Geologic and hydrologic forces shaped the Spanish Lake Basin, a "bowl" of soil and water sculpted by rivers weathering and eroding the land over tens of thousands of years. Beginning 100,000 years ago, river systems built up a high terrace in the area of the Spanish Lake Basin and carved the valley that later became the Basin. Later, the salty waters of the Gulf of Mexico entered the valley and, as sea levels dropped, it filled with sediment again.
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