SEARCH · USGS Science
Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Many areas of schist in the southern Black Hills, South Dakota, have a thin zone of disintegrated rock that is 4-10 feet below the ground surface and parallel to it. Fresh, undeformed schist overlying this zone has moved downhill in a mass-wasting process since the present surface was formed.
Most river curves have nearly the same value of the ratio of curvature radius to channel width, in the range of 2 to 3. Meanders formed by meltwater on the surface of glaciers, and by the main current of the Gulf Stream, have a relation of meander length to channel width similar to rivers. Because such meanders carry no sediment, the shapes of curves in rivers are evidently determined primarily by the dynamics of flow rather than by relation to debris load. Velocity distributions along river curves provide a generalized picture of flow characteristics. Evidence on flow resistance in curved channels suggests that a basic aspect of meander mechanics may be related to the distribution of energy loss provided by a particular configuration or curvature. No general theory of meanders is as yet satisfactory, however; in fact, present evidence suggests that no single theory will explain the formation and characteristics of all meanders and that few of the physical principles involved have yet been clearly identified.
Potassium-argon determinations on micas from Precambrian granitic rocks of the Quadrilátero Ferrífero, Minas Gerais, Brazil, suggest three ages of intrusion: 2400 m.y. determined from gneiss within the Bação complex; 1350 m.y. from rocks in the northern part of the Bação complex and also in a region 7 km north of nearest known Minas series metasedimentary rocks (Late Precambrian age ); and 450-550 m.y. The last has been determined from (1) gneiss of the eastern part of the area, (2) granitized beds of the Minas series, and (3) granite between Minas beds and the 1350 m.y. granite in the western part of the area. Other ages, ranging between 595 and 1080 m.y., may represent the effects of a younger metamorphism on older granitic rocks that are found between the peripheral area of the 1350 m.y. granites and Minas metasedimentary rocks.
Some of the classic closed-basin lakes of the western United States have been resampled, and the waters have been analyzed by modern wet-chemical methods. Included are waters from Borax and Little Borax lakes and Mono Lake in California; Big Soda, Pyramid, and Walker Lakes in Nevada; Abert Lake, Oregon; and Great Salt Lake, Utah. Tributary streams and springs have also been sampled and are reported upon. © 1961, The Geological Society of America, Inc.
Their stratigraphic setting demonstrates that three batholithic intrusions in northern and central Chile are respectively pre-Jurassic, Jurassic, and Cretaceous. Radiometric age determinations, by the lead-alpha (Larsen) method, indicate ages of 265 ± 30, 120 ± 15 to 125 ± 15, and 95 ± 10 to 130 ± 15 million years respectively for samples from the batholiths whose stratigraphic setting has been established or is inferred. With one exception, the radiometric ages are consistent with the stratigraphy. The exceptional determination of 130 ± 15 million years was for a sample of granite that intrudes Cretaceous rocks. Three other samples of Cretaceous granitic rocks were within the range of 95 ± 10 to 105 ± 10 million years. Angular unconformities whose ages are correlative with the intrusions suggest that the three batholiths can be related respectively to Hercynian, Late Jurassic, and middle Cretaceous orogenies.
The " pseudogabbro " associated with peridotite at Tinaquillo , Venezuela , is believed to be of magmatic rather than metamorphic origin principally because: (1) there are major mineralogic and compositional differences between the " pseudogabbro " and the most highly metamorphosed country rock; (2) rocks intermediate in compositional between peridotite and " pseudogabbro " are present; (3) augen of lamellar pyroxene which occur in the " pseudogabbro " but not in the country rock are of a kind found elsewhere rarely except in igneous rocks; (4) gabbroic rocks are associated with peridotite elsewhere in Venezuela and in the Carribbean region; (5) textural and structural features of the kind found in the Tinaquillo rocks are characteristic of alpine-type peridotite-gabbro complexes.
