Subaerial Erosion Research Articles

A uniform measure of subaerial erosion

Research Article| January 01, 1976 A uniform measure of subaerial erosion NEL CAINE NEL CAINE 1Institute of Arctic and Alpine Research and Department of Geography, University of Colorado, Boulder, Colorado 80302 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1976) 87 (1): 137–140. https://doi.org/10.1130/0016-7606(1976)87<137:AUMOSE>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation NEL CAINE; A uniform measure of subaerial erosion. GSA Bulletin 1976;; 87 (1): 137–140. doi: https://doi.org/10.1130/0016-7606(1976)87<137:AUMOSE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract It is suggested that observations of contemporary sediment movement in subaerial situations be estimated by the physical work (in joules) that they represent. This can be conveniently done by estimating the change due to erosion in the potential energy of the landscape. An example from an alpine area in southwestern Colorado is used to illustrate the use of geomorphic work as a measure of erosion and to show how it may be compared to other environmental exchanges of material and energy. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Permian-Triassic boundary in eastern Nevada and west-central Utah

The Permian-Triassic boundary in eastern Nevada and west-central Utah is placed at a disconformity above the Gerster Formation (Wordian) and below the Thaynes Formation (Smithian and Spathian). Evidence of subaerial erosion at the disconformity includes local truncation of Gerster beds by channels filled with chert and quartzite-pebble conglomerate. Westward onlap of Lower Triassic sedimentary deposits, as demonstrated by conodont zones, suggests an east-facing paleoslope over which the Early Triassic sea slowly advanced. The Sonoma orogeny, which coincided with the Late Permian—Early Triassic hiatus, may have been the cause of regional upwarping in eastern Nevada that produced the eastward paleoslope.

Spatial Point Processes Applied to the Distribution of River Junctions

The notion of randomness has been extensively applied to topological (nondimensional) properties of drainage networks. The spatial (dimensional) organization of five fluvial hierarchies is examined herein through the application of quadrat analysis to random and clustered spatial probability models. The Poisson, Polya‐Aeppli, and negative binomial models are compared with point‐pattern distributions of river junction location for three basins in Indiana and two fossil systems on an erosion surface in semiarid Australia. The negative binomial model best fits all five networks, suggesting that the branching behavior of fluvial systems follows the mathematical precepts leading to clusteredness of junctions. The degree to which the Polya‐Aeppli model fits the data suggests the imposition of a temporally limited set of environmental conditions optimally suited for network growth. Only for the two fossil systems does the Poisson model agree. It is speculated that the effect of prolonged subaerial erosion may be to shift a clustered distribution towards the random state as the surface approaches a pediplained state.

Stratigraphy of the Ordovician St. George Group in the Port-Au-Choix Area, Western Newfoundland

Stratigraphic sections were measured in the very complete coastal exposures around Port-au-Choix village in western Newfoundland. A formal stratigraphic subdivision of the Ordovician St. George Group into three formations is proposed. They are, from oldest to youngest: the Barbace Point Formation, a 21 + m thick unit of dense dolostone, showing diagenetic solution features; the Catoche Formation, 100 m thick, consisting of well bedded limestones with shallow-water indications; the Port-au-Choix Formation, 35 m thick, massive sugary dolostones grading into silty dolostones, with diagenetic solution features. Detailed descriptions of all new formations are given, as well as the rationale behind this new nomenclature. The Early Ordovician depositional history of the area is seen as a succession of two erosional and depositional cycles, not completely preserved in the record, whereby the Barbace Point Formation was subjected to subaerial erosion, creating a hiatus between this formation and the overlying Catoche Formation; the latter represents the beginning of a new depositional cycle which was possibly interrupted before the youngest Port-au-Choix Formation was deposited. After Port-au-Choix times, renewed erosion took place before the Table Head Group was formed.

