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An Ohio Geomystery

Cross-posted at Highly Allochthonous. There are some good comments there.

I had the good fortune of going out in the woods a few days ago with colleagues here at Kent State University. We were in a second growth forest, probably fairly typical for this part of northeastern Ohio. The upland forest had lots of maple trees, and the bottomland forests had cottonwood and sycamore. The forest is underlain by many meters of till (with silicic clasts) and below that are various sedimentary rocks. I was there to take a look at some small streams and wetlands as potential field and teaching sites. Towards the end of our tour, my colleague brought us past this site:

My first view of the geo-mystery

My second view of the geo-mystery

My colleague described the site as the ruins of a “sugar shack”, which I connected with the maple trees to mean that this was the foundation of a small-scale maple syrup or sugar production facility.

But what really caught my eye were the tabular black rocks, which seemed completely out of character for the region.

Close-up of the black rocks. Wading boot for scale.

So, I know what the black rocks are and I have a pretty good idea of why they are there, but I don’t know where they originated. I’d like to hear from our readers what they know or can deduce about these mysterious black rocks of northeastern Ohio, so share your thinking in the comments. I bet together we can get to pretty good story of the human history of these geopuzzling erratics.

Geology is destiny: globally mapping permeability by rock type

Cross-posted at Highly Allochthonous

Permeability (the ease with which a fluid moves through a material) is the ultimate goal of many hydrogeologic investigations, because without that information it is impossible to quantify subsurface water and heat flow rates or understand contaminant transport. Yet permeability is notoriously difficult to quantify, both at the local-scale and the landscape-scale. Permeability varies over 13 orders of magnitude across rock and sediment types, because of differences in pore sizes, geometry, and connectedness. Loose gravel could have permeability as high as 10-7 m2, but unfractured igenous and metamorphic rocks could be as low as 10-20 m2. The diagram below is an example of the sort of relationship between rock type and permeability shown near the beginning of every major hydrogeology textbook.

Typical ranges of permeability for different rock types, usually based on hydraulic measurements made at wells.

Typical ranges of permeability for different rock types, usually based on hydraulic measurements made at wells.

Most of the time, hydrogeologists are happy to just to get permeability to within an order of magnitude or two. Knowing permeability is not just useful for those interested in in water supply problems and transport of contaminants. For scientists who model watersheds or land-atmosphere interactions in climate models, being able to easily estimate landscape-scale permeability would be incredibly helpful.

In a new paper in Geophysical Research Letters, scientists from Canada, Germany, the Netherlands, and the US have just done a big favor for those scientists. Gleeson et al. (2010) compiled the first regional-scale maps of permeability for the North American continent and the terrestrial globe. They are interested in permeability in the uppermost 100 m of the subsurface, but below the water table, where all pore spaces are saturated with water. They defined regional-scale as greater than 5 km, because they wanted to avoid influences by things like individual fractures. Using previously published hydrogeologic models, in which permeability was calibrated against groundwater flow, tracers, or heat fluxes, Gleeson and colleagues identified permeability values for 230 hydrogeologic units, grouping them into seven “hydrolithologic” categories, by rock type.

The scientists compared the permeability values from the models to the expected permeabilities for each rock type based on smaller-scale measurements (like those used to make the graph above), and they found reasonably good correspondence. They also examined whether permeability values within each hydrolithologic category were correlated with the scale of the model used to generate them. They found that permeability was scale-independent above 5 km, except in carbonates, where large karst features may result in changing permeability with increasing area.

Using pre-existing geologic maps for North America and the world, Gleeson and colleagues divided the Earth into their hydrolithologic categories. For each category, they calculated the geometric mean of the modeled permeability values, and applied that mean permeability to all of the map units in that category. The resultant maps show the distribution of permeability across the land surface.

Portion of Figure 3c from Gleeson et al. (2010, Geophysical Research Letters).

Portion of Figure 3c from Gleeson et al. (2010, Geophysical Research Letters) showing the permeability distribution across North America. North of the dashed line is continuous permafrost and in that region, the map likely significantly over-estimates permeability.

The global map uses a single geology dataset, so there are no weird boundaries in the data, but it is of coarse resolution. The North American map (shown above) is at much finer resolution (75 km2 mean polygon area, with 262,111 polygons), but it has a few odd edges that correspond to state and national borders. The authors point to these boundary problems in their discussion of caveats, along with the problems associated with permafrost, deep unsaturated zones in arid areas, and deep weathering in the tropics. In addition, the use of a single permeability value for each category will necessarily lump together some terrains with similar rock types but differing geologic histories and resultant permeabilities (e.g., the High and Western Cascades in Oregon).

