EM 1110-2-1100 (Change 1)
31 Jul 03
U
UNCONFORMITY
A surface that represents a break in the geologic record, with the rock unit immediately above it being considerably
younger than the rock beneath. There are three major aspects to consider: (1) Time. An unconformity develops during a
period of time in which no sediment is deposited. This concept equates deposition and time, and an unconformity represents
unrecorded time. (2) Deposition. Any interruption of deposition, whether large or small in extent, is an unconformity. This
aspect of unconformity pre-supposes a standard `scale' of deposition which is complete. Major breaks in sedimentation can
usually be demonstrated easily, but minor breaks may go unrecorded until highly detailed investigations are made. (3)
Structure. Structurally, unconformity may be regarded as planar structures separating older rocks below from younger rocks
above, representing the `break' as defined in (1) and (2) above. A plane of unconformity may be a surface of weathering,
Erosion or denudation, or a surface of non-deposition, or possibly some combination of these factors. It may be parallel to the
upper strata, make an angle with the upper strata, or be irregular. Subsequent Earth movements may have folded or faulted it.
UNCONSOLIDATED
In referring to sediment grains, loose, separate, or unattached to one another.
UNDERCUTTING
Erosion of material at the foot of a cliff or bank, e.g., a sea cliff, or river bank on the outside of a meander.
Ultimately, the overhang collapses, and the process is repeated.
UNDERTOW
(1) A current below water surface flowing seaward; the receding water below the surface from waves breaking on a
shelving beach. (2) Actually undertow is largely mythical. As the BACKWASH of each wave flows down the BEACH, a
current is formed which flows seaward. However, it is a periodic phenomenon. The most common phenomena expressed as
"undertow" are actually RIP CURRENTS.
UNDERWATER GRADIENT
The slope of the sea bottom. See SLOPE.
UNDISTURBED WATER LEVEL
Same as STILL WATER LEVEL.
UNDULATION
A continuously propagated motion to and fro, in any fluid or elastic medium, with no permanent translation of the
particles themselves.
UPCOAST
In United States usage, the coastal direction generally trending toward the north.
UPDRIFT
The direction opposite that of the predominant movement of littoral materials.
UPLAND
Dry land area above and landward of the ORDINARY HIGH WATER MARK (OHWM). Often used as a general
term to mean high land far from the COAST and in the interior of the country.
UPLIFT
The upward water pressure on the base of a structure or pavement.
UPRUSH
The rush of water up the FORESHORE following the breaking of a wave, also called SWASH or RUNUP.
Appendix A Glossary of Coastal Terminology
A-85
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