The Adelaida District's Limestone Layers Laid Bare
February 28, 2023
On Friday, Winemaker Neil Collins poked his head into the office we share, looking excited, and said, "Hey, you got a minute?" I did, and we hopped into the ATV and Neil piloted us south across the creek and along the south side of the reservoir the property's previous owner made back in the 1950s. He stopped near the dam, and we headed out on foot. What he had found was remarkable: that Las Tablas Creek had become an exhibit for the local geology.
This has been a banner year for Las Tablas Creek. After three years where it barely ran, the series of storms that we got in late December and early January got it flowing fast:
During a break in the storm Viticulturist Jordan Lonborg got down to shoot this video of Las Tablas Creek… and it’s moving fast, has filled up our reservoir and is now flowing through the spillway for the first time since early 2019! pic.twitter.com/mTWzT1o9hJ— Tablas Creek (@TablasCreek) January 5, 2023
A week later, after another storm had dumped six more inches of rain in about 24 hours, it burst its banks and flowed over Adelaida Road just outside the winery, producing impressive enough footage that it made it onto several national networks as an illustration of the widespread California flooding:
All that water flowing down the creek and into (and out of) the reservoir changed the landscape in in visible ways. In the creekbed it's clear how high the water came because everywhere below that line it scoured away the topsoil and exposed the limestone layers underneath:
At the far end of the lake is a spillway through which the water flows once it has filled the reservoir. It's a remarkable illustration of the local geology. Most of the calcareous soils that underly the Adelaida District are soft, as much clay as rock, which has given rise to the (incorrect) theory that none of it is limestone1. While it's a good thing that we don't have solid limestone underneath us, as limestone is too hard for vines' roots to break up or break through, there are bands of limestone that run throughout the region. The previous owner made use of one of these layers in the creation of the spillway, which follows the slanting descent of the layer from dam-level down to the original creekbed. Here are two views: on the left from above the spillway in late January, and on the right from below last week:
In both photos, though most clearly from below, you can see the many layers of softer rock that the water has eaten away over the years, while staying above the harder limestone layer.
A side-stream that flowed into the creek showed another good example of the mix of harder and softer calcareous layers, and the step-like pattern that is repeated in creekbeds throughout the region:
While the softer layers crumble and decompose, the harder limestone bits stay in the topsoil. Most wineries remove them before planting anything, as otherwise they chew up tractors at an alarming rate. The rocks that are removed are the raw materials for the walls you see at Tablas Creek and around the Adelaida District:
All this rock is sitting there year-round, just a few feet below the surface. Thanks to the rain we've received (and continue to receive) this winter it's easier to see than ever.
- If you're interested into a deep-dive into the chemistry and geology of the calcareous soils out here, check out my 2020 blog Why Calcareous Soils Matter for Vineyards and Wine Grapes.