Abstract Miocene strata exposed in the Calvert Cliffs, along the western shore of the Chesapeake Bay, Maryland, have a long history of study owing to their rich fossil record, including a series of spectacular shell and bone beds. Owing to increasingly refined biostratigraphic age control, these outcrops continue to serve as important references for geological and paleontological analyses. The canonical Calvert, Choptank, and St. Marys Formations, first described by Shattuck (1904), are generally interpreted as shallowing-up, from a fully marine open shelf to a variety of marginal marine, coastal environments. More detailed paleoenvironmental interpretation is challenging, however, owing to pervasive bioturbation, which largely obliterates diagnostic physical sedimentary structures and mixes grain populations; most lithologic contacts, including regional unconformities, are burrowed firmgrounds at the scale of a single outcrop. This field trip will visit a series of classic localities in the Calvert Cliffs to discuss the use of sedimentologic, ichnologic, taphonomic, and faunal evidence to infer environments under these challenging conditions, which are common to Cretaceous and Cenozoic strata throughout the U.S. Gulf and Atlantic Coastal Plains. We will examine all of Shattuck‚s (1904) original lithologic “zones” within the Plum Point Member of the Calvert Formation, the Choptank Formation, and the Little Cove Point Member of the St. Marys Formation, as well as view the channelized “upland gravel” that are probably the estuarine and fluvial equivalents of the marine upper Miocene Eastover Formation in Virginia. The physical stratigraphic discussion will focus on the most controversial intervals within the succession, namely the unconformities that define the bases of the Choptank and St. Marys Formations, where misunderstanding would mislead historical analysis.

Abstract The Chesapeake Bay Bridge, a $45,000,000 structure, dedicated July 30, 1952, extends 4.3 miles across Chesapeake Bay, linking Maryland's Western Shore with Del-marva Peninsula. Superstructure consists of simple and cantilever deck spans, 2920-foot suspension main bridge, and 1720 feet through cantilever span. Substructure is entirely on steel piles up to 135 feet long and penetrating to lowest elevation–203 feet M.S.L. Two alternate locations were investigated by 38 borings, totaling 7372 linear feet; the deepest reached elevation–355 feet. Representative grain-size analysis, time-consolidation, Atterburg Limit, compression, and related laboratory tests on 501 ordinary and 93 undisturbed samples furnished quantitative data for substructure design. To enable three-shift drilling, and eliminate shore-based survey crews and delays during periods of restricted low-level visibility, barges were positioned by use of special sextant charts which afforded virtually instantaneous graphical fixes based on observed angles between night-lighted, elevated shore signals. Two shore-to-shore geologic sections were developed, depicting unconsolidated Coastal Plain formations penetrated, including Recent and Pleistocene silts, sands, and gravels; the Aquia (Eocene); and the Monmouth, Matawan, and Magothy (Upper Cretaceous). Explorations of the drowned Pliocene (?) valley of the Susquehanna encountered 1.5 miles west of the Eastern Shore beneath 65-90 feet of water were of critical engineering importance and are of unique geologic interest. Former channel, approaching 5000 feet in width, bottom at elevation-170, and filled with up to 100 feet of highly compressible organic silt, necessitated use of longest piles on project. Integrated geologic and soils studies proved exceptionally valuable in all phases of the project.

Seismic investigations of various water-covered areas by the refraction method are described. These areas were chosen for their diverse geologic columns. The instrumentation was such that frequencies from 10 cycles per second to 10,000 cycles per second were investigated and analyzed. Intensity measures of the various frequencies were also made. Recordings were made using geophones and hydrophones for sound receivers. The water-borne sounds in 10 and 20 fathoms of water showed marked frequency dispersion, although sounds originating in very deep water showed no dispersion when recorded in shallow water. The dispersion appears to be governed by the water depth and the bottom material. No water-borne sounds were originated when shots were placed on land or when land intervened on a straight path from shot to receiver. High frequencies disappear more rapidly with distance than low frequencies. Oscillations of the explosive gas bubble were observed and time intervals checked with present theory. The pulse sounds produce all the phenomena of the original explosion except that each succeeding pulse seems to have a lower high-frequency limit. All the important ground-borne sounds were found to be below 100 cycles per second. No correlation of intensity to transmission layer could be discerned except in one case. Refraction profiles were laid as near as possible along the strike of the structures. Reversals on profiles were available only twice, so that slopes were not determined. Depths to the basement complex were 3130 feet at Solomons, Md., shoal; 3080 feet at Solomons, Md., deep; 6400 feet at Jacksonville, Fla., shoal; 7730 feet at Jacksonville, Fla., deep; 1380 feet at Virgin Islands, shoal; 1710 feet at Virgin Islands, deep; > 16,600 feet at Barbados; 12,210 feet at Orinoco, shoal; 22,790 feet at Orinoco, deep. Travel-time curves are included for each location. Interpretation of the seismic results is made with some aid from the known geology.

