The Ludlow Formation (lower Paleocene) of southwestern North Dakota contains elongate lenticular light- to medium-gray sandstone bodies several kilometers long, hundreds of meters wide, and more than 30 meters thick. Internal sedimentary structures, relation to surrounding beds, and fossil content indicate a fluvial-channel origin for these sandstone bodies. Marginal and bottom relations between these channel-sandstone bodies and the enclosing rocks are typically crosscutting, with the sandstone being inset in trough-shaped channels.

One sandstone body, otherwise typical of the above sandstone bodies, is not inset but is a thick lens between marker lignite beds. Lignite beds are separated by less than 1.5 m of sediment at the margins of this body. A kilometer distant, the lignite beds are separated graphically by more than 30 m of channel sandstone. The underlying marker lignite beds are not eroded or crosscut but remain intact and are split or wedged from the overlying marker lignites by the sandstone body. This relation indicates that the channel-fill sandstone occupies a preexisting, largely non-stream-eroded depression, and lies with minimal erosional unconformity on the underlying sediments. The occupied depression was probably formed as a result of localized compaction of sediments. Other possibilities for t e development of the depression include intermittent deformation associated with the nearby Cedar Creek anticline, or the possible solution of underlying salt beds of Paleozoic and Mesozoic ages. Whatever the origin of the depression, it was occupied and back filled by channel deposits of a stream. Other channel-sandstone bodies in this area combine the inset and wedging relationship in the interval between marker lignite beds.

This rapid, large-scale wedging part of marker lignite beds creates problems in detailed correlation within the Ludlow Formation.

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