8 Desert varnish – oxidation of iron and manganese, useful for dating desert surfaces
9 Arid Landscapes: Basic Concepts I. Most deserts exhibit highly angular landscapes.mechanical weathering dominatesangular particles of weathered rock; bedrock outcrops with wind erosionlittle soil or vegetation to soften landscapeII. Softer, more rounded landscapes appear where deposition is predominant.dunesalluvial fansIII. Desert rainfall is infrequent, intense, and unreliable.intense flash flooding, intense erosionephemeral streams
14 The Nile River meanders through the Sahara A braided stream near Chinle, Arizona
15 FLUVIAL LANDFORMS IN ARID REGIONS Fluvial Erosion – sporadic nature of desert runoff (rare, intense flash floods), and lack of vegetation to protect surface materials causes rapid erosion.Ephemeral stream channels – washes or arroyos (orwadis in the Middle East, and barrancas in Latin America)Even streams originating in humid regions cannot maintain sufficient discharge and eventually disappearSteep slopes – steep-walled canyons Mesas and Buttes
27 The Badlands of South Dakota – Impermeable clays andlack of soil coverproduced rapid runoff,and a dense network ofbarren ridges dissectedby a maze of steep, drygullies and ravines –Badlands topography.Fluvial Transportation – large amounts of rock and sand moved short distances during sporadic flows
28 Alluvial fans Bajada Piedmont Alluvial Plain Fluvial Deposition – alluvium deposits at the base of mountains or in closed basins – does as much to level the land as does erosion – differentiated from coarse to fineAlluvial fans Bajada Piedmont Alluvial PlainRock debris in piedmont region Debris flow fansDesert basins of interior drainage – Bolsons PlayasAlso playa lakes, clay pans, salinas, salt flats (or salt-crust playas)
29 Piedmont – gently sloping land that extends out from the base of Piedmont – gently sloping land that extends out from the base of mountains, composed of a series of alluvial fans – (depositional).Pediment – sloping bedrock at base that develops due to gradual retreat of the adjoining mountain slope – (erosional).
34 EOLIAN (WIND) PROCESSES AND LANDFORMS Eolian Erosion - much less effective than fluvialdeflation – picking up and moving small particles of rocks;helps produce deflation hollows and desert pavementsabrasion – like sandblasting effect; polishes and etches exposed surfaces; produces ventifacts and yardangsEolian Transportation - moves nothing bigger than coarse sandduststorms - can extend thousands of feet upsand storms - only very near the surfaceEolian Deposition - dunes form and shift where winds and terrain slow the winds or block movement of dunes; loess deposits – wind-deposited silt ← dust storms
37 EOLIAN (WIND) EROSION FEATURES “Desert pavement” of pebbles and stones– result of deflationall small materials and sand removed by windvery thin coating of stones
38 Desert pavements: Also known as: “gibber” in Australia “reg” in North Africa
39 Wind Erosion Features: An Yardang in the KhargaDepression, EgyptWind Erosion Features:Ventifacts,Yardangs,Pedestal RocksA sand-blastedrock, or Ventifact,on MarsPedestal Rocks(Rock Mushrooms)in Utah’s Goblin Valley S.P.
40 EOLIAN (WIND) DEPOSTION FEATURES Active/Shifting/MigratingDunes
41 LONGITUDINAL DUNES (SEIFS) Types of DunesBARCHANTRANSVERSE DUNESLONGITUDINAL DUNES (SEIFS)
42 Types of DunesBarchans - migrating crescent dune with horns pointing downwind; form where strong winds blow in a consistent direction. Move fast. Common in central Asia and Sahara. Can reach hundreds of feet in height.
43 Transverse Dunes - parallel waves of crescent dunes perpendicular to wind direction; migrate downwind. Found in areas with large supply of sand.
44 Seifs – multiple, very long, narrow, parallel dunes Seifs – multiple, very long, narrow, parallel dunes. They occur in areas with at least two dominant wind directions. Can be tens of miles long. More common outside the U.S.
46 Stabilized Dunes – when plants establish themselves in a dune area
47 Loess Deposits: Wind-deposited silt, transported by dust storms; vary in thickness from few inches to over 300 feetGully eroded thick loess in Northern ChinaUnstable loess bluffsin Vicksburg, Mississippi
48 Major Loess Regions of the World (Most loess deposits are peripheral to deserts and recently glaciated areas)
49 Finally,Combined effects ofWeathering, Mass Wasting + Fluvial and Eolian processes!Delicate Arch, Arches National Park, Utah