Chesapeake Bay, the large arm of the sea in eastern Maryland, is a classic drowned river valley and one of the nation's preeminent wetlands. You can learn more detail about Maryland geology at the state geological survey site, where this map is presented in county-sized chunks at full fidelity.
This map was published by the Maryland Geological Survey in 1968. I have produced two versions that include the key to the different rock units, at 1200x850 pixels (500 KB) and 2050x1420 pixels (680 KB), which is 2.5 times as big on your screen as the paper original.
More Maryland resources on About.com:
About Washington DC
Maryland Geography, State Symbols & Facts
Maryland National Parks