Douglass Creek basin lies west of the Continental Divide in the northern part of the Rocky Mountain physiographic province. Numerous minor environmental differences exist between the Douglass Creek area and the Pipestone Springs and Canyon Ferry areas east of the Divide. In the 19th century, however, the three areas had identical mammalian species representation, although not equally dense populations. Fossils of an early Oligocene biota have been collected from the Douglass Creek basin . Presence of all but one of the Douglass Creek mammalian species in the Pipestone Springs-Canyon Ferry early Oligocene fauna suggests that the three ancient ecosystems resembled each other in much the same way as the 19th century systems. The early Oligocene deposits and biota of the Douglass Creek basin indicate a moist, temperate climate with seasonal variations. Sediment size and distribution suggest that the cross-valley relief was no greater than it is now. The fish and invertebrate faunas show that a shallow, hard-water lake existed in the area. The flora included a lowland, lake-border association and an upland coniferous forest. Although the ancient Douglass Creek biota doubtless included many species not represented in the fossil collections, most of the mammalian species are probably represented in the combined Douglass Creek , Pipestone Springs, and Canyon Ferry fossil assemblages. If so, the number of mammalian species was about the same as in the 19th century ecosystem.
Seat rocks, including underclay, underlie coal beds and show features such as roots, profiles similar to water-logged soils, lack of bedding, soil-like fracture, and gradation into normally bedded sedimentary rocks indicating that they were once soils. Coarse-grained seat rocks range from argillaceous to nearly pure quartz sandstone (ganister). Seat rocks composed of clay-sized particles (underclays) range from mixtures of illite, mixed-layer clay, and kaolinite to essentially pure kaolinite. Variations in the quartz or kaolinite content suggest that there are degrees of alteration in the formation of a seat rock and that all kinds of seat rock were formed by leaching of the substratum in a swamp. The increasing crystallinity of the clay minerals from plastic underclays to semiflint clay to flint clay and the corresponding increase in titanium support this conclusion. These two features suggest strongly that flint clays formed in place in an acidic swamp environment. The Olive Hill clay shows residual acidity. Seat rocks are affected by the chemical environments in the swamp before peat accumulation, during peat accumulation, and during coalification. Kaolinite is forming in present-day swamps and in wet acidic soils in Hawaii and probably formed in late Paleozoic swamps. The facts can be interpreted to show that all kinds of seat rocks could have formed in Pennsylvanian swamps and probably most did.
Normal faults offset a bedrock surface scoured by Pleistocene ice in several areas within and south of Yellowstone National Park . Recurrent earthquake shocks and fresh appearance of some scarps suggest that movement is continuing along some faults . Four systems of faults are described. Quaternary movement occurred along more than 60 faults on the Mirror Plateau, 15 miles northeast of Yellowstone Lake. Faults trend northwest, and several are more than 6 miles long. Maximum displacement exceeds 250 feet. The majority have northeast blocks downdropped, but some grabens and horsts are present. Eocene to Pliocene igneous or pyroclastic rocks are displaced. Ice moved southwest and south from the Beartooth and Absaroka ranges, nearly at right angles to the fault trends. Drainage in many ice-scoured valleys was disrupted by faulting, and small lakes (such as Mirror Lake) formed on downthrown blocks. Thermal activity occurs along some of these faults . Directly east of Mirror Plateau, the Lamar normal fault has a displacement of 1300 + feet; perhaps 1000 feet of this may have occurred during Quaternary time. The Yellowstone Falls fault system cuts Pliocene rhyolite southeast of the Upper Falls of the Yellowstone River. Faults trend northwest; maximum displacement exceeds 200 feet. The Solfatara fault system trends north-northwest, cuts Pliocene rhyolite, and has a maximum Quaternary displacement of about 200 feet. The Hering Lake fault system is a northern extension of the Teton fault , trends northward, and cuts Pliocene rhyolite and rhyolitic welded tuff. Maximum displacement is about 200 feet. West-flowing streams established on bedrock scoured by ice were disrupted, and Beula, Hering, and South Boundary lakes formed on the downthrown (east) blocks. The sharp angular unstepped appearance of fault scarps 50 to 200 feet high in these fault systems suggests that each scarp of this type was formed by one continuous movement. The displacement along faults associated with the Hebgen earthquake of August 1959 is commonly less than 20 feet. The abundance of Quaternary faults and the record of 18 earthquakes in historic time suggest that additional faulting and earthquake activity can be expected in the future. Recognition of this probability should influence the location and type of construction of buildings and other facilities.
