Window history
Sinkhole at college offers glimpse of Sierra’s prehistoric past
Asinkhole that recently opened up next to the San Diego Ditch on the Columbia College campus has provided a fresh, dynamic opening into a subterranean Mother Lode story 300 million years old and older, back to when the Central Sierra was flat and lay beneath a prehistoric ocean.
The sinkhole appeared in November and remained active as of this week, according to Jeff Tolhurst, PH.D., with the college’s Geosciences, GIS, and GPS programs.
The edges of the mouth of the active sinkhole are dirt and rock, and sunlight illuminates the depths of the hole for a few feet, and it continues opening up into darkness below.
Aside from scaring a few people and alarming others, the sinkhole has drawn the interest of scientists like Tolhurst and Tom Hofstra, PH.D, head of the college’s Forestry and Natural Resources Program.
The sinkhole when it was first discovered Nov. 23 was about 22 feet long, 12 feet wide, and 12 to 15 feet deep, Tolhurst said.
Tuolumne Utilities District workers brought in a 37-foot length of flexible pipe to bridge the sinkhole and restore flow in the San Diego Ditch in late November. TUD workers have since filled in most of the sinkhole, with about 250 cubic yards of boulders, rocks, and dirt, but a section of the fill was collapsing Wednesday.
Flow to the San Diego Ditch was interrupted last week when a storm-blown tree fell on a flume section of the PG&E Tuolumne Main Canal below Lyons Dam. The canal has since been repaired, while flow into the San Diego Ditch remained at a standstill Wednesday. Sections of the San Diego Ditch are fenced off to keep curious people away. People on residential properties downstream from Columbia College rely on San Diego Ditch water.
It’s unlikely that another sinkhole will open up so suddenly somewhere else on the Columbia College campus that anyone would get hurt, but fatal sinkhole incidents do happen, Tolhurst said.
In April 2006, a 27-year-old man died after being covered by rubble when he fell into a sinkhole opened in the middle of a house in Alta in Placer County.
Hofstra and Tolhurst know the history of sinkholes in the area. There are others along the San Diego Ditch upstream from the current one, elsewhere on the campus, in the labyrinth of hydraulically-mined rocks between the college and Columbia State Historic Park, as well as other places in the Columbia area.
Hofstra and Tolhurst showed where another sinkhole was discovered back in 1968 when workers were first building Columbia College, below the San
Diego Reservoir and its earthen dam at the center of campus.
One of the landforms at Columbia College is so riddled with sumps and sinkholes and subterranean passages that it was dubbed Honeycomb Hill by the Gold Rush miners who preceded the college by more than a century.
Honeycomb Hill stands above where the current sinkhole has opened on the San Diego Ditch.
Most sinkholes in Columbia do not result from mining activities, including hydraulic mining and miners’ installation of ditches to bring water to separate gold from rock ore, according to Tolhurst and Hofstra.
The Columbia Basin, surrounded by hilltops and mountains, includes former ocean reefs of limestone that, through heat and pressure from tectonic activity, has changed and recrystallized to become marble. The soft marble is prone to erosion, which has led to numerous sinkholes that Gold Rush miners used to call sumps.
Tolhurst held up a fragment of white rock upstream from the fenced-off sinkhole on Wednesday and explained it was marble that used to be limestone from the Calaveras Formation.
“The original limestone had amphipora fossils in it,” Tolhurst said, referring to invertebrate organisms that geologists and other scientists call rock-building organisms. “Amphipora were similar to coral or sea sponges, that make the mineral calcite, which is what marble is composed of. Calcite dissolves in weak acidic rainwater and groundwater, which can cause the marble to form voids underground. These caves and caverns can then form sinkholes at the earth’s surface, where they collapse.”
The entire Calaveras Formation runs north-south about 100 miles, 30 to 60 miles wide on the west slope Central Sierra foothills from north of Placerville to Mariposa. It’s estimated by geologists to have formed 300 million to 400 million years ago, based on ages of fossilized creatures including jawless, vertebrate eels called conodonts, the amphipora, and igneous, fire-formed rocks in the formation, according to Tolhurst.
Hofstra and Tolhurst recounted how Chilean miners were among the first in the Columbia area in 1848 at the start of the Gold Rush.
The Chileans set up camp in what they called Santiago Gulch, named for the capital and largest city of Chile, and helped build Santiago Ditch on a sloping side of Honeycomb Hill. Over time, anglicized pronunciations of Santiago changed to San Diego, so the ditch and the reservoir are called San Diego to this day.
Among the caves in Honeycomb Hill is Honeycomb Cavern, mapped by the late Ralph Squire, who owned the Marble Quarry Campground near Columbia College. Squire was a member of a caving group called the Columbia Grotto.
Hofstra walked along a ridge on Honeycomb Hill, pulled back some barbed wire fencing and showed the entrance to Honeycomb Cavern. The original limestone of Honeycomb Hill, which subsequently metamorphosed into marble, likely formed in warmer, tropical or subtropical ocean waters, likely south of the equator, Tolhurst said.
These carbonate rocks were then transported, along with other big pieces of current-day California, to their current location during the past billion years of tectonic plate movement.
In summary, from Tolhurt’s perspective, TUD has filled in the recent sinkhole on San Diego Ditch, and people are likely safe from sinkholes at Columbia College. Most of the buildings on campus are not on Honeycomb Hill and they were not built on marble, the rock that dissolves to form sinkholes.
Sinkholes will continue to form on Honeycomb Hill over geologic time, Tolhurst said. Tolhurst used geospatial technology to assess the impacts of the recent sinkhole on San Diego Ditch, and to tell the story of the specific natural hazard. For more information visit https://bit.ly/3356jmh online.
Hofstra and Tolhurst are both viewing the recent sinkhole as an opportunity to recruit students of all ages who want to learn more about drone mapping, geographic information systems, geology, geography, forestry, fire science, fuels management, and firefighting.
Visit www.gocolumbia.edu for more information.