Discussion

The President (SIR PETER KENT) welcomed this contribution with the comment that a Permo-Triassic age would be in line with evidence elsewhere of an early breakdown of the Atlantic continental margin, long pre-dating the lateral translation marked by magnetic stripes etc. He was however somewhat concerned that other authorities (also familar with other New Red Sandstone outcrops in the Hebrides) had not accepted such a late date, and he asked whether there was direct evidence from palynology or other studies. He also commented that thicknesses of the order estimated for the Stornoway conglomerates (4 km) were usually associated in other places with relatively local contemporaneous trapdoor type faulting. Dr Audley-Charles remarked that studies of clast composition in the thinner English Permo-Trias deposits have been interpreted as indicating deepening and widening levels of erosion. The present interpretation of the Stornoway Formation as approximately 4 km thick and consisting mainly of rudites would seem to require deep sub-aerial erosion of the adjacent landmass contemporaneous with accumulation of these rudites. Had the authors made any statistical study of clast composition? Such a study might reveal evidence of systematic changes in clast composition through the accumulated succession, resulting from erosion having removed successively deeper levels of the nearby landscape from which these rudites were derived. Would the authors accept that the lack of such composition changes could be used as an argument for the Stornoway Formation being much thinner than they had estimated? Professor J. Watson had examined the Stornoway Conglomerate in the field

Holocene Shoestring Sand on Inner Continental Shelf off Long Island, New York

Two common kinds of shoestring sands are deposits of stream channels and of shoreline barriers. The shapes of transverse sections through shoestring sands of channel origin are thought to be flat on top and concave up at the base. By contrast, sections through shoestring sands of barrier origin are regarded as flat on the bottom and convex up at the top. Fragmentary evidence from the Holocene shelf sediments off Long Island, New York, suggests that a shoestring sand of barrier origin is present, but that its transverse section is flat on top and concave up at the base. This evidence consists of two continuous seismic-reflection profiles and six cores of Holocene bottom sediments. The seismic profiles, made nearly perpendicular to the shore, display a lenticular sedimentary body 600 to 800 m wide and a maximum of 10 m thick. The shape of this body is flat on top and concave up at the base. The cores of Holocene sand collected seaward of the lens consist of sediment that we interpret as diagnostic shoreface storm deposits. The Holocene cores collected landward of the lens consist of materials containing diagnostic evidence of origin in back-barrier area. In the absence of more complete field data, it is impossible to present a unique interpretation of the geologic relations among the lens-shaped sediment body, the cores seaward of it, and the cores landward of it. One possibility is that they are all of different ages and the lens originated by subaerial erosion into exposed shelf sediments of a channel parallel with shore and that this channel later was filled by sediments. Another possibility, preferred by us, is that the lens and the belts of unlike sediment flanking it, all formed at the same time near a barrier that existed 7 km offshore when sea level was 24 m lower. If so, the lens best can be interpreted as the product of inlet migration parallel with shore.

Holocene Shoestring Sand on Inner Continental Shelf off Long Island, New York

Two common kinds of shoestring sands are deposits of stream channels and of shoreline barriers. The shapes of transverse sections through shoestring sands of channel origin are thought to be flat on top and concave up at the base. By contrast, sections through shoestring sands of barrier origin are regarded as flat on the bottom and convex up at the top. Fragmentary evidence from the Holocene shelf sediments off Long Island, New York, suggests that a shoestring sand of barrier origin is present, but that its transverse section is flat on top and concave up at the base. This evidence consists of two continuous seismic-reflection profiles and six cores of Holocene bottom sediments. The seismic profiles, made nearly perpendicular to the shore, display a lenticular sedimentary body 600 to 800 m wide and a maximum of 10 m thick. The shape of this body is flat on top and concave up at the base. The cores of Holocene sand collected seaward of the lens consist of sediment that we interpret as diagnostic shoreface storm deposits. The Holocene cores collected landward of the lens consist of materials containing diagnostic evidence of origin in back-barrier area. In the absence of more complete field data, it is impossible to present a unique interpretation of the geologic relations among the lens-shaped sediment body, the cores seaward of it, and the cores landward of it. One possibility is that they are all of different ages and the lens originated by subaerial erosion into exposed shelf sediments of a channel parallel with shore and that this channel later was filled by sediments. Another possibility, preferred by us, is that the lens and the belts of unlike sediment flanking it, all formed at the same time near a barrier that existed 7 km offshore when sea level was 24 m lower. If so, the lens best can be interpreted as the product of inlet migration parallel with shore.

