lengths of about 1 or 2 m so that a long core can be laid into a rack that will 

 allow photography of the entire sequence. 



A sample core description sheet is shown in Figure 27. Important char- 

 acteristics of the sedimentary sequence that need to be described include grain 

 size variations, sedimentary structures and directions, and occurrences of 

 cyclic bedding such as varves. Evidence of plant roots and features such as 

 color changes, mottling, discontinuities, and other variations in physical char- 

 acteristics may be indicators of key changes. Roots and rooting, for example, 

 often correspond to marshes in coastal sequences. Fossils and pollen in strati- 

 graphic sequences are indicative of paleoenvironmental characteristics and 

 changes. Techniques for analysis and interpretation of such evidence can be 

 found in Faegri and Iverson (1975), and Kapp (1969). 



Variations in grain size in cores can yield much information about the sedi- 

 mentary environments and thus the geologic history of the region. Coarser 

 fractions settle first, followed by silts and clays. This separation is a function 

 of particle settling velocities, which vary depending upon particle size, 

 density, shape, and the nature of the transport media. Changes in the environ- 

 ment of deposition can result in the clay fraction being separated from granu- 

 lar material both spatially and temporally. For example, clay deposits are 

 usually deposited further from shore than granular material and usually appear 

 on top of granular material. 



X-ray radiography is an imaging method that amplifies contrasts in grain 

 size, mineralogical composition, packing density, water content, diagenetic 

 products, sedimentary structures, and geochemical inclusions in cores that 

 otherwise appear homogeneous (Roberts 1981). Being able to distinguish 

 these features may assist in understanding the sequence of geomorphic changes 

 that occurred at that site. For example, the scale and direction of bed forms 

 can be used to estimate paleocurrents. Marker horizons are related to a date 

 or a significant event. Peat indicates stability and growth at or near sea level. 

 Radiography is based on the differential transmission of X-ray radiation 

 through a sample onto sensitized X-ray photographic film. Variations in tex- 

 ture as well as chemical composition throughout the sediment result in differ- 

 ential attenuation of the incident X-ray radiation before it reaches the 

 underlying film. Samples of uniform thickness of about 1 cm that are cut 

 longitudinally with a wire knife provide the best results in radiography 

 (Roberts 1981). 



The occurrence of paleosols in cores may also provide important informa- 

 tion toward assessing the geologic history of coasts. In terrestrial coastal 

 environments, there may be prolonged periods of minimal sedimentation dur- 

 ing which soil development may occur, followed by periods of relatively rapid 

 sedimentation without soil development 1 . This scenario is characteristic of 



70 



The term "soil" in this context refers to unconsolidated surficial sediment which supports 

 plant life. This is a more restrictive definition than the one typically used in engineering texts, 

 which refer to soil as any unconsolidated material, even if barren of plant life. 



Chapter 4 Laboratory Techniques and Approaches 