A wave-theoretical interpretation is given of pressure waves generated in shallow water by explosions of charges of T.N.T. ranging from 0.5 to 300 lbs., and recorded by Ewing and Worzel. ( See accompanying paper, Explosion sounds in shallow water. ) The normal mode theory of propagation of sound in layered media, which was developed by the writer in 1941, was extended to cover the case of explosive sound, and the predictions of the theory about the shape and variation of amplitude in the received pressure pulse were investigated in detail. It was found that the theory predicted the existence of a series of readily identifiable new features in the pressure wave, each of which is characteristic of the depth of water and the structure of the bottom. A study of the original records, some of which are reproduced on Plates 1-11, revealed the presence of all the predicted phases. The characteristics of these phases were then measured, and the data were interpreted in terms of the structure of the bottom at the various stations. The deductions about the distribution of sound velocity in the bottoms, based on an analysis of the various features of the pressure waves, are given in Table A, and it will be seen that they agree among themselves. The following results were obtained: (1) A study was made of the dominant periods in the ground waves which are propagated along the various interfaces in the layered bottom, in order to verify the theoretical prediction that the deeper the interface (higher sound velocity) the longer should be the periods. A verification of this theoretical prediction is well illustrated in Figures 1 and 2, and to a lesser extent in Figure 3. (2) An extensive investigation, covering an analysis of more than 40 records, was made of the dispersion in the water wave (which is illustrated by the third trace from the bottom on Plate 11). A technique was developed for determining from the records the speed with which each frequency in the water wave is propagated. The discovery made empirically by Ewing that this speed is a function of frequency only {see accompanying paper, Explosion sounds in shallow water ) and is independent of the range was confirmed in all the records, as is shown in Figures 6-19. The shape of the mean dispersion curve at each station was successfully interpreted by an application of the normal mode theory in a layered liquid half-space. Theoretical dispersion curves form the background in Figures 6-19, and, with the aid of these, deductions were made about the sound-velocity distribution in the top layers of the bottom. The conclusions are given in columns 6 and 7 of Table A and in Table 1. (3) The theory of normal modes was developed by the writer to a stage which enables one to compute the actual curve of pressure variation, as recorded by various types of receivers, due to an arbitrary explosion. A sample of such a theoretical pressure wave is shown in Figures 24A, 24, and 25. (4) The following new features of the pressure waves were predicted by the theory of normal modes of a layered liquid half-space and were subsequently discovered and analyzed by the writer: A) In case of a uniform bottom extending down to a depth many times the depth of water, the ground wave should begin with a so-called limiting period which is characteristic of the depth of water and the sound velocity in the bottom. The limiting period was identified and measured in the records taken at the Solomons Shoal station where the bottom is known to meet the requirement stated above, and the results are shown in Table 2. The value of 1.29 for c 2 /c 1 obtained from the average observed limiting period, where C 1 and c 2 denote the sound velocities in the water and in the bottom, is slightly higher than the values deduced from the other features quoted in Table A, but this small discrepancy can be explained by the effect of the deep layers. B) The water wave should arrive riding on a low-frequency wave called the rider wave ; the frequency of the rider wave just prior to the arrival of the water wave is determined by the depth of water and the distribution of sound velocity in the bottom. The rider wave was identified and its period measured on all records taken at Solonons Shoal, Jacksonville Shoal, and Jacksonville Deep. The results are set out in Tables 3-5, and the resulting conclusions about the sound velocity in the bottom are quoted in Table A. Some illustrations of the rider waves can be seen in the records reproduced on Plates 1-9. C) The amplitude of the water wave should increase with time to a maximum value and should decrease thereafter, while the period should remain constant after the maximum is passed. The value of this period, which will be referred to as the Airy period , is again characteristic of the depth of water and the structure of the bottom. Values of the Airy period are given in Tables 2, 3, 5, and 6, and the interpretation of the average values is given in Table A. D) A three-layered medium in which the thickness of the intermediate layer is only of the order of the depth of water should possess dispersion characteristics similar to those of a medium with a uniform bottom. The existence of the intermediate layer should therefore not be revealed by a secondary arrival. Theory also predicts that the amplitude of the rider wave should be relatively low in such a medium (by a factor of 1/5 to 1/10), while the water wave should be of normal intensity. The stations of Virgin Islands Shoal and Virgin Islands Deep which, judged by the combined evidence from the refraction data and the dispersion data in the water wave, have a veneer of mud of a thickness of the order of the depth of water covering a high-speed coral base, would be expected to fall into this class. The records taken at these stations were found to be lacking in secondary arrivals and to be devoid of rider waves, as is illustrated in Plates 8 and 9. The success of the theory in explaining the appearance of the records taken at the Virgin Islands, which were entirely different from the records taken at all the other stations, is very encouraging. (5) Theoretically the maximum amplitude in the water wave should vary like the inverse 5/6-th power of the range, whereas the observations of Ewing and Worzel indicate that in some stations the maximum amplitude varies like the inverse square of the range. We have, of course, neglected absorption and scattering, but, as I have already suggested, it would be interesting to check the experimental determination of variation of intensity with range. (6) Our study shows that in all stations the speed of sound in the first 30 feet of the bottom is no more than about 10 per cent greater than in water. This result conforms with Ewing's finding that all bottom samples were muddy. (7) A complete theory of propagation of sound, both of single-frequency and of the explosive type, in layered media is developed in Part II of this paper. This includes a discussion of the "ray theory" and the wave theory. One interesting theoretical result is that in case of a density discontinuity at the bottom the normal modes are not orthogonal, nor is their amplitude, in case of a point source, correctly given by standard theory of normal modes. Another of the new results arrived at is that, when the wave length of sound is of the order of the depth of water, the amplitude of the pressure should decrease at large ranges like the inverse square of the range, as in the Lloyd Mirror Effect. The asymptotic expressions given in Eqs. (32) and (33) are strikingly verified in Figure 23, in which they are compared with values obtained by numerical integration of the integral in the exact solution.