It was stated above that the definition of unconformity, as given, would include certain structures, which, nevertheless, must be distinguished from it: one of these is contemporaneous erosion. This structure is produced when a current of water excavates channels for itself in the still soft and submerged mass of sediment. After the current has ceased to flow, renewed deposition fills up the hollow with the same or a different kind of material as was thrown down before. This structure requires only a short pause in deposition, not a long, unrecorded break, and does not necessarily involve movements of elevation and depression. Furthermore, contemporaneous erosion is a local phenomenon, and though in a limited section it may not always be easy to distinguish it from an unconformity, the difference becomes apparent when a wider area is examined. If the structure be one of contemporaneous erosion, the two series of strata will be conformable except along the line of the channel or channels. Fig. 204 is an example of this structure and shows where a channel in an ancient sea-bottom was filled up by a later deposition of material.

The clay "horses" (as miners call them), which frequently interrupt coal beds, are the channels of streams which meandered through the ancient peat bog, and which were filled up with sediment when the swamp became submerged. The "horses" are usually of the same rock as that which forms the cap or root of the coal seam.

Contemporaneous erosion, cnannel in wall of Niagara Gorge.

Fig. 204. - Contemporaneous erosion, cnannel in wall of Niagara Gorge.

(U. S. G. S).

Horizontal And Oblique Bedding

Another kind of deceptive resemblance to unconformity is occasionally caused by the alternation of horizontal and oblique bedding, a horizontal bed resting upon a series of inclined layers. A conspicuous example of this is given by the Le Clair limestone of Iowa, which was at one time altogether misunderstood, but the deception is seldom one that a little care will not expose.