Substrata of impermeable rock and stone can trap water and retain it in pockets, or on long faulting subsurface ridges or volcanic dikes water can collect and percolate to the surface.
12.
The southern part of the valley consists of Triassic limestone, and it is only in the north that late Paleozoic impermeable rock is exposed, which is already part of the Karawanks.
13.
Some aquifers are predisposed towards this type of intrusion, such as the impermeable rock or clay layer separates fresh groundwater from saltwater, isolated cracks breach the confining layer, promoting upward movement of saltwater.
14.
The rains cause a peculiar phenomenon : fresh water collects in the valleys between sand dunes and is prevented from percolating down by a layer of impermeable rock which lies underneath the sand.
15.
The Arbuckle is a porous rock group that contains saline water unfit for human consumption in that area of the state and is separated from shallower freshwater aquifers by thousands of feet of impermeable rock.
16.
This latter unit is the principal groundwater aquifer for the region, and its freshwater wells are protected from seawater intrusion by the uplift along the More Ranch Fault, which has placed relatively impermeable rock units between it and the ocean.
17.
In addition to the aquatic environment, which is usually a sea, but might also be a river, lake, coral reef or algal mat, the formation of an oil or gas reservoir also requires a sedimentary basin that passes through four steps : deep burial under sand and mud, pressure cooking, hydrocarbon migration from the source to the reservoir rock, and trapping by impermeable rock.
