Besides being obstructed where crossed by the terminal moraine, the valley of the Baraboo was clogged to a less extent by drift deposits between the moraine and the Lower narrows. At one or two places near the City of Baraboo, such obstructions, now removed, appear to have existed. Just above the Lower narrows (c, Plate XXXVII) there is positive evidence that the valley was choked with drift. Here in subsequent time, the river has cut through the drift-filling of the preglacial valley, developing a passage about twenty rods wide and thirty-five feet deep. If this passage were filled with drift, reproducing the surface left by the ice, the broad valley above it would be flooded, producing a shallow lake. The retreat of the ice therefore left two well defined drift dams in the valley, one low one just above the Lower narrows, and a higher one, the moraine dam, just west of Baraboo. Disregarding the influence of the ice, and considering the Baraboo valley only, these two dams would have given rise to two lakes, the upper one behind the higher dam being deeper and broader, and covering a much larger area; the lower one behind the lower dam, being both small and shallow. Up to the time that the ice retreated past the Lower narrows, the waters of the upper and lower lakes were united, held up to a common level by the ice which blocked this pass. After the ice retreated past the Lower narrows, the level of the Baraboo lake did not sink promptly, for not until the ice had retreated past the site of the Wisconsin was the present drainage established. Meantime the waters of the Baraboo lake joined those of Wisconsin lake through the Lower narrows. If the lakes had been before connected at some point farther west, this connection through the narrows would not have changed the level of either. If they were not before connected, and if the Wisconsin lake was lower than the Baraboo, this connection would have drawn down the level of the latter.