No abstract available.
A fragment of an oddly sculptured glass disc found on the cliff of Gay Head, on Martha's Vineyard, Massachusetts, is thought to be a tektite. Unless carried to Gay Head by man from one of the known tektite fields, it raises to three the number of tektite localities in the Western Hemisphere. The freshness of surface features and the clarity of glass indicate a recent origin. If, however, it weathered out of the cliff, it may be Late Cretaceous, Miocene, or Pleistocene in age. © 1961, The Geological Society of America, Inc.
Recent age determinations indicate that at least two groups of " alkalic " igneous rocks exist in New England , with ages of about 185 and 270 million years. Because of their lithologic and geologic similarities, all these rocks had previously been grouped with the White Mountains plutonic-volcanic series of New Hampshire. Until reliable petrographic or geologic criteria become available for distinguishing these age groups, caution should be exercised in assigning any undated body of " alkalic " rock to one or the other group.
Summit elevations in the Lebanon Valley, part of the Great Valley, range from 440 to 720 feet above msl (mean sea level). This range cannot be accounted for adequately by the peneplain concept. Although accordant summits, the chief evidence for peneplains, occur over large areas, summits are not accordant between adjacent areas within the valley. The Lebanon Valley is underlain in the south by carbonate rocks and in the north by shale. The major stream valley in the carbonate area is now partly occupied by segments of two streams, but at one time it was the location of one major stream -the ancestral Quittapahilla Creek-which was beheaded by a tributary to Swatara Creek. Landforms of the Lebanon Valley are probably the result of erosion within two separate stream systems-Swatara and ancestral Quittapahilla creeks-in which streams and interfluvial areas were in a state of erosional equilibrium. The land surface in equilibrium with the ancestral Quittapahilla Creek lies at a higher elevation than adjacent land surfaces that were in equilibrium with Swatara Creek. The land surface on the carbonate rocks, which is in the ancestral Quittapahilla Creek system, lies at a lower elevation than shale within the same system, but it commonly lies at a higher elevation than shale in adjacent parts of the Swatara Creek system. Accordance of summits is the result of uniform erosion of uniform rocks in basins whose discharge points are at the same elevation. Lack of accordant summits on uniform rocks is the result of erosion in basins whose discharge points differ in elevation.
A trend toward increasing use of test drilling and subsurface exploration is resulting in a decreasing reliance on human experience in coping with problems in hydrogeology . Application of "observation and inference" and discreet abstraction of readily available data will solve more problems than is now realized.
Precise measurements of fluctuations of ground-water temperature based on monthly readings in shallow glacial -outwash aquifers may be used to study ground-water movement and recharge. An increase in storage in the shallow basin of Okabena Lake, Worthington , Minnesota , permits the water to cover a much larger area and probably increases the opportunity for infiltration of lake water into aquifers connected with the lake. In a shallow artesian aquifer confined by relatively impervious glacial till, water moves toward the land surface in response to steep temperature gradients in the winter, and cool water descends to the aquifer when the upward temperature gradient is lowered or eliminated. Thermometry may also be used in hydrogeologic studies of limestone terrane.
