Thorn Creek Watershed Based Plan December 2005
2–4 Watershed Resource Inventory
ure 2-1 (Appendix A) provides a color-classified map of elevations, as well as floodplains, within the wa-tershed.
The elevation change from the headwaters, at 790 feet above sea level, to the mouth, at 585 feet,
is a little more than 200 feet along a horizontal distance of about 20 miles, an average gradient of 10 feet
per mile. The general shape of the profile (Figure 2-2, Appendix A) illustrates the substantial differences
in stream gradient between the upper morainal portion of the watershed, where the gradient is over 17
feet per mile, and that of the Lake Chicago Plain to the north, where it is less than 3 feet per mile.1
There are two soil associations in the upper (southern) morainal portion of the Thorn Creek watershed,
one formed in silty clay loam till, the other in heavier till of silty clay texture (Figure 2-3, Appendix A). A
common soil of the latter association is Bryce silty clay (235), a hydric soil of small depressions and drain-ageways,
formed under wet prairie or marsh vegetation. Another common soil of this association is
Frankfort silty clay loam (320A–C soil type), an upland soil of level or gently rolling terrain. Frankfort
probably supported prairie and/or savanna vegetation. Two other soils, Napanee silt loam (228B-C) and
Chatsworth silty clay (241D–F) are found along the wooded slopes of the upper Thorn Creek Valley.
Soils of the silty clay loam association are widely distributed in this part of the watershed. The common
hydric soil is Ashkum silty clay loam (232), found in small drainageways and wet spots throughout. Two
soils, Blount silt loam (23) and Morley silt loam (194C–F), are characteristic of the extensive woodlands of
this area. Blount is found on level uplands, while Morley is the soil of slopes. Two other widely distrib-uted
soils, Beecher silt loam (298) and Markham silt loam (531), are also located on uplands, but probably
originated under prairie and/or savanna vegetation. They are the loamy analogues of Frankfort.
The major soil of the Thorn Creek floodplain is Sawmill silty clay loam (107). It occurs both in the upper,
morainal reaches of the creek and in the floodplain of Thorn Creek as it traverses the Chicago Lake Plain.
Within the watershed, the boundary between the Tinley ground moraine and Chicago Lake Plain runs in
an arc from just south of Route 30 at the Indiana state line in the southeast to the intersection of 183rd
Street and Chicago Road in the northwest.
Two groups of soils are characteristic of the Chicago Lake Plain: soils of the lake plain proper, and those
of the two beach ridges associated with lake plain. Additionally, soils developed on bedrock outcrops
may be found in several places, most notably in the vicinity of the Thornton Quarry.
Major soils of the lake plain itself are Milford silty clay loam (69), Martinton silt loam (189), and Del Rey
silt loam (192). Milford is a poorly drained soil of flats, shallow depressions and drainageways. Martinton
and Del Rey occur at slightly higher elevations, and are somewhat better drained. Martinton developed
under prairie vegetation; Del Rey may have originally supported savanna or open woodland. Morley, a
principal forest soil of the moraines, also occurs on lake plain, in narrow strips along the wooded slopes
of the Thorn Creek valley. Conversely, Milford, Martinton and Del Rey are found in the morainal region,
occupying the basin of Glacial Lake Steger, an ice-front lake, whose basin lies between the Tinley moraine
and Valparaiso ground moraine.
1 Sources: Thorn Creek: An Inventory of the Region’s Resources, 2000. Illinois Department of Natural Resources, Critical Trends As-sessment
Program. Bretz, J. Harlan. 1955. Geology of the Chicago Region. Part II – The Pleistocene. Illinois State Geological Survey Bulle-tin
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