Page 560 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

524 part III The earth–atmosphere interface
▲Figure 16.32 Cross bedding in sedimentary rocks. The bedding pattern of cross stratifica- tion in these sandstone rocks tells us about dune environments before lithification (hardening into rock). [Bobbé Christopherson.]
Coastal Dune Geomorphology
Coastal sand dunes originate from sediment supplied by the work of ocean waves (and waves on large lakes) and by fluvial processes that move sediment onto deltas and es- tuaries. Once sand is deposited on shore, it is reworked by wind pro- cesses into the shape of dunes. Dunes along seacoasts are either fore- dunes, where sand is pushed up the seaward-facing slope (Figure 16.33), or backdunes, which form farther away from the beach and are pro- tected from onshore winds; back- dunes are more stable and may be hundreds of years old. Most areas of coastal dunes are relatively small in size (especially when compared with desert dune fields that may cover large portions of continents).
In a natural cycle, coastal dunes grow by accumulating sand which is then returned to the littoral environ-
contrast to star dunes, crescentic dunes form from a single principal wind flow.
Ancient sand dunes can be lithified into sedimen- tary rock that carries patterns of cross bedding, or cross stratification. As the ancient dune was accumulating, the sand that cascaded down its slipface established dis- tinct bedding planes (layers) that remained as the dune lithified. These layers are now visible as cross bedding, so named because they form at an angle to the horizontal layers of the main strata (Figure 16.32). Ripple marks, an- imal tracks, and fossils also are found preserved in these desert sandstones.
▲Figure 16.33 Coastal foredune. Marram grass (Ammophila brev- iligulata) forms a mono-specific vegetation community where sand burial and salt spray create harsh conditions on these foredunes at Pointe de l’est, iles de la Madeleine, Québec. [Philip giles.]
ment periodically by wave erosion. Over the longer term, foredunes move inland as sea level rises and storm en- ergy increases with climate change. In developed areas, the foredunes cannot retreat inland without impinging on human development. When storms occur, dune move- ment is intensified, and either dune erosion or sand de- position, or both, occurs.
The effectiveness of dune systems for contributing to protection of developed areas from wave erosion and storm surge was observed by local residents and scien- tists during and after Hurricane Sandy on the U.S. East Coast in October 2012. In the largest storms, dunes are not a guarantee of protection from erosion, but they help to absorb much of the impact. Thus, maintaining dune systems should be given priority; however, this can be a controversial decision in developed areas. Property own- ers sometimes favour ocean views in the short term over obstructed views and storm protection that dunes pro- vide in the longer term.
Loess Deposits As discussed earlier, wind can trans- port smaller particles such as dust and silt long dis- tances. In many regions of the world, fine-grained sediments (clays, silts, and fine sand) have accumu- lated in unstratified, homogeneous (evenly mixed) deposits called loess (pronounced “luss”), originally named by peasants working along the Rhine River Val- ley in Germany. Loess deposits form a thick blanket of material that covers previously existing landforms. Figure 16.34 shows the worldwide distribution of these accumulations.
The loess deposits in Europe and North America are thought to be derived mainly from glacial and peri- glacial sources, specifically the alluvial deposits from