The whole story of this mostly sand-and-gravel material below the loess is still not perfectly clear, but it is known that its history is completely different from the loess above it. Much of it is much older than our Peoria loess, and it was put there by processes very different from our loess's deposition by wind.
For example, at Natchez the topmost gravel is of Ice-Age, or Pleistocene, age, in the general neighborhood of 700,000 years old. Earl Manning at Tulane makes the interesting comment that "The reason that you can find agates in [the gravel] at Natchez, is that it's been washed south from the Lake Superior area (where the agates originally came from) by the Mississippi River.
In fact it's wonderful that the loess-zone's ravines and bayous often cut into these strata, for in the resulting streambed gravel and sand, some of which is pictured in a close-up at the left, there are fascinating discoveries to be made.
For example, there are semiprecious stones such as agates, carnelian, quartz crystal, jasper, and there is even petrified wood and fossils of other animals far, far older than the large mammals we list elsewhere as having been discovered buried in our loess's lower levels, such as Woolly Mammoths and Ground Sloths.
An interesting publication you might look for is D.T. Dockery III's "Rocks and fossils collected from Mississippi gravel" in Mississippi Geology, v. 16, no 2, pp 25-42, 1995.