The work of Gleeson and colleagues represents an important first step in translating regional-scale geologic data into permeability fields. These maps will be useful for continental-scale and larger earth system models and for data sparse regions. Their methodology also raises some interesting possibilities for subdividing the hydrolithologic categories in areas where there are more hydrogeologic model data available, but where there hasn’t been comprehensive hydrogeologic modeling. Finally, their finding that regional-scale model values are in accord with the ranges reported in every hydrogeology textbook is a significant confirmation of the fall-back position of many students of hydrogeology: “If you have no data from wells in your field area, use a textbook to estimate permeability from the rock type.”

Gleeson, T., Smith, L., Moosdorf, N., Hartmann, J., Dürr, H., Manning, A., van Beek, L., & Jellinek, A. (2011). Mapping permeability over the surface of the Earth Geophysical Research Letters, 38 (2) DOI: 10.1029/2010GL045565

Castle Geology

Cross-posted at Highly Allochthonous

Being a giant geo-nerd, I tend to pepper my travels with a lot of geologically or hydrologically interesting places. A recent trip brought me to the UK and included a meetup with my Highly Allochthonous coblogger in Edinburgh. Being an American tourist, I also felt compelled to visit at least one castle during my time in the UK, so I dragged Chris to Edinburgh Castle…where we naturally we ended up talking about the geology.

Edinburgh Castle from Princes Street

Edinburgh Castle from Princes Street. The low area with gardens in the foreground used to be a loch/lake. (Photo by A. Jefferson)

First off, Edinburgh Castle sits atop the core of a Carboniferous (340 million year old) volcano, now called Castle Rock. The rock is impressively elevated above the surrounding glacially sculpted cityscape. The volcanic root formed a bit of a speed bump for Pleistocene glaciers moving from west to east. Castle Rock was more erosion resistant than surrounding sedimentary rocks, so it was left standing higher than the surroundings. But it also protected the sedimentary rocks in its lee – forming a giant crag and tail structure. The Royal Mile, which stretches from the Castle to Holyrood Palace rides on the tail of this structure – which gradually decreases in height and width away from the castle. In the Google Earth image below, I’ve maximized the vertical exaggeration and you can get some sense of it, though I can tell you that a more visceral understanding is achieved by walking up or down the Royal Mile.

Crag and tail of Edinburgh Castle and the Royal Mile

Crag and tail of Edinburgh Castle and the Royal Mile (image from Google Earth)

Within the castle itself, the glacial legacy is on display in the building stones, which show a remarkable diversity of lithologies, both those found locally and those farther afield in Scotland. It is likely that castle builders would have made use of the rocks left on the landscape when the glaciers retreated, but the ice caps would have given them plenty of variety. In the middle of the photo below, I think we are looking at the Old Red Sandstone, which has an important place in the history of geology and of paleontology.

Stonework on St. Margaret's Chapel, the oldest extant building in Edinburgh Castle

Stonework on St. Margaret's Chapel, the oldest extant building in Edinburgh Castle (photo by A. Jefferson)

The stone construction seemed to vary between buildings and even parts of buildings within the castle. The photo above is from St. Margaret’s Chapel, built in the 12th century, and you can clearly see how the big rocks are matrix-supported by a sand and gravel cement. This wall is actually even different from other walls on the same building, where stones are much more rectangular, probably cut, and there is much less matrix, as shown in the photo below. The informative castle guidebook suggests that the wall pictured above may have been originally part of another structure, onto which St. Margaret’s chapel was built adjointly. I should also point out that the walls in St. Margaret’s chapel are incredibly thick (1 m or more), so there’s a lot of stone work we’re not even seeing from the outside.

St. Margaret's chapel showing variety of stone work

St. Margaret's chapel showing variety of stone work (photo by A. Jefferson)

Portcullis Gate and the Argyle Tower

Portcullis Gate and the Argyle Tower (photo by A. Jefferson)

In other parts of the castle, and indeed in the surrounding city, it was fun to watch for places where old doorways or windows had been filled in or where additions had been added to stone buildings. These were usually pretty easily spotted by a change in the stone construction style. For example, in the photo on the left, you can see the main gated entrance to the castle (the Portcullis Gate), which was constructed in the late 16th century. On top of it is the Argyle Tower, one of the youngest buildings in the castle, constructed in 1887. To me it also looks like there might be an intermediate strata between the cut and tightly placed blocks immediately around the gate and the much more modern top part.

Wall adjacent to Lang Stairs

Wall adjacent to Lang Stairs. The plaque on the wall commemorates the successful recapture of the castle from the English in 1314. (photo by A. Jefferson)

One of the most impressive features of the castle was the way the stone architecture worked with the irregular topography of the bedrock surface. Walls had uneven bottoms, and even, in some places, sides, like this wall near the Lang Stairs.

