Dolomite and limestone of the Black River Group of Ordovician age overlie Cambrian rocks. They are lithologically similar to the Newark Supergroup, which consists of red sandstone, shale, and conglomerate and fills a number of fault-block basins that extend from New Jersey to South Carolina. These basins are discussed in Chapter L of this Atlas. The rocks in the trough dip eastward at about 15 degrees. They feather out to the west to expose the underlying pre-Mesozoic bedrock.
Glacial deposits overlie the early Mesozoic rocks.
Because they are hard and resistant to erosion, the lava flows typically form topographic ridges. The softer sandstone, which is easily eroded, typically forms lowlands and rarely crops out. The New Haven Arkose is the lowermost formation and consists of all sedimentary rocks below the lowermost lava flow. The Meriden Formation consists of the three lava flows and interbedded sandstone. The Portland Arkose is the uppermost formation and includes all sedimentary rocks above the uppermost lava flow.