to place, not only in their names (see the table below) but also in their properties. Nevertheless, it is possible to separate the groups of units that form aquifers from those that form confining units.
Figure 4. Rock units for the St. Peter - Prairie du Chien - Jordan aquifer (between the lines). This figure shows the variation of rock units of this aquifer throughout Southeastern Minnesota, Iowa, Wisconsin and Upper Peninsula of Michgan.
It is possible to map groundwater flow directions on a variety of scales from water level data in wells and in surface water.
Figure 5. The horizontal flow direction of the St. Peter - Prairie du Chien - Jordan aquifer. Regionally, water in this aquifer unit flows southeastern toward the Illinois Basin and eastward into the Michigan Basin. Locally, water in the aquifer moves toward centers of pumping at Minneapolis-St. Paul, Minn., Green Bay and Milwaukee, Wis., and towards major drains including the Mississippi, Minnesota, Wisconsin, and Rock Rivers.
Figure 6. Bedrock Hydrogeology Map (Prairie du Chien-Jordan aquifer) for the Bridgewater township region. In most of the western 2/3 of the area, groundwater flows toward the Cannon River. In most of Bridgewater Township, the aquifer is confined by overlying glacial deposits.
Scale: grid spacing = 1 mile
Symbols:
Black Contours: elevation of potentiometric surface of the Prairie du Chien – Jordan aquifer
Arrows: Direction of groundwater flow
X – location of wells
Shades of blue: saturated aquifer thickness
Stippled area: aquifer is partly unconfined
Colored circles: locations of water tests
For full caption, full map and explanation, see Moira Campion, 1997, Rice County Geologic Atlas, Part B, Plate 8: Bedrock Hydrogeology.
A summary of some interpretations of this map can be found here.
The following cross-section extends northward through Dodge, Goodhue and Dakota Counties. It shows the layers of rocks in a vertical slice through the earth. Water flow lines are shown in blue. Note that water travels both vertically and horizontally through the aquifers, but mainly vertically through the confining layers.
Figure 7. Vertical flow direction of the St. Peter - Prairie du Chien - Jordan aquifer. Water moves downward through glacial deposits and the vertically stacked aquifers and leaky confining units, and then horizontally toward major drainages where it is discharged.
Aquifers are characterized by several properties. In general, the higher the transmissivity (the product of rock unit thickness and hydraulic conductivity), the more water an aquifer can supply for human uses. Though the properties of the rocks change from place to place, both the Prairie du Chien and the Jordan rock units are very useful sources of water.
Figure 8. The transmissivity, hydraulic gradient and storage coefficient of the St. Peter - Prairie du Chien - Jordan aquifer in various locations. Note the transmissivity of this aquifer ranges from less than 100 to 33,000 feet squared per day based on aquifer and specific-capacity tests.
Figure
9. The St. Peter - Prairie du Chien - Jordan aquifer's transmissivity and hydraulic
conductivity compared to other aquifers. Note that the Jordan aquifer has the
lowest transmissivity of them all (because it is relatively thin), but has a
higher hydraulic conductivity than the Mt. Simon aquifer.
Sources:
Unless otherwise noted, all figures were taken from USGS Groundwater Atlas