Finally, I would be remiss if I didn’t point out one important hydrogeological influence on Edinburgh Castle and its history. As I mentioned, the castle is much higher than the surrounding topography (peak is 134 m above sea level) and made of dense volcanic rock. This has advantages from the defensive point of view, but a significant disadvantage from the “having enough water to stay alive” point of view. During peace times, castle residents presumably brought water up the hill from the loch or from wells in town. But during sieges, they were reliant on the Fore Well. This well, constructed in the 14th century, is an impressive 34 m deep (through solid rock! in the 14th century!), but only the bottom 3 m of the well are below the water table. The well could provide 11,000 liters – but that’s not a lot to supply a bunch of soldiers and their animals. During at least one siege, most of the deaths seem to be attributable to lack of adequate water, rather than the warfare itself.

So that’s what happens when you take an American geo/hydro nerd to Scotland…she looks at castles and thinks about rocks and water.

New publication: Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA

How does a landscape go from looking like this…

<2000 year old landscape on basaltic lava with no surface drainage

~1500 year old basaltic lava landscape with no surface drainage

to looking like this?

2 Million year old landscape on basaltic lava

2 Million year old landscape on basaltic lava. Note steep slopes and incised valleys

Find out in my new paper in Earth Surface Processes and Landforms.

Hint: Using a chronosequence of watersheds in the Oregon Cascades, we argue that the rates and processes of landscape evolution are driven by whether the water sinks into the lava flows and moves slowly toward springs with steady hydrographs or whether the water moves quickly through the shallow subsurface and creates streams with flashy hydrographs. Further, we suggest that this water routing is controlled by an elusive landscape-scale permeability which decreases over time as processes like chemical weathering create soil and clog up pores in the rock. And as a bonus, because of the high initial permeability of basaltic landscapes, the formation of stream networks and the dissection of the landscape appears to take far longer than in places with less permeable lithologies.

Jefferson, A., Grant, G., Lewis, S., & Lancaster, S. (2010). Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA Earth Surface Processes and Landforms, 35 (7), 803-816 DOI: 10.1002/esp.1976

Chris Rowan speaking today in the department

I’m delighted to be hosting Dr. Chris Rowan of the University of Edinburgh. Chris’s specialty is paleomagnetic applied to both neotectonic and paleoclimatic problems, and he’s worked in some fabulously exotic locations. Chris is also the lead blogger at Highly Allochthonous, where I occasionally contribute posts as well.

Dr. Rowan will be giving a 2 pm seminar in McEniry 401 with the title: “In search of good palaeomagnetic data: a romp through New Zealand, South Africa and Oman” This talk is aimed firmly at the non-expert.

Dr. Rowan and I will also be convening an informal discussion called “Beyond LOLcats: Earth Science in the Internet Age” at 11 am in McEniry 401. We’ll be discussing how tools like RSS feeds, Google Wave/Docs and Twitter can enhance facilitate collaboration and enhance research productivity.

If you can, please join us for one or both of these interesting seminars.

GSA Abstract: Sediment size distributions in forested headwater streams of the North Carolina Piedmont

The Watershed Hydrogeology Lab is going to be busy at this year’s Geological Society of America annual meeting in Portland, Oregon in October. We’ve submitted four abstracts for the meeting, I’ll be co-convening a session, and I’ll be helping lead a pre-meeting field trip.

New lab member Cameron Moore has been busy working all summer long at our Redlair field sites, and he’s become an expert at Wolman pebble counts. We think it’s pretty exciting to have such a high density of data in a small area in small streams. Here’s his abstract:

Sediment size distributions in forested headwater streams of the North Carolina Piedmont

Cameron Moore and Anne Jefferson, Department of Geography and Earth Sciences, University of North Carolina at Charlotte, 9201 University City Blvd., Charlotte, NC 28223, fax: 704-687-5966, phone: (704) 687-5973

Headwater streams constitute more than 70% of total stream channel length in North America, yet geomorphic controls on such streams are still poorly understood in many regions. For example, close coupling between hillslope, debris flow, and fluvial processes in headwater streams may counter general downstream-fining trends. The goal of this study was to perform an intensive analysis of sediment size distributions in moderate-relief headwater streams, something rarely performed at such a small (<5 km2) scale. Twenty-seven Wolman pebble counts were conducted on 13 first to third-order streams within 3 km of each other in Gaston County, North Carolina.  The watersheds are forested and represent relatively undisturbed conditions for the Carolina Piedmont. The underlying rock types are felsic metavolcanic rocks and quartz-sericite schist, and each stream drains only one lithology. Ongoing data analysis will relate sediment size distributions to watershed area and channel slope. Median (D50) grain size in all the reaches where Wolman pebble counts were performed ranged from 12 – 46 mm, and averaged 26.9 mm overall. Of the ten streams where multiple counts were conducted, six display a distinct trend toward downstream coarsening. Grain sizes in the lower reaches of two streams may be influenced by a possible backwater effects from the South Fork of the Catawba River. Uniformity coefficients ranged from 1.89 – 9.00, showing a pattern of increasingly well-sorted bed material in the downstream direction in eight of the ten streams sampled multiple times. The data also show that woody debris jams lead to accumulations of poorly sorted sediment with low D50 values relative to the mean D50 value.  Between-stream variability approaches the magnitude of longitudinal variability in any single stream. This suggests that extremely local geomorphic history exerts a strong influence on headwater stream sediments.