Deep Eustatic Terrace Levels: Further Speculations

Criteria for recognizing the maximum glacial eustatic lowering of sea level during Pleistocene time depend on the interpretation of terrace features on high-energy coastlines in structurally stable regions of relatively low sedimentation. A variety of factors favor the Bering shelf and southern Australia as areas likely to retain evidence of a deep terrace record. In both localities, there is strong morphologic evidence for a continuous terrace along the upper continental slope at about 240 m. Intermediate terraces are also displayed at about 165 and 200 m below a very uniform shelf-edge break at 146 m. A eustatic lowering of sea level to approximately 240 m is supported by deep submersible observations of terraces and gravel accumulations off lower California, subaerial erosion features on carbonate banks, and survey data from selected shallow seamounts.

Pliocene-Pleistocene Carbonate Rocks of Bonaire, Netherlands Antilles

Research Article| August 01, 1974 Pliocene-Pleistocene Carbonate Rocks of Bonaire, Netherlands Antilles CHARLES A. BANDOIAN; CHARLES A. BANDOIAN 1Dames & Moore, 6 Commercial Drive, Cranford, New Jersey 07016 Search for other works by this author on: GSW Google Scholar RAYMOND C. MURRAY RAYMOND C. MURRAY 2Department of Geology, Rutgers University, New Brunswick, New Jersey 08903 Search for other works by this author on: GSW Google Scholar Author and Article Information CHARLES A. BANDOIAN 1Dames & Moore, 6 Commercial Drive, Cranford, New Jersey 07016 RAYMOND C. MURRAY 2Department of Geology, Rutgers University, New Brunswick, New Jersey 08903 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (8): 1243–1252. https://doi.org/10.1130/0016-7606(1974)85<1243:PCROBN>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation CHARLES A. BANDOIAN, RAYMOND C. MURRAY; Pliocene-Pleistocene Carbonate Rocks of Bonaire, Netherlands Antilles. GSA Bulletin 1974;; 85 (8): 1243–1252. doi: https://doi.org/10.1130/0016-7606(1974)85<1243:PCROBN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A sequence of Pliocene-Pleistocene carbonate rocks preserved on the western side of the island of Bonaire, Netherlands Antilles, represents a thick build-up of sediment deposited mainly on the flanks of the Cretaceous volcanic core of the island. This sediment built up and developed a shallow-water shelf similar to the one that surrounds the island today. Sediments at the top of the section and the updip portions of beds lower in the section have been dolomitized in two episodes of dolomitization. Minor sea-level fluctuations occurred during deposition of this sequence, and a major lowering of sea level terminated sedimentation.Carbonate rocks and terraces were superimposed on Pliocene-Pleistocene rocks as the island was subjected to uplift and (or) sea-level fluctuation during Pleistocene time. Seven well-developed terraces are present and range from 5 to 90 m above present sea level. Two distinct episodes of terrace formation are represented. The two highest terraces at 90 and 70 m were formed during the earlier episode. Relative sea level was lowered at least 60 m, exposing Pliocene-Pleistocene dolomite and limestone to subaerial erosion. A rise in sea level to approximately 50 m was accompanied by deposition of carbonate sediment on the erosion surface. Marine sediment filtered down into fractures and solution cavities in the underlying rock. This cavity-filling sediment and the lowermost limestone deposited on the erosion surface have been dolomitized. Subsequent uplift of the island and (or) eustatic lowering of sea level were accompanied by extensive deposition of eolianite at elevations above 50 m. Terraces of erosional and depositional origin were formed during this regression at elevations of 50, 30, 20, 10, and 5 m. Deposition of sediments associated with the present 5-m terrace occurred during a high stand following a major lowering of sea level. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Drowned and buried valleys on the southern New England continental shelf

