Through shell mound dating, scholars noted three periods of ancient Bay Area history, as described by F.M. Stanger in "La Peninsula": "Careful study of artifacts found in central California mounds has resulted in the discovery of three distinguishable epochs or cultural 'horizons' in their history. In terms of our time-counting system, the first or 'Early Horizon' extends from about 4000 BCE to 1000 BCE in the Bay Area and to about 2000 BCE in the Central Valley. The second or Middle Horizon was from these dates to 700 CE, while the third or Late Horizon, was from 700 CE to the coming of the Spaniards in the 1770s."