Page:The Journal of geology (IA journalofgeology11893univ).pdf/155

lin observed decisive evidences of the lapse of a considerable interval between the formation of a moraine lying east of Mad river, near Urbana and Springfield, and the moraines on either side of it, the moraine on the east being one of the later Scioto moraines, while that on the west is a Miami moraine. This older moraine proves to be the outer moraine of the Miami lobe (see map). While this moraine was being formed the Miami lobe occupied the Mad river drainage area, and the waters from the melting ice-lobe were forced toward the south into the Little Miami valley, passing just east of Springfield. The course is well defined, there being a gravel plain leading south along the east side of this moraine. The altitude of this gravel plain is much greater than that of the immediate bluffs of Mad river valley, but is lower than the water-shed between the Mad river and the Scioto river system, just east of it. It consequently presents somewhat the appearance of a broad irrigating ditch, following the face of a slope at a considerable altitude above the stream. When the ice had retreated from the Mad river basin the drainage of the high country to the east of Mad river soon opened channels directly across this gravel plain and the moraine west of it down to the trough in which the river flows. Similar channels were formed by streams leading down to Mad river from the elevated country west of its basin, and a broad valley was opened along the axis of the trough. When a fresh advance of ice occurred the Miami lobe came nearly down to the Mad river valley from the west and covered the upper portion of the western tributaries. Its moraine, in crossing the interglacial valleys, descends into them, but only partially fills some of them, thus repeating the phenomena of the outer moraine, in the Whitewater valley, as noted above. Similarly, the Scioto lobe trespassed on some of the eastern tributaries of Mad river, and its moraine partially fills the interglacial valleys. It should, perhaps, be stated that these interglacial valleys do not follow preglacial troughs, but instead, have bluffs standing as high as the interfluvial portions of the slopes of the basin. Their excavation began with the retreat of the ice-sheet from the outer Miami