require shovels, hand augers, posthole diggers, or similar hand-operated 

 devices. Larger scale efforts may involve trenches, pits, or other large open- 

 ings created for visual inspection, sample collection, and photography 

 (Figure 18). Often, undisturbed chunk or block samples and disturbed jar or 

 bag samples are hand carved from these excavations and taken back to the 

 laboratory. 



Rates and patterns of sedimentation can be determined using marker 

 horizons. Marker horizons may occur in relation to natural events and 

 unintentional human activities or they may be directly emplaced for the 

 express purpose of determining rates and patterns of sedimentation. Recently, 

 several studies have estimated rates of sedimentation in marshes by spreading 

 feldspar markers and later measuring the thicknesses of materials deposited on 

 the feldspar with cryogenic coring devices. 



The petrology and mineralogy of rock samples can be used to identify the 

 source of the sediment. This can indicate if river flow has changed or if 

 coastal currents have changed directions. 



Direct sampling of subbottom materials is often essential for stratigraphic 

 studies that extend beyond historic time scales. Table 5 lists details on a num- 

 ber of subaqueous sediment sampling systems that do not require drill rigs. 

 One system listed in Table 5, the vibracorer, is commonly used by geologists 

 to obtain samples in the marine and coastal environments. Vibratory corers 

 consist of three main components: a frame, coring tube or barrel, and a drive 

 head with a vibrator (Figure 19). The frame consists of a quadrapod or tripod 

 arrangement, with legs connected to a vertical beam. The beam supports and 

 guides the core barrel and vibrator and allows the corer to be free-standing on 

 the land surface or seafloor. The core may be up to 3 or 4 m long, which is 

 adequate for borrow site investigations and many other coastal studies. Heavy 

 duty, longer corers are available. 



While common vibratory corers are capable of penetrating 5 m or more of 

 unconsolidated sediment, actual performance depends on the nature of the 

 subbottom material. Under unfavorable conditions very little sediment may be 

 recovered. Limited recovery occurs for several reasons, chief among these 

 being lack of penetration of the core barrel. In general, stiff clays, gravel, 

 and hard-packed fine to very fine sands are usually most difficult to penetrate. 

 Compaction and loss of material during recovery can also cause a discrepancy 

 between penetration and recovery. In comparison with rotary soil-boring 

 operations, vibratory coring setup, deployment, operations, and recovery are 

 quite rapid. Usually a 3-m core can be obtained in a matter of minutes. 

 Longer cores require a crane or some other means of hoisting the equipment, 

 a procedure that consumes more time but is still comparatively rapid. Success 

 with vibracoring depends on some knowledge of soil type beforehand. 



Cores can be invaluable because they allow a direct, detailed examination 

 of the layering and sequences of the subsurface sediment in the study area. 

 The sequences provide information regarding the history of the depositional 



Chapter 3 Field Data Collection and Observation 



43 



