Finer sediment will be carried and then laid down as the energy of the water drops below its capacity to carry the weight of heavier grains. Cross beds can therefore form in any environment with moving water, such as braided or meandering rivers, shorelines of lakes or oceans, or in submarine settings which are affected by deep sea currents. When cross-bedding forms, sand is transported as sand-dune like bodies (sandwave), in which sediment is moved up and eroded along a gentle upcurrent slope, and redeposited (avalanching) on the downcurrent slope (see upper half of picture at left).
These tilted layers contained within larger layers are termed cross bedding. If you go dig trenches into modern sediments, you will find that cross-beds form a part of ripples and dunes. When cross beds form, the grains avalanche down the face of the form as previously mentioned. Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments. Ripple foresets that climb on the backs of their predecessors are indicative of waning flow conditions and rapid sediment fallout, such that sediment drops out of suspension as fast as it can be molded into a bedform. These mud-draped ripples are linguloid in form, indicating shallow, rapid flow within the ripple stability field.
In wind-blown or water-lain sandstone, a form of erosion during deposition of shifting sand removes the tops of mounds to produce what are called cross-beds. Thus ripples form which eventually shift or move downstream. Note the trough cross-bedding on the cut that is perpendicular to flow (i.e. the v-w cut) This figure shows flow along the bed across a set of sinuous-crested bedforms. The stoss angle, climbing ripples can be classified as subcritically-climbing, critically-climbing, or supercritically-climbing. Beach and Longshore Bar Cross-bedding. Sand Dune Cross-bedding. These ridges are separated by low linear furrows; both form by disruption of the viscous sub-layer. You can see the edge of the channel in the upper left corner and a quarter for scale in the right-central part of the photograph.
Graded bedding is shown in this example where pebbles form the lower part of the bed and are replaced by sand and pebbles towards the top, (King George IV Lake area, central Newfoundland). Cross-beds can be used to determine bedding tops because of the truncation of the beds. Answer to How do the cross beds form? Close. Home home / study / science / earth sciences / questions and answers / how do the cross beds form? Cross beds form as high-velocity currents (wind or water) transport sediment grains over an obstacle such as a boulder or ridge. Wind-formed cross beds, in comparison, can be quite large because air flow can be constant for hours or even days, producing sand dunes composed of sets of cross beds up to 100 feet thick. 3) as the waters slow, larger clasts are deposited in the form of gravel closest to the source, then sand falls or accumulates, then clay eventually may accumulate in the flat areas bordering the streams. How can you read the current direction from the cross beds? Ripples and. Cross Bed Formation. Sand dune. Wind. Migration of ripples and dunes. Sand is. Sand avalanches. transported up. down the slip. the lee face of.