The Plant Communities of Rocky Glen Park

The varied plant communities of Rocky Glen Park represent one of the Glen’s great treasures, along with its unique geology and historic sandstone carvings. There are three areas of the Glen: the actual Glen, the surrounding remnant prairie meadows on the Glen’s upland slopes and the wooded uplands beyond the prairies.

The main ravine of Rocky Glen has steep walls, exposed sandstone and a north south orientation. There is a perennial stream that along with the exposed rock provides habitat for plants not commonly found in central Illinois. Some of these plants include bishop’s cap (Mitella diphylla), hepatica (Hepatica acutiloba), bulbet bladderfern (Cystopteris bulbifera), various mosses and liverworts.

The meadows exist on the upper slopes that surround all three sides of the Rocky Glen canyon. They total about 5 acres in 6 separate areas. Forty years ago these meadows contained many varieties of grasses and flowers, including spring ephemeral flowers. Originally part of a much larger plant community, all of the meadows have become heavily shaded by brush and small trees. Restoration of the prairie meadows is a primary focus of volunteer workers during monthly restoration workdays. Removal of non-native plants such as bush honeysuckle and regular prescribed burnings will be done to rejuvenate the native prairie plants.

The flat plateau east of Rocky Glen is heavily overgrown with brush and woods so that is nearly impassable except for several ATV trails. This area was a former horse pasture until abandoned in the 1960s and has been seriously disturbed. The plateau ends on the east side with a very steep drop off of 120 feet to the Kickapoo Creek floodplain. There are several small areas of native plant communities on the edge of the drop off and occasional slopes.

North and northwest of Rocky Glen there are two streams that form the Glen waterfalls. This land is mostly flat, dissected by two large gullies. Much of the landscape is a mature mixed hardwood forest of oak and hickory. There is no evidence of logging having occurred in these areas of Rocky Glen.

Most of the flat terrain directly west of the Glen was farm field until the 1950s. It is now a nearly closed forest canopy with a variety of bushes and trees. This area over time is becoming a mature forest that includes both understory canopy trees and a variety of ground cover. The plant community is a mix of both native and non-native plants. To the west and south of the Rocky Glen Park is an additional 55 acres of natural area owned by the Peoria Park District. To the north of the property is approximately 15 acres of natural area owned by the Illinois Department of Transportation.

What follows is a technical description of specific plant communities at Rocky Glen as they looked nearly 40 years ago, taken from the field work and written description by John White, biologist for the Illinois Nature Preserves Commission in his summary of the property in March, 1973. Mr. White’s description is entirely about the privately owned land that is now Rocky Glen Park.

Character of the area

The lower slopes (of Rocky Glen), are forested with a mixture of mesophytic species, including red oak (Quercus rubra), sugar maple (Acer saccharum), and basswood (Tilia americana). There are some sycamores (Platanus occidentalis) and American elms (Ulnus americana) along the intermittent stream. The understory on the lower slopes mainly consists of hophornbeam (Ostya virginiana), with hydrangea (Hydrangea arborescens) and musclewood (Carpinus caroliniana) on moister sites. The upper slopes are dominated by white oak (quercus alba) and black oak (Quercus velutina).

Where the forest borders the prairie, prairie grass grows beneath the oaks, particularly on the east side of the Rocky Glen. The thickets are of sassafrass (Sassafras albidum), smooth sumac (Rhus glabra), dogwood (Cornus Sp), wild black cherry (Prunus serotina), and big tooth aspen (Populus grandidentata).

Prairie Location

The prairie remnants occupy sloping prominences between side-ravines of Rocky Glen. They are spurs of the upland prairie, which has been destroyed by farming. They are abruptly truncated by fences that separate the prairie from the fields. There are 8 prairies, ranging from 0.1 acres to 1.9 acres, in total 5.6 acres, excluding those areas that have been overgrown by woody plants. The largest prairies face west to southwest, but all slope aspects are represented, because the prairies also occupy north and south-facing slopes.

Prairie Grasses

Little bluestem (Andropogon scoparius) is the dominant grass; less-common prairie grasses, which might not occur on every prairie include Indian grass (Sorghastrum nutans), big bluestem (Andropogon gerardi), prairie dropseed (Sporobolus heterolepis), and side oats grama (Bouteloua curtipendula).

