Complex sentence: The initial four properties were built between 1906-1908, an additional four in 1911, and the bulk of the remainder built in the 1920s, slowed down during the Great Depression, and then concluded in the 1940s.
Split into simpler sentences: The initial four properties were built between 1906-1908, an additional four in 1911, and the bulk of the remainder built in the 1920s. Construction slowed down during the Great Depression, and then concluded in the 1940s.