Geological formations in Tennessee largely correspond with the state's topographic features, and, in general, decrease in age from east to west. The state's oldest rocks are igneous strata more than 1 billion years old found in the Blue Ridge Mountains, and the youngest deposits in Tennessee are sands and silts in the Mississippi Alluvial Plain and river valleys that drain into the Mississippi River. Tennessee is considered seismically active and contains two major seismic zones, although destructive earthquakes rarely occur there. The Eastern Tennessee Seismic Zone spans the entirety of East Tennessee from northwestern Alabama to southwestern Virginia, and is considered one of the most seismically active zones in the Southeastern United States, frequently producing low-magnitude earthquakes. The New Madrid Seismic Zone in the northwestern part of the state produced a series of devastating earthquakes between December 1811 and February 1812 that formed Reelfoot Lake near Tiptonville.