Inspiration in ancient rocks and simple physics

If you ask my mom how I got started in geology, she’d tell you that it began with her taking 3-year-old me to see landslides coming off steep hillslopes during the spring thaw. That makes a nice story, but its not the real reason I got sucked into geology. Truth be told, I picked geology because it was the field of science my parents knew nothing about.

In my hometown public school system, the smart kids were herded towards doing in-depth middle school science fair projects. There was a wonderful teacher who helped us find projects and mentors, and taught us the art of visual displays and public teaching. As the child of two scientists, I was a natural fit for the program. There was only one problem: I didn’t want to do anything with which my parents could help. That was my mild form of early teenage rebellion. With my parents’ expertise in biology, chemistry and computer science, I felt I only had one choice: physics. But physics had too much math for my taste. (Little did I know just how mathy geology can be.)

Then a family friend suggested a geology project, I took it and ran with it, and the rest is history. My family friend was a resident of the Bayfield Peninsula, which juts up into Lake Superior from northern Wisconsin. Our friend was a sailor and nature enthusiast, and he pointed out that all of the rock cliffs along the lakeshore had right-angle fractures. He wanted to know why.

Figure 1. Shoreline at Big Bay State Park, Madeline Island, Wisconsin.

Figure 1. Shoreline at Big Bay State Park, Madeline Island, Wisconsin. Photo by Anne Jefferson, July 2007.

That question was the inspiration for my first real science fair project was “Fracture characteristics and geologic history of the Chequamegon Sandstone (Bayfield Group, Late Precambrian).” I collected dozens of stones from the rocky beaches of Madeline Island, where the Chequamegon Sandstone is exposed. I measured the angles between all sides of the stones, and tried to correlate them with grain size, induration and other characteristics. I made my first and last thin sections and I sieved samples using the same sort of Ro-Tap machine I now teach students to use. I also learned about things like properties of non-crystalline materials, the North American Mid-continental rift sytem, paleocurrents, and Pleistocene glaciations.

Figure 2. More shoreline made of Chequamegon Sandstone in Big Bay State Park, Madeline Island, Wisconsin. Glacial scour marks are visible on some of the rock surfaces.

Figure 2. More shoreline made of Chequamegon Sandstone in Big Bay State Park, Madeline Island, Wisconsin. Glacial scour marks are visible on some of the rock surfaces. Photo by Anne Jefferson, July 2007.

I don’t think my conclusions were particularly startling to people who knew anything about rocks. The rocks generally broke along their bed planes, and then at 90 degrees from their bedding, with more than 50% of the rocks exhibiting fractures between 80 and 100 degrees from bedding. Secondary modes were 60 and 120 degrees from bedding. More tightly indurated rocks had a higher propensity to have obtuse fracture angles.

Figure 3. The young scientist at work.

Figure 3. The young scientist at work. Photo by Carol Jefferson, August 1991.

That first project led to a second project, a year later: “Strength, porosity and fractures in the Chequamegon, Mount Simon, and Eau Claire Formations,” in which I contrasted the materials properties of two building stones and an aquifer. Then the Mississippi River floods of 1993 pretty permanently steered my interest from ancient rocks and materials properties towards the more dynamic modern landscape. I’ve never again worked on rocks within an order of magnitude as old as my first rocks, and these days I’m more apt to think about the water flowing over and through rocks than the rocks themselves. But sometimes I’m in the field, and my eyes will be drawn to an outcrop, boulder, or piece of float. And I still find myself silently inspired by the amount of geologic history that rock has experienced to end up in the stream bed, hillslope or lakeshore obeying simple laws of physics.

Figure 4. Perpendicular joints in the Chequamegon Sandstone at Big Bay State Park, Madeline Island, Wisconsin.

Figure 4. The adult scientist still inspired by those perpendicular joints in the Chequamegon Sandstone at Big Bay State Park, Madeline Island, Wisconsin. Photo by James Jefferson Jarvis, July 2007.