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How the salt got there

Sidebar by Ed Quillen

Salt Works – July 1996 – Colorado Central Magazine

Geologists say the salt of South Park has been there for a long time — since the Pennsylvanian Period of the Paleozoic Era about 300 million years ago.

Like today, Colorado then boasted two parallel north-south mountain ranges: Uncompahgria and Frontrangia. Unlike today, these mountains were islands in a vast sea.

Erosion from these high lands settled in the sea, leaving thick layers of sediment. Some of that sediment was compressed into sandstone, and then rose 70 million years ago when the current Rockies were born.

That’s what you see at Red Rocks park west of Denver, as well as the nearby hogback. It’s also the horizontal sedimentary rock, which looks like masonry, visible between Tennessee Pass and Minturn across Battle Mountain.

As water eroded sediments from the Ancestral Rockies, it also dissolved minerals from the rocks. When the water reached shallow bays and inlets, it evaporated, leaving behind the dissolved minerals (“evaporites”). The gypsum deposits near Eagle and by Coaldale are one result.

Prominent among those dissolved minerals are salts. To most of us, salt is just one chemical compound, sodium chloride. But to a chemist or geologist, there are thousands of salts, ranging from close relatives like potassium chloride to controlled substances like cocaine hydrochloride.

In most geologic accumulations of salts, though, our familiar table salt is a major component. That was the case in a basin that sat between the two island ranges of the Ancestral Rockies 300 million years ago.

The tidal basin was like today’s Red Sea — relatively shallow, without much of an outlet. Sea level apparently fluctuated considerably then, so sometimes the basin may have had no outlet at all.

As water evaporated, it left great layers of salt (think of the salt flats in Utah) that were later covered.

Move forward to modern times, and the salt beds are lifted near the surface. Outcrops might even have been exposed briefly until detritus from the hills above tumbled down and covered the salt.

Rain and melted snow seep down into the gravel slopes above the salt outcrops, dissolve the salt, and flow underground until coming out at one of the brine springs at the southwest corner of South Park.

Deeper underground, the accumulating sediments over the salt beds increase their pressure and squeeze out any water from the salt.

This probably explains why the 1,000-foot-well at the Salt Works produced better brine than the springs. That water might have been down there for millions of years, accumulating its full share of salt.

In either case, the brine that was boiled at the Salt Works represents one of the most ancient economic mineral deposits in Colorado, and a crystallized remnant of an earlier mountain range that rose 200 million years before the Rockies.

It is also a geologic process that continues to this day. Water runs across the rocks of the mountains around South Park and dissolves minerals. Some water flows into shallow ponds and evaporates, leaving the white crusts of salt and other minerals that you see when driving.

–Ed Quillen