Starved Rock

The sandstone which constitutes Starved Rock took millions of years to be formed and subsequently deformed and just a few thousand years to be carved away, leaving the landscape that we can see today.

Approximately 505 to 438 mya during the middle of the geologic time period referred to as the Ordovician, this portion of Illinois was located approximately 20 degrees south of the equator on the fringe of a shallow water marine environment. The sandstone cliffs at Starved Rock were formed out of this environment as evident by the constitution of the sandstone. An examination of the sandstone with a hand lens reveals well-rounded well-sorted grains, made almost entirely of pure quartz. These features of the sand are the result of washing around in the surf, which sort and round the quartz grains while washing the clays and other impurities out to sea. The sandstone, called St. Peter Sandstone after the St. Peter River in Minnesota where it was first identified, was deposited in some areas as thick as 520 feet.

In the area surrounding Buffalo Rock State Park, the layers above the sandstone consist of Pennsylvanian age coal and shale. In the Pennsylvanian period, approximately 326 to 286 mya, Illinois was located just south of the equator in a swampy environment. This swampy environment facilitated the deposition of coal, as discussed earlier, and shale which is formed by the consolidation of clay. The layers in this area progress from the Ordovician aged sandstone to the Pennsylvanian aged coal and shale. There is no evidence of Silurian, Devonian, or Mississippian aged rocks in this sequence, however, Silurian and Devonian aged rocks are found both to the east and west of this area. This evidence suggest that the Silurian and Devonian aged rocks must have been brought to the surface in this area and eroded away before the Pennsylvanian strata was deposited. The local uplift was brought about by the formation of the La Salle Anticlinorium. Due to the erosion of Devonian and Silurian aged strata prior to the deposition of Pennsylvanian aged strata, the time period for the formation of the La Salle Anticlinorium is estimated at 360 to 326 mya, during the Mississippian period.

Although these layers of sandstone appear to be flat they are actually forming the east limb of the La Salle Anticlinorium, dipping approximately 5 degrees to the east. The sandstone at Starved Rock is approximately 100 feet above ground, however, three miles to the west this same layer of sandstone is found 1,000 feet below the surface. This structure of layers found in the Starved Rock area is called an anticline because older rocks are found on the inside of the fold and younger rocks are found on the outside of the fold. In contrast, a syncline is a fold in the earth in which the younger rocks are found on the inside of the fold and older rocks are found on the outside of the fold.

During the Wisconsinan glaciation, this area surrounding the Illinois river valley was buried with glacial sediment, leaving the ground elevation relatively flat like most of northern Illinois. This being the case, why does Starved Rock State Park look much like you would expect an Illinois landscape to look before the ice ages? The reason for this landscape is due, in a large part, to a major flood called the Kankakee Torrent and to a lesser degree, following floods caused by the periodic draining of Lake Chicago and Lake Nipissing.

Approximately 14,000 to 15,000 years ago the melt waters from the Wisconsinan glacier became too great for the glacial moraines north and east of Starved Rock to hold. The water broke through the moraines and flowed to the headwaters of the Vermillion River, a tributary of the Illinois River, causing what is referred to as the Kankakee Torrent. Since the main outlet for these waters was the Illinois River, the flood became concentrated in the Illinois valley, scouring out glacier deposits and bedrock. It is believed that the current Illinois River was the Kankakee Torrent's inner channel, increasing the width of the Illinois valley by a half mile. Over the next 10,000 years, periodic flooding of Lake Chicago and Lake Nipissing deepened the Illinois River Valley an additional 60 feet.

Today erosional processes continue to wear away the sandstone and widen the river valley although at a much slower pace comparative to what the major floodwaters did. The planks and staircases around Starved Rock State Park were built to prevent the effects of erosion from Humans. Because the sandstone is loosely cemented, it is easy to imagine the effects of thousands of people walking on top of the sandstone, where each step erodes away more of the stone. Erosional processes that can not be protected against (without spoiling the natural beauty) are the effects of water and wind on the sandstone. Although it is an erosional process, wind has little effect on the sandstone, at least in the perspective of our lifetime. You can however, see the effects water has had on the sandstone by looking at the rocks below you. The sandstone displays an uneven surface because some areas are cemented stronger than others.