Prairie Flowers

Even a partial list of the prairie forbs is large: compass plant (Silphium laciniatum), prairie dock (Silphium terebinthinaceum, smooth aster (Aster laevis), heath aster (Aster ericoides), showy goldenrod (Solidago speciosa), downy gentian (Gentiana puberula), wild indigo (Baptisia sp), bush-clover (Lespedeza capitata), rattlesnake master (Eryngium yuccifolium), fox-glove ( Penstemon sp), coreopsis (Coreopsis palmata), milkwort (Polygala sp), purple coneflower (Echinacea purpurea), blazing stars (Liatris aspera.

Woody plants characteristic of the prairies are leadplant (Amorpha canescena), prairie rose (Rosa Carolina), and prairie willow (Salix humilis). The following woody plants are invading the prairie: blackberry (Rubus sp), big-toothed aspen, smooth sumac, hawthorn (Crataegus sp), and oaks, particularly black oak (Quercus velutina).

The herbaceous flora of Rocky Glen is very rich; notable speices include bishop’s cap (Mitella diphylla), hepatica (Hepatica acutiloba), bulbet bladderfern (Cystopteris bulbifera), and sand phlox (Phlox bifida). The flowering of woodland spring ephemerals is especially spectacular.

Other plants once known in the area by Virginius Chase ( 19th Century Peoria naturalist), that are unique to Peoria County include small flowered agrimony (Agrimonia parviflora), broomsedge (Adropogeon virginicus), the sedge Carex communis, sand phlox (Phlox bifidi), the bluegrass Poa chapmaniana, milkwort (Polygala sanguinea), lousewort also known as wood betony (Pedicularis canadensis) and nutgrass (Sclera triglomerata).

Area Resources and Values – Natural Quality and Rarity

Both the forest and the prairie at Rocky Glen are of high natural quality; the existence of both together is remarkable because prairie in general has been practically eliminated in Illinois, and the original forest – prairie border is rarely found in a natural state.

There are many indications that the forest at Rocky Glen has never been logged. Although the larger trees average only about 1.5 feet in diameter, and the largest are 2 feet in diameter, they are fine old-growth trees. In the ravine the trunks are commonly clear of limbs for 40 feet above the ground, and the canopy is closed with the large, speading limbs that can only develop after many years of stability. Above the ravine, the oaks likewise have massive, spreading limbs. The trees are not particularly large, but this is a result of natural factors, not unnatural disturbance. The soil of the steep slopes is in many places very thin, hardly more than weathered bedrock ( shale). Red oaks grow on this thin soil, though their growth must be slow, and it is readily apparent that the trees topple by the time they are a foot in diameter, due to the shallow rooting depth. There are no stumps in the tract, although old stumps may be found in the adjoining woods, indicating that if trees had been cut, their stumps might still be visible. The composition and structure of the ravine forest and its understory it that of a mature forest, and the dense, young thickets that have invaded the upland are not the result of cutting, but probably the result of fire suppression. Otherwise, there are no great numbers of secondary successional species in the forest; and, although the source is nearby, there are no weedy trees such as white mulberry (Morus alba), Osage orange (Maculara pomifera), and black locust ( Robinia pseudoacacia) in Rocky Glen. The difference between the forest at Rocky Glen and the disturbed forest on the bluff direct adjoining Rocky Glen on the east are quite apparent.

Diversity of Habitats

Rocky Glen is diverse because it has both prairie and forest. There a number of differnt enviornments because of the varying slope aspects and differences in substrata ( loess, glacial drift and various bedrock exposures). However the ravine bottom is so narrow that the stream occupies essentially the whole bottom in the upper part while the mine dumps have destroyed the ravine-bottom community at the lower end, if the bottom ever was broad enough to support a separate and distinct community. Therefore such species characteristic of ravine bottoms such as black walnut (Juglans nigra) and Kentucky coffeetree (Gymnocladus dioica), are absent from Rocky Glen.

One of the two largest prairies along Rocky glen is in or near the proposed right-of-way of the highway to be built north of Rocky Glen. The highway should be built so that the least disturbance would be made to the priaire and adjoining woods. Potential damages include direct destruction if the highway is built across the area, instead of moved to the north, and damages from moving or parking heavy equipment in the prairie. Erosion from the earthmoving activites could possibly result in siltation downstream in the scenic waterfall and rock shelter area of Rocky Glen.