ORIGIN OF A LOW-LYING BOULDER FIELD, WESTERN MAINE

An extensive boulder field of uncertain origin is located between two large lakes in western Maine. Trending east to west between Rangeley Lake and Mooselookmeguntic Lake, the boulder field is mapped as a meltwater channel (Caldwell, 1974), but the absence of a pronounced topographic channel warranted further research into the origin of this feature. Recent work on Rangeley Lake documents evidence for a post-glacial lake larger than present (Metcalfe, 2007). An alternate origin for the boulder field is as a lag deposit in an outlet channel associated with this lake.

The boulder field is divided into two visible areas (7800 m² and 4000 m²) separated by a road and forest. Boulders size range from 3 m to 50 cm long at the surface and decrease in diameter at depth. The two boulder lithologies present are schist and granite. These rocks derive from the Dead River Formation and Adamstown pluton, outcropping immediately to the west-northwest of the boulder field. Boulders are sub-angular to well rounded, and winnowed of fines. The thickness of the boulder field is greater than 2 meters. Forest vegetation obscures most of the lateral contact between the boulder field and adjacent till. The till provides a source material for the development of soil and subsequent growth of vegetation which is now encroaching the boulder field. The surface between the boulder field and the till zone show no change in elevation. The elevation of the boulder field surface decreases to the southwest and has less then a 1 percent gradient. The orientation of the long axis of the boulders tends to be predominantly to the west.

The shape and orientation of the boulders support the idea of a lag deposit. However, if this is a lag deposit, a significant volume of interstitial finer material originally found between the boulders must have been removed. These fines may have been transported into Mooselookmeguntic Lake. Furthermore, the removal of so much material would be expected to result in the development of a topographic channel, but no distinct channel is observed in the boulder field.