STRUCTURE 661 



presence of a thick, continuous bed of sand which thins out, or becomes 

 lenticular, along- the edges of the pool. Between the productive pools the 

 sands are either very thin or entirely absent. Besides the larger pro- 

 ductive pools, there are a number of smaller pools which lie at about the 

 same level as the larger pools, but are entirely distinct from them, or are 

 connected with them by thin, unproductive beds of sand which are thinner 

 and lower than the productive beds in either the larger or the smaller 

 pools. In all the producing pools in the east side of the axis of the La 

 Salle anticline the sand lies at approximately the same elevation above 

 sealevel. It has not been possible to detect with certainty any folding 

 which may account for the location of the productive pools. Any differ- 

 ences in elevation due to folding of the strata are without doubt less than 

 the known differences in elevation due to the irregularities in the sand 

 lenses. Another feature which appears to have a direct bearing on the 

 origin of the sands is that in the smaller sand masses there is commonly 

 a tendency to cut down into the underlying beds and to present a convex 

 surface downward, whereas the large sand masses which compose the 

 Birds, Parker, and Flat Eock pools have their surfaces convex upward. 



MODE OF ORIGIN OF THE SANDS 



It is of the utmost importance that the mode of origin of the sands 

 responsible for the accumulation of oil shall be known before any. attempt 

 is made to locate new favorable territory or the possible extension of 

 older fields, such as those of Crawford County. After a careful consider- 

 ation of all possible conditions governing the deposition of the Eobinson 

 sand. Doctor Rich reaches the conclusion that these sands may be part of 

 a great delta formation in which are combined river-channel deposits, 

 offshore sand-bars thrown up by the waves along the front of the delta, 

 and wave-worked sands spread out over the adjacent ocean bottom. Rich 

 writes : 



"The present deltas of the earth, where exposed to wave action, are much 

 modified along their margins by the waves. The materials supplied by the 

 rivers are picked up and strewn along the coast by waves and currents and 

 built up into sand-bars which differ, however, from typical offshore bars in that 

 they are smaller and more irregular, and, furthermore, in that the constant 

 building out of the delta. front causes new bars to be thrown up at intervals 

 outside the older ones, thus producing a more or less parallel series of discon- 

 tinuous sand-bars, the inner and older of which are protected from wave ero- 

 sion and, in the normal coui-se of events, are finally buried under delta de- 

 posits and are preserved intact. On the modern deltas irregular shifting of 

 the distributary streams constantly alters the form of the delta front ; incloses 

 lakes here and there by building out irregularly; fills others with sand, and 

 gives rise to numerous channel deposits in the upper beds of the delta. 