Late Cretaceous Tuscaloosa Formation occurs as discontinuous remnants that cap many of the ridges in the Western Highland Rim. Typically the formation consists of well-rounded, poorly sorted chert gravel which is trimodal in size distribution. The gravel fraction (mode 15 to 40+ mm) consists of Devonian and Mississippian chert and a small percentage of sandstone pebbles. The medium sand fraction (mode 0.5 mm) consists mainly of angular to well-rounded chert grains developed by attrition during transport. Well-rounded and frosted quartz grains also are present. The fine fraction (mode 0.15 mm) consists of clay, authi-genic (?) mica, and quartz. At its eastern limit the Tuscaloosa is locally well sorted and contains quartz pebbles and a large proportion of quartz sand. Also present in the same area are well-sorted, heavy-mineral-bearing sands and bimodal (0.04, 0.2 mm) siltstone, which contains sand-sized pellet aggregates and fragmentary plant fossils. The finest fraction (less than 0.044 mm) of both eastern and western facies of the Tuscaloosa consists of 60-80 per cent quartz, 5-30 per cent kaolin, and 5-30 per cent montmorillonite, all of which are present in Devonian and Mississippian bedrock. Minor exotic constituents include volcanic(?) glass and heavy minerals. The Mississippian chert gravel in the Tuscaloosa is of local origin, but the Devonian chert was transported from a western source. Other components from a western source are sandstone pebbles and frosted sand grains, both of which probably were derived from Cambrian or Ordovician formations that cropped out on the Pascola arch, an eastward-sloping extension of the Ozark dome. Quartz pebbles, heavy minerals, and some of the angular quartz sand present at the eastern edge of the Tuscaloosa may have been derived from Pennsylvanian sandstone and conglomerates that cropped out north and south of the Pascola arch. Most of the Tuscaloosa Formation is believed to be of nonmarine origin, deposited on the eastward-sloping flank of the Pascola arch. The eastern facies of the Tuscaloosa is believed to be partly marine in origin, the exotic components having been swept in by longshore currents. During deposition of the Tuscaloosa , the Ozark dome and the Cincinnati arch were connected by the Pascola arch, which is now deeply buried beneath the Mississippi Embayment, At that time the Pascola arch stood structurally about 3000 feet higher than at present, and its structural shape and dimensions were comparable to the present Nashville dome. During deposition of the Tuscaloosa the Nashville dome was structurally about 1000 feet lower than at present, and its crest probably was submerged beneath the sea.
During early Late Cretaceous the area of the northern Mississippi Embayment was occupied by a structural and topographic dome standing nearly 1000 feet above sea level. Rocks as old as Cambrian were exposed on its crest. Downwarping of the dome, which commenced in Late Cretaceous time, resulted in the development of the Mississippi Embayment syncline, now filled with sediments of Cretaceous , Paleocene, and Eocene age. The flanks of the Mississippi Embayment syncline slope uniformly about 30 feet per mile. Most of the bending occurred near the synclinal axis, which generally follows the present course of the Mississippi River. Faults cutting the Paleozoic basement and, in some areas, extending upward into the overlying younger rocks resulted from superposition of the narrow synclinal bend across the now-buried structural high. Adjustment also probably occurred by faulting of Paleozoic rocks along the Tennessee River, where the eastern flank of the syncline was bent downward. As a result of structural movements during the Cretaceous and later periods, the old dome has been depressed beneath the Mississippi Embayment and now forms the Pascola arch connecting the Ozark and Nashville domes.
During experiments on sediment transport and resistance to flow with a uniform 0.33-mm sand , data were recorded on the movement of individual rocks having intermediate diameters from about 0.1 to 0.5 foot. The experiments were conducted in a flume 2 feet wide by 60 feet long and for most runs, depth was held constant at 0.5 foot. The experiments showed that rocks on the sand bed moved downstream consistently only if the flow was in the upper regime - that is, only if the bed forms were plane bed , standing waves, or antidunes. The rocks moved at velocities that were approximately half the average velocity of the water. On all bed forms in the lower flow regime (ripples, ripples superimposed upon dunes, and dunes), the rocks moved upstream and down into the bed . That is, the rocks moved into a scour pocket that formed at the upstream side of the rock. The movement upstream and down into the bed is limited by and approximately equal to the distance below the original rock position of the minimum bed elevation plus approximately half the rock diameter. The data indicate that cross-bedded sand deposits formed by the ripple or dune phases of transport would contain few, if any, pebbles or cobbles . Because the flow, in at least the downstream reaches, of most rivers is in the lower regime, the upstream movement and scour into the bed demonstrated in these experiments is an important factor in the sorting process.