Roman Salters were not far behind the potters in thier importance to the expanding empire. Their job was to produce the salt that allowed for food preservation and supported the movement of the troops and citizens in an ever growing region. Everything from Taxes to travel routes depended on salt, and salt depended on brine supply and climate to determine how it was made.
As I mentioned in Salt Origins, all salt originally came from the sea. By the time of ancient rome however there were 4 options
a) Costal Waters - The ocean is at approximately 3.5% salinity on average but varies dramatically depending on several factors
Evaporation: High evaporation rates, like those in the Red Sea and Persian Gulf, increase salinity because the salt is left behind when water turns to vapor.
Precipitation: Rainfall and snowfall add fresh water to the ocean, which dilutes the salt and lowers salinity.
Ice melt: Melting sea ice adds fresh water, while the formation of ice removes fresh water from the ocean, concentrating the salt and increasing salinity.
River inflow: Coastal areas where large rivers empty into the sea have lower salinity due to the influx of fresh water.
b) Halite - Ancient salt captured deep within the earth this form of salt often has other minerals present changing its color and flavor. To Extract this salt you could gather it where it is near the surface, mine it, or pump water over it to disolve it and reprocess the salt using different manufacturing methods.
c) Brine Springs- in areas like Cheshire the ground water can become super saturated by the Halite that is relatively accessible and bubbles to the surface naturally in the form of cool springs that are 8 x more salty than the ocean. Brine springs can also form from geothermal activity but these hot springs tend to have a higher concentration of sulfides and are less likely to be used in salt production.
d) Salt Marshes - Coastal wetlands flooded by tides, located between land and salt/brackish water, are formed by mud and silt deposits. While this can make it harder to process the brinw, areas such as those around Venice found ways to section off marshes and form evaporation ponds.