 Every mountain except Slide (and perhaps West Kill) was covered.The glaciers left behind lakes when they melted, some of which became reservoirs in the 20th century.Some geologists also believe the glaciers scoured off sedimentary layers that may have contained coal, noting the surfeit of coal to the south in Pennsylvania and that the white quartz pebbles found atop Slide Mountain are often an indicator that one has reached the bottom of a coal bed in those regions.