USING AIRBORNE LIDAR DATA TO CREATE A SURFICIAL GEOLOGIC MAP OF THE UPPER NARRAGUAGUS RIVER WATERSHED, MAINE

Airborne laser swath mapping (ALSM) or lidar provides excellent fine-scale detail to create topographic and surficial geologic maps. We present a map of the surficial geology of the upper Narraguagus River watershed that demonstrates the potential of mapping using 1-m pixel resolution lidar digital elevation models (DEMs). Importantly, ALSM penetrates tree canopy allowing images of the bare earth not easily seen with aerial photography. ALSM technology has other distinct advantages over aerial photographs including the ability to digitally manipulate data by color coding for elevation and by increasing the contrast between slight changes in topography. This manipulation enables remote-sensing mapping of geomorphic features at finer resolution than was previously possible. Our work in the upper Narraguagus watershed used detailed lidar image analysis and ground truthing to interpret glacial and post-glacial features and history. The study area was covered by ice during the Wisconsinan glaciation. At the last glacial maximum, sea level was >100 m lower due to eustatic change, and the land surface of Maine was isostatically depressed. As the ice began to retreat, a drape of till and eskers were deposited in the upper Narraguagus watershed, and sea level rose to meet the ice front. Sediment from central Maine, far outside of the present watershed, was transported through the ice, and deposited both as eskers and lobate subaqueous outwash plains composed of gravel, sand and finer marine sediments. Then, isostatic rebound began to outpace eustatic change and relative sea level fell. The outwash delta deposited around ice blocks left behind leaving depressions, some of which are now occupied by lakes. Topographic analysis suggests that separate outwash surfaces were once continuous, and slope SSE approximately 0.25%. During the Holocene, the Narraguagus River attained its present watershed boundaries, and began to incise the late Pleistocene deposits.