Bathymetry and seismic reflection profiling have revealed a sequence of seven post-Jurassic drowned or buried drainage systems on the southern New England shelf. The basement and younger stream patterns have a dominant southward trend with preferred drainage avenues from the mainland to the middle shelf indicated by superposed valley ground positions from unconformity to unconformity over time. Fluvial action under stable tectonic conditions is inferred by low valley height/width ratios with higher ratios related to ice modification of inner shelf pre-glacial river valleys. Fluvial processes responding to sea-level withdrawals have greatly influenced the shelf's later development. Periodically during post-Paleocene time, sediment from subaerial erosion has been transported to the shelf edge by streams. Deltaic deposition on a subsiding base has controlled outbuilding on the outer shelf where the frequent presence of overlying drainage networks is the result of numerous sea-level regressions. Since Eocene time, sediment has been channelled to the deep sea via Block Canyon and its progenitor. Locally structures created by erosion and glacial deposition have governed drainage direction. On the inner shelf, late Tertiary - early Pleistocene streams were diverted southeastward and southwestward by the magnitude of Long Island's Coastal Plain escarpment and by secondary cuestas between eastern Long Island and Block Island. The probable eastern reach of Dana's southern Sound River valley can be traced from northeastern Long Island across Block Island Sound. An early Woodfordian end moraine of the Wisconsin stage impounded melt waters in Block Island and Rhode Island Sounds. Where the moraine was breached near Block Island, fans were formed adjacent to the water gaps. In Rhode Island Sound the earlier and later Woodfordian end moraines deflected some mainland drainage toward the southwest.

Classification and Relief Characteristics of Northern Alaska's Coastal Zone

Four main genetic coastal types are proposed to classify the shoreline of northern Alaska bordering the Chukchi and Beaufort Seas, which extends for more than 2,150 km. from Cape Thompson eastward to the Canadian border: Land erosion - coast marked by subaerial erosion of terrestrially shaped land forms and partly drowned by rise in sea level (8.9 per cent of coastline); River deposition - coast formed by fluvial deposition (19.9 per cent); Wave erosion - coast shaped primarily by marine agencies and expos to the open ocean, being marked by coastal retreat and negligible nearshore deposition (37.5 per cent); Marine deposition - similar to preceding except nearshore sediment deposition is pronounced (33.7 per cent). Four categories of coastal relief or sea cliff height associated with these coastal types are proposed: Low relief - less than about 2 m; Moderate relief - about 2-5 m; High relief - about 5-8 m; Very high relief - greater than about 8 m. About 1590 km or 74% of the coast has relief of 5 m or less whereas mean relief or scarp height for the entire coast is about 4 m. In general, mean scarp heights decrease to the east along the coastal plain.

Patterns of Present Sub-Aerial Erosion and Landforms in Mid-Wales

Patterns of Present Sub-Aerial Erosion and Landforms in Mid-Wales

Aspects of Some Evaporitic Structures in Western Mediterranean Sea

The stratigraphy and outcrops of the gypsum series in Sicily suggest that the interaction of tectonic and subaerial erosion may explain the genesis of perched structures. A comparison between these Sicilian evaporitic features and some submarine diapirs in the western Mediterranean Sea shows possible interesting similarities. Both Sicilian and submarine structures may be of the same origin.

Carbonate Facies in Ordovician of Northern Arkansas

The Ordovician carbonate sequence in northern Arkansas consists, in ascending order, of the Cotter, Everton, Joachim, Plattin, Kimmswick, and Fernvale formations. A comparison of the rock types, sedimentary structures, and faunal assemblages of these rocks with those of recent carbonate sediments indicates that these ancient rocks may be subdivided into lithofacies that are similar to modern supratidal, intertidal, and subtidal facies. Further comparison of these lithofacies with an epeiric sedimentation model shows that (1) the Cotter Dolomite, Joachim Dolomite, and Plattin Limestone were deposited chiefly in supratidal and intertidal environments, (2) the Everton Formation was deposited chiefly in an intertidal environment, and (3) the Kimmswick and Fernvale Limestones ere deposited in protected to open subtidal environments, above wave base. Two major unconformities in the sequence, each overlain by a sandstone (Kings River Sandstone Member of the Everton Formation and St. Peter Sandstone), probably are the result of the widespread epeirogenic movements that initiated development of the sub-Tippecanoe unconformity. Earlier workers have reported paraconformities and microdisconformities within the post-St. Peter sequence that supposedly represent periods of subaerial exposure and erosion. Our premise is that these surfaces were integral to the sedimentary environments and were, in fact, developed by shifts in laterally adjacent, coexisting facies; these shifts were not related to uplift or to subaerial exposure of preexisting rocks.

