July 21, 2020

Is this Florida’s most famous landslide?

Posted by larryohanlon

By Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab

Are there even landslides in Florida?

I have always been interested in the surprisingly incised topography in parts of Florida’s panhandle, and I recently wondered if the combination of weathered, weak Miocene-Pliocene sedimentary deposits, extreme rainfall, and locally steep slopes and bluffs might come together to trigger occasional landslides. I asked Google and was rewarded with a vintage paper called…wait for it…”A Florida Landslide.” Written in 1948 by Richard Jordan of Florida State, the paper describes a surprisingly impressive landslide that occurred in Gadsden County, Florida in early April of that year. The map below shows the slide’s location on the Florida panhandle–it can be found at 30.608743N 84.760314W, between Greensboro and Lake Seminole. Today, it is noted as “Pitts Point” on Google Maps, apparently named for the landowner at the time of the slide.

A photograph of the slide soon after it occurred is shown below, and the 1948 paper (the source of the image) can be found at this link.

The slide scar is about 560 ft (170 m) across, and the headscarp is 45 ft (14 m) high. There is approximately 100 ft (30 m) of relief between the farm field containing the headscarp and the valley floor containing the deposit. The deposit fans out along the stream valley, and can be seen to contain lots of jumbled tree trunks. According to the paper, the slide occurred after an exceedingly rainy 12-month period (rainfall ~50% above normal), with 16 in (41 cm) of rain in the month prior to the slide. The substrate was described as “partially indurated clayey sands” with no obvious clay detachment plain and no significant dip towards the stream valley. The slide scar is entirely forested today, but shows up nicely in a 1-meter LiDAR hillshade from The National Map, which I turned into a .kmz overlay.

The LiDAR hillshade shows some nice kinematic details of the slide, which Jordan describes as an earthflow. Slide blocks are still visible at the foot of the headscarp, and flow lines in the deposit are also visible, particularly where the slurry-like material escaped through the narrow outlet and began to spread along the valley floor. In the image below, I have highlighted the deposit with a transparent pink overlay.

Some of the slide blocks are curved and bent due to their progress towards the outlet, and their en echelon arrangement is highlighted in the paper as evidence of how the slide evolved its interesting broad scar and narrow outlet. The image below outlines two of the curved blocks.

Jordan suggested that failure began along the low bluff that runs left to right through the center of the image. After the initial failure, blocks began to detach from the entire length of the curved scarp. Because the land surface above the bluff (into which the scar grew) has very little slope, retrogressive block detachment did not move preferentially uphill (there isn’t much of an uphill!), allowing the headscarp to retrogress in all directions. The scar is slightly elongated back into the flat upland towards the top of the image, and this may reflect the very gentle slope that is present. The blocks, which were saturated, “liquefied” into a slurry after detachment and flowed out the narrow outlet. Uniform outward growth of the scarp, along with the liquefying of the detached blocks, allowed a large, mostly evacuated scar to form with a comparatively narrow outlet. I attempt to illustrate this in the conceptual GIF below.

Jordan suggested that because the geologic and topographic conditions at the slide are present over much of the area, similar events could occur in the future. I checked out the hillshade topography in the area on The National Map viewer, and I think some other slides are visible not far to the southwest of the 1948 slide. They don’t share its unusual geometry because they occur on steeper and more steadily sloping surfaces. Two are pointed out below; I think the one on the left would be interesting to see in 1-meter hillshade imagery, which I was unable to obtain for this location.

I think the broad scar/narrow outlet shape of the 1948 slide is its most interesting feature (in addition to its being within the borders of Florida!). It is reminiscent of some much larger plateau-edge slides in Jamaica, where weathered Miocene limestones overlie some gypsum and clay-rich horizons, which may offer low-friction detachment surfaces. These slides are huge and descend about 2,000 ft (~600 m), but they share the rounded or horseshoe-shaped headscarp and comparatively narrow outlet of the 1948 feature. The first one shown below is near Ewarton, Jamaica and can be found at 18.184930N 77.111085W. The scar is 3,000 ft (918 m) across, and the slide descends 1,800 ft (550 m) to the valley floor.

The second feature is at 18.100158N 77.603985W, northwest of Mandeville. Its scar is 3800 ft (1150 m) across, and the slide descends 2,100 ft (640 m) to the valley below.

This post was originally published on The Geo Models Blog, and reproduced here with permission.