In the deeps of the geologic timescale the two shorelines of the Lehigh River occupied by the 19th-century towns of Mauch Chunk and East Mauch Chunk were situated on the bottom of an ancient river-fed tarn, a mountain lake which filled the valley on the west bank and covered the relative flatlands on the east bank. The muddy bottom of that high tarn (the range then rivaling the Himalayas in size), where the waters pooled at a lower elevation amongst the twisted folds of four near-parallel ridgelines, created a level region whose settlements became the relatively flat lands on either bank of the Lehigh. The ridgelines, which run east-northeast to west-southwest, are (from north to south) Broad, Nesquehoning, Pisgah, and Mauch Chunk ridges (or Mountains)—each of which runs over west to the gaps cut by the Schuylkill River.