<P> The shape of the grains and the sorting and composition of sediment can provide additional information on the history of cross beds . Roundness of the grains, limited variation in grain size, and high quartz contents are generally attributed to longer histories of weathering and sediment transport . For example: well - rounded, and well - sorted sand that is mostly composed of quartz grains is commonly found in beach environments, far from the source of the sediment . Poorly sorted and angular sediment that is composed of a diversity of minerals is more commonly found in rivers, near the source of the sediment . However, older sedimentary deposits are frequently eroded and re-mobilized . Thus, a river may well erode an older formation of well - rounded, well - sorted beach sands of nearly pure quartz...</P> <P> Flows are characterized by climate (snows, rain, and ice melting) and gradient . Discharge variations measured on a variety of time scales can change water depth, and speed . Some rivers can be characterized by a predictable seasonably controlled hydrograph (reflecting snow melt or rainy season). Others are dominated by durational variations characteristic of alpine glaciers run - off or random storm events, which produce flashy discharge . Few rivers have a long term record of steady flow in the rock record . </P> <P> Bed forms are relatively dynamic sediment storage bodies with response times that are short relative to major changes in flow characteristics . Large scale bed forms are periodic and occur in the channel (scaled to depth). Their presence and morphologic variability have been related to flow strength expressed as mean velocity or shear stress . </P> <P> In a fluvial environment, the water in a stream loses energy and its ability transport sediment . The sediment "falls" out of the water and is deposited along a point bar . Over time the river may dry up or avulse and the point bar may be preserved as cross bedding . </P>

How is cross-bedding used by geologists to interpret geologic history