Long Green Valley Historic District is a national historic district in Baltimore County, Maryland, United States. It is an approximately 6,066-acre (24.55 km2) rural agricultural area to the northeast of the city of Baltimore. The valley has a distinct physical unity created by gently rolling fields dotted with crossroads villages such as Glen Arm, Baldwin, and Hydes and farm complexes. Its architecture covers the 18th, 19th, and early 20th centuries and reflect major architectural styles popular in the United States from the Neoclassical of the 18th century to the Georgian Revival of the pre 1935 period.