Overlying sediments are divided into Quaternary and Tertiary sedi- 

 ments, the latter questionably identified as the Yorktown Formation and 

 Miocene in age. The contact between the two lies between 26 and 32 meters 

 (85 and 105 feet) below sea level along the Maryland coast. This contact 

 between the eroded Tertiary and Pleistocene sediments is also the base of 

 the Pleistocene aquifer, a sand and gravel ground-water unit. The seismic 

 reflection horizon that appears to correlate with this surface is horizon 

 B-^ . A structure contour map of horizon Bj , best identified in the 

 southern part of the study area, shows that the surface is rather steep 

 and even with a southeasterly slope (Fig. 15). Locations of borings and 

 the depth to the base of the Pleistocene along the barrier island, as 

 defined by Rasmussen and Slaughter (1955), are also shown in Figure 15. 

 That the horizon could not be traced throughout the study area indicates 

 either a lateral acoustic change in the nature of the horizon or abrupt 

 changes in elevation of the horizon. 



Reflection unit A is Quaternary in age and units B and C are inter- 

 preted as Tertiary. Most of the horizons correspond to hydrologic units 

 and some of the correlations are uncertain due to the variation in ele- 

 vation and discontinuous nature of many of the horizons. Tentative 

 correlations are shown in Table 3. 



Table 3. Correlation of acoustic horizons with significant 

 stratigraphic and hydrologic surfaces. 



Stratigraphic units Acoustic horizons Hydrologic units 



Quaternary A„ 



Holocene , 

 A 3 



Pleistocene A, „, . _, . _ 

 ,„„,.„ . 4 Pleistocene aquifer 

 (Columbia Formation) R 

 1 



Tertiary _ _ ,, _. . , , 



B„? Upper confining bed 



B 3 



B ? Pocomoke aquifer 



Miocene r 



(Yorktown Formation?) ! Lower confining bed 



? 



Ocean City aquifer 



c 2 



Lower confining bed 

 C 3 or C^ 



Manokin aquifer 



Miocene 



(St. Marys Formation) 



42 



