San Diego's Spanish and Mexican roots are most evident in Old Town, the first European settlement in Southern California. Old Town San Diego's first houses, of sun-dried adobe bricks arranged around a central plaza, began to appear in the 1820s; by the 1850s, after the discovery of gold drew prospectors to California from around the globe, they began to be replaced with wood-frame structures. In the 1860s, however, the advent of Alonzo Horton's New Town to the southeast stole thunder from Old Town, which began to wither. Efforts to preserve it began early in the 20th century, and when it became a state historic park in 1968, the process of restoration gained momentum.