. Geology and underground water resources of northern Louisiana . w whichthe enormous pressure of the upper layers of rocks prevents theexistence of spaces of any character between the rock position of this zone of no pores has been estimated by VanHise to be about 6 miles from the surface. For another type of springs see ; Prof. Pap.,No. 46, p. 76,^Sixteenth Ann. Rept. U. S. Geol. Survey, pt. i, 1896, p. 593. 320 vkatch] Underground Water of Northern La. 73 MAIN ground-water TABLE. The upper limit of this zone of complete saturation is knownas the main ground-wate
. Geology and underground water resources of northern Louisiana . w whichthe enormous pressure of the upper layers of rocks prevents theexistence of spaces of any character between the rock position of this zone of no pores has been estimated by VanHise to be about 6 miles from the surface. For another type of springs see ; Prof. Pap.,No. 46, p. 76,^Sixteenth Ann. Rept. U. S. Geol. Survey, pt. i, 1896, p. 593. 320 vkatch] Underground Water of Northern La. 73 MAIN ground-water TABLE. The upper limit of this zone of complete saturation is knownas the main ground-water table. In regions of heavy rainfall itis relatively near the surface, while in areas of light precipitationit is deep in the ground. The possibility of obtaining waterbelow this main ground-water table depends on the location ofthe coarse beds, such as sand and gravel, which will yield theircontained water readily. Clay beds in this zone, though alsocompletely saturated, release the water extremely slowly if at all,and have, therefore, no water-bearing 1 mile From U. S. Geol. Surv. Fig. 28.—Cross section on Long Island, New York, showing the relation of a perched water table to the main water table and the production of springs dependent on a perched water table. PERCHED GROUND-WATER TABLESAbove this zone of complete saturation and separated from itby nonsaturated strata, there are, in regions containing irregularclay or relatively impervious beds, more or less elevated, limited,and disconnected zones of saturation which may be termed perched water tables. They supply local shallow wells, andwhen cut by valleys produce springs of greater or less import- 321 74 Geol. Surv. of lyA. Report of 1905 [bui,i<. 4 ance (fig. 28). Wells- dependent on perched water tables are ingeneral much less satisfactory than those which pass below themain water table, as they derive their supply from more or lesslimited bodies of saturated strata which are quickly affected byperiods of
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