A Highland Fling (Part 2): Learning from LiDAR

The Field Methods class finished our late Winter/early Spring fieldwork at William & Mary’s Highland in March. Over the course of four field excursions, we put in ~530 people hours at Highland – learning to do science in the field and ultimately collecting data on vegetation, soils, water, and rocks (of course).

Since Spring Break, we’ve completed draft geologic maps and, in small groups, are working on specific research projects. On April 28th, we’ll present our findings and move that content to a class website.

Shaded relief map of Williamsburg & Mary's Highland

Shaded relief map of William & Mary’s Highland and the surrounding area (from LiDAR topographic data). The small red boxes are detailed maps that are illustrated below.

In this post, we discuss some of what we’ve learned using LiDAR (Light Detection And Ranging) topographic data from Highland. These data were collected by the Commonwealth of Virginia in 2016/17 and are publicly available, although the learning curve for data processing is not trivial.

We use the bare earth LiDAR data which effectively strips away vegetation such that the Earth’s surface features are quite evident. Our dataset has a 0.0278” (second) resolution, which translates to individual pixels ~1 meter (3’) on a side with an elevational accuracy of ~10 cm (~4”) enabling us to build high-resolution digital maps of the Highland terrain.

The movie below is a rotating 360˚ digital panorama that’s centered on Highland, its perspective is from an oblique vantage point ~1500 meters (4000’) above the terrain.  I could watch this movie all day, as the texture and topography of the landscape almost jump out from the screen. The Highland property extends from the slopes of Carter Mountain eastward to a set of low linear ridges. The topographic grain trends in a northeast-southwest direction– why might that be? The fine-scale texture of the surface (pasture or a vineyard or a pine stand) is also evident.

The detailed map below is a shaded relief map that employs both oblique directional and hill slope shading to bring out landscape features. We obliquely illuminated the digital landscape from the northwest, such that slopes facing to the northwest are bright while slopes facing to the southeast are in varying degrees of shadow. The hill slope shade makes steep areas darker and flat areas brighter. Collectively, this combination of shading, on this mostly southeast facing slope, yields the ‘brooding shade’ because of its high contrast and dark overtones. But this style of shading also makes many landforms pop!

Hill shade/slope shade map of the southeastern side of Carter Mountain. Identifiers include stone walls, outcrops, farm pond, terraced slope in apple orchard, debris flow and debris flow snout

Brooding hill shade image on the southeastern side of Carter Mountain. Note the many landscape details.

In this map, the terraced slopes of the orchard on Carter Mountain are quite apparent, as is the massive stone wall that separates the orchard property from Highland. We hypothesize that when the terracing for the orchard took place, rocks were removed and piled up along the property line. Shading also reveals large natural outcrops of the underlying bedrock – during our geological mapping these were features of interest.

In the steep valley below the farm pond, we’ve identified a landslide deposit, likely a debris flow as it’s got the characteristic form that indicates earth materials moved downslope in mass. We’ve verified this interpretation with field observations – the base of the deposit has a bouldery snout while higher up in the valley there are low scarps created as the mass movement eroded down to bedrock. We’ve characterized the geometry of this deposit, but we don’t know the age of this mass movement event. Was it a historic event (within the past 2 or 3 centuries) or pre-historic movement (thousands of years old)?

The eastern side of the Highland property is dominated by large pastures both to the north and south of the graceful ash-lined entrance road. The North Pasture is interspersed with patches of woodland. The largest of these woodlands hides an extensive gully system that is beautifully revealed on the LiDAR imagery. How did these erosional gullies come to be? In spots, the gully is 3 meters (10’) deep, you could lose a cow in there!

Aerial photo and topographic map of the East Gully. Identifiers include The East Gully Woods, channel headwall, deeply eroded channel and shallow gully heads

The East Gully. Top- Google earth imagery. Bottom- LiDAR derived 1-meter contour map.

It’s likely that the gullies were created by poor land use management, perhaps from over grazing by livestock or careless soil tillage, which caused extensive runoff and focused erosion to carve the gully network. The age of the forest tells us something about the age of the gully network, and our tree coring efforts indicate that most of the trees are less than 70 years in age. An aerial photograph from the 1930s shows a gully free pasture in that area.

Highland is an active farm and in recent years they’ve taken numerous steps to mitigate the impact of cattle in the riparian areas. Effectively all of the wooded area in the figure above is currently fenced such that cattle are excluded from the woodland and gully network – that’s good. Highland’s been proactive in the development of riparian buffers and has earned high marks for their ongoing land stewardship efforts.

Massey Creek is a small creek that flows off the crest of Carter Mountain, as its gradient becomes progressively less steep it develops a low-slope alluvial floodplain. But then something curious happens, the creek drops over a small (~1.5 m), but suitably dramatic waterfall. For geologists, the waterfall is a knickpoint ­– a source of both joy and questions.

Below the knickpoint, Massey Creek has deeply eroded into its alluvium and forms an incised channel as is colorfully evident on the LiDAR-derived topographic map below. What’s the origin of this waterfall? Over time will this knickpoint remain stationary or will it migrate upstream?

So many questions.

Massey's Creek map

Massey Creek LiDAR derived topographic map with colored slopes illustrating alluvium and knickpoints.

The LiDAR topographic data enables us to ‘see’ many small features on the Highland landscape. Collectively, these landforms provide evidence to understand both the active processes as well as the landscape history at Highland.

I’ll finish with a few pictures of 2022 Field Methods class (aka ‘the Fun Bunch’) during their numerous people hours at Highland.

Students examining an outcrop at Highland.

Bundled up and busy on a cold day at Busted Bivalve Rock.

Students examining the remains of a soil core.

Field Methods posers after describing soil cores, the contents of which were laid out on the blue tarp.

Students on the lawn at Highland having their lunch.

Lunchtime at Highland.

Faculty and students examining a tree core.

Professor Dom Ciruzzi shows off a tree core obtained in the East Gully woods.

Marble statue of James Monroe with W&M students examine the rock texture.

Contemplating the mysterious marble statue of Monroe.

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