Depositional patterns of the upper Devonian Waterways Formation, Swan Hills area, Alberta

ABSTRACT The Waterways Formation is comprised of deeper-water sediments which envelop the Swan Hills Formation, an early Upper Devonian reef complex, in west-central Alberta. The interbedded shales and limestones of the formation range in thickness from less than 20 ft in the area of maximum reef development in the Swan Hills fields to more than 750 ft in the central portion of the eastward from the area of study. On the basis of electric-log correlations, the Waterways Formation was divided into eight major units (Units I-VIII). Isopach maps constructed from an overlying datum to the upper surface of each unit illustrate the depositional topography on which each subsequent unit was deposited. Stratigraphic cross-sections constructed perpendicular to regional strike illustrate a prograding type of basinfill process compatible with the Rich (1951) model. Sediments of the Waterways Formation in the Swan Hills area appear: (1) to have been transported from an eastern source area; (2) to have been deposited below wave base in an intermediate- to deep-water environment; and (3) to be significantly younger than the reef complex comprising the Swan Hills Formation. The lack of apparent shallow-water sediments or erosional surfaces suggests starved basin conditions in the area at the time of reef development, and supports the suggestion that the cessation of reef growth was due to drowning rather than subaerial erosion.

Depositional patterns of the upper Devonian Waterways Formation, Swan Hills area, Alberta

ABSTRACT The Waterways Formation is comprised of deeper-water sediments which envelop the Swan Hills Formation, an early Upper Devonian reef complex, in west-central Alberta. The interbedded shales and limestones of the formation range in thickness from less than 20 ft in the area of maximum reef development in the Swan Hills fields to more than 750 ft in the central portion of the basin eastward from the area of study. On the basis of electric-log correlations, the Waterways Formation was divided into eight major units (Units I-VIII). Isopach maps constructed from an overlying datum to the upper surface of each unit illustrate the depositional topography on which each subsequent unit was deposited. Stratigraphic cross-sections constructed perpendicular to regional strike illustrate a prograding type of basinfill process compatible with the Rich (1951) model. Sediments of the Waterways Formation in the Swan Hills area appear: (1) to have been transported from an eastern source area; (2) to have been deposited below wave base in an intermediate- to deep-water environment; and (3) to be significantly younger than the reef complex comprising the Swan Hills Formation. The lack of apparent shallow-water sediments or erosional surfaces suggests "starved basin" conditions in the area at the time of reef development, and supports the suggestion that the cessation of reef growth was due to "drowning" rather than subaerial erosion.

Evidence of Foundered Continental Crust beneath the Central Tyrrhenian Sea

THE dredging of schists, phyllites and marbles from the faulted margin of a tilted crystal block in the central Tyrrhenian Sea shows that the acoustical basement beneath the centre of this sea basin includes a sequence of rocks similar or perhaps identical to the Palaeozoic and Triassic schists and phyllites of the adjacent Apennine, Calabrian and Sicilian chains, the Pontian Islands1 and Sardinia. Even the low to medium grade metamorphism observed must have occurred beneath the Earth's surface and following metamorphism and deformation we infer that these rocks were uplifted, denuded by subaerial erosion and finally foundered more than 3,000 m below sea level. The Neogene subsidence is still continuing. The metamorphic rocks obtained from the Tyrrhenian acoustic basement appear to support the former existence of the Tyrrhenides and indicate that this ancient upland was underlain by continental crust.

Relationships Between Bottom Topography and Marine Sediment Properties in an Area of Submarine Gullies

ABSTRACT A precise bathymetric survey, including the collection of 21 cores, was accomplished by the submersible DEEP QUEST along a gullied section of the upper San Diego Trough slope off Del Mar, California. The primary purpose of the study was to investigate the relationship between topography and the mass physical properties of marine sediments. A secondary objective was to gain more information on the nature and origin of these submarine gullies. The cores were obtained along precisely controlled transects down the gully axes as well as on transects approximately perpendicular to the axes. Sediment samples from the cores were tested in the laboratory for shear strength, water content, bulk density, grain size distribution, and mineralogic composition. Submarine gullies within the area appear to begin at the edge of the shelf and trend almost directly westward. The lateral extent of these gullies is from 800 to 1200 meters. Width of the larger gullies varies from 203 to 274 meters. The gullies have a relief of 18 to 30 meters. The morphology of the gullies strongly suggests that they were originally formed by subaerial erosion. Within the submarine gullies the only sediment property which appears possibly to be related to the topography is the grain size distribution. The expected progressive decrease in the grain particle size occurs in the seaward direction. A trend was observed in the water content-sediment depth profiles, which is believed to be a result of the grain size distribution and not of the topographic changes. This trend indicates water content values decreasing as the size of the sediment particles increase. Water content values also indicate a non-linear decrease with sediment depth. X-ray diffraction and heavy mineral analysis indicate that the sediments are very homogeneous in mineralogic composition throughout the area studied. Quartz was usually identified as the major constituent of the entire sample. Illite and mixed layer micas were the predominate clay minerals in the clay size fraction. The principal heavy minerals were amphiboles, pyroxenes and opaques.

