The best exposure of the breccia beds is that of the cut of the Chicago, Milwaukee and St. Paul Railway west of Linn Junction. The brittle lower Davenport has here been broken and rebroken into a mass of small sharp-edged fragments, while the tough heavily bedded upper Davenport ledges have been fractured to large blocks, which sliding on each other have smoothed and grooved their sides. The breccia beds may be seen in the upper eleven feet of the Kenwood cliff, at Troy Mills, and in the cliffs along the Wapsipinicon valley as far down as near to Central City. THE CEDAR VALLEY LIMESTONES The Otis, Independence, and Davenport limestones form a group called the Wapsipinicon, from its outcrop along the river of that name in Linn county. The remaining limestones of the Devonian are grouped together under the name of the Cedar Valley. These consist of limestones of various types, sometimes crowded with fossils, and sometimes destitute of any trace of ancient life. They occupy the western townships of the county. THE CARBONIFEROUS At the close of the Cedar Valley stage the sea retreated westward from our area, and Linn county became dry land. For long ages its rocks were covered with rich soils supporting a luxuriant vegetation, probably tropical in its aspect. We know that running water channelled this ancient land, for when at last in _Pennsylvanian_ (Coal measure) time the land sunk slowly beneath the sea, there were deposited in such channels clays and sandstones, which perhaps are only remnants of wide sheets of similar deposits now removed by denudation. A mile and a half south of Marion (southeast quarter of section 12, Rapids township) a well twenty-three feet deep penetrated a bed of dark shale which carried leaf impressions of a number of ferns characteristic of the undergrowth of the Carboniferous forests. A third of a mile southeast of Lisbon, and again about two miles south of the same village, at Bertram at the east end of the railway bridge, and on the old county road between Cedar Rapids and Marion, are exposures of sandstone which in some instances contain fragments of the logs drifted from perhaps distant uplands and water-logged and sunk in these ancient sand beds. The Bertram outlier contains many rolled coral fragments and worn bits of shells of the Devonian, included in Carboniferous deposits, much as the same fossils may now be found in the river deposits of the present age in the sand bars of the Cedar. MESOZOIC AND TERTIARY