Since I was down in Washington D.C. the other day, this article caught my eye - “Capitalsaurus,” A D.C. Dinosaur at Smithsonian.com.
image at left).
The Triassic Period (around 200-250 million years ago) was the time the supercontinent of Pangaea began rifting apart. Prior to this, the continent of Africa was connected to eastern North America.
The rift basins, shown in red at left, are essentially fossil "stretch marks" in the crust. Further to the east, the crust broke, seafloor crust formed from ascending basaltic magma, and eventually opened up into the Atlantic Ocean as Africa drifted away.
figure at right shows how Pangaea rifted apart, formed an ocean basin as the two continents drifted away from each other, and left behind a rift basin (to the left of the ocean).
Since these rift basins are places where sediment accumulates, they are areas where fossils can be preserved and the Triassic Period was the initial time of the dinosaurs (which arose, lived, and died in the Mesozoic Era - a time consisting of the Triassic, Jurassic, and Cretaceous Periods).
Anyway, that's why we have some dinosaur trackways and fossils here in the eastern part of the U.S.
This dinosaur, informally dubbed Capitalsaurus, was discovered in 1898 when sewer workers were excavating at First and F Streets SE (see map) - the corner is now called Capitalsaurus Court. What was found are a few vertebrae and bone fragments of a therapod (two-legged, meat-eating dinosaur). The Smithsonian article, linked above, discusses some of the confusion about its correct classification. Capitalsaurus is an informal name, not a scientific one, since it was not officially described in a peer-reviewed journal. The District of Columbia, in the Official Dinosaur Designation Act of 1998, made Capitalsaurus the official dinosaur of D.C. (January 28 is Capitalsaurus Day).