Origin of the outer part of the Laurentian Channel

In the past, four different modes of origin have been proposed for the Laurentian Channel: structural, subaerial erosion, glacial erosion, and erosion by turbidity currents. To extend our knowledge of the outer part of the Laurentian Channel and further elucidate the problem of the origin of this important topographical feature, we have employed seismic-reflection profiling techniques.Two significant sub-bottom surfaces are recognized from the profiles: (1) A more or less smooth glacial pavement across Carboniferous, Cretaceous, and Tertiary (?) strata that conforms closely to the shape of the present sea floor; (2) an older, irregular, erosional surface cut into Carboniferous and Cretaceous rocks. These channels are infilled with Tertiary sediments.Based on the evidence gained from our interpretation of the seismic records, it appears likely that the outer channel is essentially a product of subaerial and subsequent glacial erosion. It does not appear to be structurally controlled, nor does it appear to have been formed by turbidity currents.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Eastern margin of the Red Sea and the coastal structures in Saudi Arabia

Results of many investigations since 1950 show that the eastern margin of the Red Sea and associated coastal structures in Saudi Arabia have a long geologic history, starting with the deposition of Precambrian eugeosynclinal sedimentary and volcanic rocks before 1000 Ma ago and extending to recent geologic time. The northeastern flank of the Red Sea rift valley is in a shield area affected by possibly four plutonic events at 1000, 720 to 735 (?), 660 to 670, and about 570 Ma. Cratonization of the shield occurred during all or part of the span 520 to 590 Ma. Nubian-type sandstone of Cambrian and Ordovician age laps up on the shield from Jordan southeastward around the rim of the Great Nafud basin, and along the eastern edge of the shield southeastward to 45° E longitude where it is overlapped by Permian limestone. The sandstone reappears to the south and extends southward and westward to the Asir Mountains at the Yemen border. Isolated sandstone outliers are present in the central shield, proving that lower Paleozoic sandstone covered most, if not all, of the basement as now exposed. The Mesozoic era was almost entirely a time of uplift and non-deposition except a middle to late Jurassic fringe marine invasion in the south and a possible narrow invasion from the Gulf of Suez at the end of the era. Marine and non-marine sedimentary deposits of middle and late Tertiary age are found along the Red Sea coast, and Oligocene basaltic flows are present at both low and high altitudes in the coastal ranges. Evidence for important volcanism during Oligocene and earliest Miocene time is widespread, and within the eastern rift fault zone early Miocene hypabyssal intrusives cut the shear zones. Major rifting occurred just before or during early Miocene when the flanks of the rift valley were ramped upward. Shortly after this volcanism a thickness on the order of 3500 m of middle Miocene marl and evaporite beds filled the Red Sea trough. Evidence also exists for widespread subaerial erosion in the Pliocene. Younger lava flows are Pliocene in age but the youngest, near Al Medinah, came as late as A. D. 1250. Lake-bed deposits are very probably in large part Pliocene throughout the shield. The Red Sea coastal plain in Saudi Arabia rises gently eastward from a 3 m littoral surface, generally underlain by dead reef from the Yemen border northward to Al Wajd, a distance of 1400 km. At Jizan, in the south, a salt dome has pushed the 3 m surface up to an elevation of about 50 m. From Al Wajd northward, Pleistocene terraces have been faulted, culminating in several surfaces as high as 520 m above the Red Sea at Tiran Island. Ramping of major fault-bounded blocks along the eastern side of the Red Sea trough—the Midian block in the north, a poorly defined central block, and the Asir block in the south—is connected with renewed movement on regional Precambrian faults. Drainage patterns of wadis in these blocks are characteristically affected by the ramping, and stream capture is common in the Midian and Asir blocks.