DROPSTONES BEYOND THE GLACIAL MARGIN IN KENTUCKY

Exotics located away from the glacial margin are rare in Kentucky, but do occur. In eastern Kentucky, boulder size exotics have been documented to have weathered out of Devonian black shales and interpreted as glacial dropstones. Cobble size exotics also occur due to Native American activities. Recent surficial geologic mapping in Kentucky has led to the discovery of pebble to boulder size exotics away from glacial drainages. The boulders weigh ≈150 lbs or more and have not been identified to be associated with any known cultural sites. They are also not located near Devonian black shale outcrops. The surface drainages associated with the finds are tributaries to glacial drainages, but were not glaciated themselves.

The lithologies of the exotics are meta-sediment, gneissic granite, and granite pebbles to boulders. Three were located along the Green River in western Kentucky. One of these was a granite pebble extracted from a core near Calhoun, KY. Additional sites include a granite pebble field near Lexington, KY in the Kentucky River basin, and a gneissic granite boulder near Virginia, KY in the Tradewater River basin in western Kentucky. Other occurrences have been noted in the Tradewater River Valley near Sturgis, KY.

These examples are seldom seen in-situ as they are often incorporated into the discoverer‘s landscaping and only seen by investigators thereafter. Most are found, or have been reported to have been found, near the surface. Two in-situ examples are the gravel from a core near Calhoun, KY, and the gneissic granite boulder from Virginia, KY. The gravel from the core was found at 79 ft. and is likely Illinoian in age while the Virginia boulder was exposed in a corn field rill indicating a late Wisconsinan age. These exotics likely represent dropstones that were carried down glacial drainageways and then diverted into slackwater lakes in tributary valleys. Possible mechanisms for the transport up valley are either by wind action, fluvial transport during flooding events, or a combination of the two.