CONTROLS ON TRELLIS DRAINAGE PATTERNS OF GLACIAL LAKE ALBANY SEDIMENTS, WASHINGTON COUNTY, NEW YORK

During deglaciation of the Laurentide Ice Sheet, the Battenkill and Hoosic Rivers deposited deltaic sediments into Glacial Lake Albany in southern Washington County, New York. Incised into the topographically smooth lake and deltaic sediments are at least five trellis-shaped drainage networks. The main stems of the drainages are incised into, and have orientations down, the steepest gradient. However, the main tributaries to each channel are approximately perpendicular to the steepest slope and do not have topographic controls at the surface that influence drainage.

In order to understand the formation of the regularly-spaced trellis drainages, we collected map and field relations of the networks. The main tributaries perpendicular to the steepest slope have a strike of N15E, approximately parallel with the mountains bounded by the Taconic Frontal Thrust System ~3 km to the east. All of the trellis drainage networks head on the Glacial Lake Albany sediments and are disconnected from the Taconic Mountains. The spacing of the main stem channels is ~1.5 km.

Approximately ~10 km to the north there is a similar network of streams that lie above the maximum lake level obtained by Glacial Lake Albany. These streams feature a trellis-shaped drainage pattern that is controlled by the blocks of the Taconic Frontal Thrust. We propose that these drainage networks are an analog to those incised into the Glacial Lake Albany and deltaic sediments. The morphology of the trellis patterns incised into the glacial lake sediments are controlled not by the sediment, but by the underlying geology of the Taconic Frontal Thrust. Groundwater flow from the Taconic uplands to the Hudson River is blocked by the subsurface ridges that are sub-parallel to the Taconic Frontal Thrust and effectively form groundwater dams. The groundwater flow is thus focused and directed through the gaps between the bedrock ridges. Down gradient this groundwater formed springs that through sapping developed into headward eroding channels.

In order to more fully test our hypothesis we will collect drill logs to determine the depth to bedrock and nature of the sediment-bedrock interface. We will also make additional observations in stream channels to map the presence of bedrock outcrops.