Fluvial Terrace Mapping

written by Matt T. Bleakley

Left:Drainage Basins Using ArcInfo

Eastern Pennsylvania has experienced several glaciations throughout its recent history. This study examines the effects of the latest of the glaciations on the Lehigh Valley, and more specifically the Jordan Creek. Most recently ~17 Kya. The Laurentide Ice sheet extended down to ~40 km to the north of Allentown. The ice sheet did not extend into the Lehigh Valley during this advance. The previous glaciation to effect the area was the Illinoian. At ~200 Kya. The Laurentide Ice sheet extended down to Blue Mountain, but did not quite cross into the Lehigh Valley. The terminal moraines for this glaciation, among other glacial features, can be found just to the north of the Lehigh Valley, and are rather well preserved. However there exists evidence for a still more ancient glaciation. Less obvious features are present in the Lehigh Valley due to their much more ancient age. Paleomagnetic studies (Richmond and Fullerton, 1986) have constrained their ago to be within 770 – 970 Ka. The exact timing of these events is difficult to obtain due to the severely weathered state of most of the glacial features. At its furthest extent the ice sheet would have blocked the flow of the Jordan Creek into the Lehigh River. The Jordan would then have emptied into a ppro-glaciallake extending up the Lehigh Valley to the west. The presence of the lake at this time would have allowed for the development of outwash features, some of these features still somewhat visible today. Sediments derived from the ice mass flowed out into the lake and were deposited as the present day valley floor. Several relatively low relief areas in the valley can be correlated to create a profile of how this pro-glacial lake may have filled, and subsequently drained. This study focuses more deeply on the draining of that lake, and its effect on the Jordan Creek. As the lake drained to the west, and the ice front began to recede the lake level would have dropped. The base level for the Jordan Creek would have followed along with the draining of the lake. The drainage would cause the Jordan to now cut down through the newly deposited sediments along the valley floor. The lake more than likely drained in stages, those stages are represented by the terraces present along its current channel. These terraces were correlated and used to look at how this pro-glacial lake may have drained.