The geology of Franklin Creek is of considerable interest as the valley is deep enough to expose three distinct rock strata. New Richmond Sandstone, a soft rock of Lower Ordovician age, is exposed at the bottom of the gorge. These are some of the oldest rocks anywhere in the state, dating back to approx. 485 million years ago. Above the New Richmond sandstone lies the Shakopee Formation, consisting of sandy dolomite and also of Lower Ordovician age, albeit slightly younger. This strata is capped with St. Peter Sandstone, which is of Middle Ordovician age, approximately 458-453 million years old. Away from the outcrops along the creek, which have been exposed by erosion, the bedrock is covered with a deep till from the Wisconsin glaciation. Finally, this is mantled with a layer of loess from which the upland soil is developed.
The occurrence of vegetation is influenced by soil characteristics, topography and moisture. Low lying areas along the creek bed support a bottomland forest dominated by silver maple and hackberry. Slippery elm and Kentucky coffee tree also occur here. Ravines support mesic (intermediate between wet and dry) upland forest characterized by sugar maple and basswood. Pawpaw, an understory shrub of more southernly distribution, is found here. On drier sites, there is a dry-mesic forest of white oak, red oak, black oak, shagbark hickory, and hop trees. May is a great month to see the spring ephemerals, especially the bluebells that blanket these wooded areas.
A small glacial drift hill prairie remnant is present in the preserve. Indian grass and tall dropseed are characteristic plants of this habitat. Other prairie species present include false toadflax, flowering spurge, and hoary puccoon.
High quality, undisturbed cliff communities are present on the bedrock outcrops. Canada yew, an evergreen shrub of more northern woods, is common here. Bladdernut, bishop's-cap, and shooting star are among the many wildflowers found here, as well as several species of ferns.
A vegetation survey was conducted in 2018 by Chris Benda. The plant list can be found below.
The forested areas provide habitat for many species of non-game wildlife. Woodpeckers, flycatchers, thrushes, vireos, and warblers are among the breeding birds on the site. The area provides critical stop-over habitat for many more species during migration. Great blue herons and green herons have been observed here. Mammals utilizing the area include raccoons, chipmunks, gray and fox squirrels, 13-lined ground squirrels, beavers, and deer. Shrews, white-footed mice, weasels, and foxes may also be seen. A patient and observant eye may note toads, bullfrogs, water snakes, and several kinds of turtle. Thirty species of fish have been recorded from Franklin Creek. One of our ongoing projects is creating a spreadsheet that hopes to record every animal that has been observed at FCSNA. This spreadsheet is updated frequently.