Marl is lime rich mud (typically gray) that is found around most of the lakes in the county. It is extremely common around lakes in areas where the glaciers came (all of Michigan) and I’m going to try and explain why Marl forms. I also wanted to put a picture here of what Marl looks like but I couldn’t find anything on-line. Basically it looks like gray ooze. Sometimes there’s shells and other stuff in it but the grayness of it, and how it just seeps through your hand is the most distinctive thing about it. I did find a nice picture of Silver Lake though – thanks to Rob Raux on Flickr
As an example of how common Marl is, the below image is taken from the 2013 MDNR Fishery Resource Report for Silver Lake near Pinckney and shows the entire lake ringed with marl.
As people who have tried to build houses around local lakes may know, marl deposits can be thick and are usually very unsuitable for building on. Before the advent of cheaper cement and fertilizer/conditioner options, marl was extensively dredged/mined from glacial lakes in the Midwest for use in Portland Cement and farmer’s fields. “The Lakes of Northern Indiana and their Associated Marl Deposits” (1900) by George Ashley extensively describes this industry in Northern Indiana using similar marl deposits to those likely found in Washtenaw County at the time. In 1954, 218,429 short tons of Marl were dredged from all lakes in Michigan.
Several lakes in Washtenaw County had enough deposits of marl to be dredged for economic uses.
Four Mile Lake near Chelsea was extensively dredged for its Marl by the West German Portland Cement Co./Millen Portland Cement Co./Michigan Portland Cement Co. in the early 20th Century
The most fascinating thing to me about marl is its formation however. Like peat, it is still being formed to this day and represents an on-going, post-glacial depositional process. The marl that we see and feel between our toes today in any of the lakes in the county was not brought here by the glaciers but was slowly deposited on a yearly cycle after the lakes were made. Marl (in the form of Calcium Carbonate) is deposited out of the water column in warm temperatures (summer months) in lakes with higher Ph (above 7)
The chemical pathway to marl formation in our lakes is:
Carbon Dioxide (C02) + Water (H20) → Carbonic Acid (H2C03) → H+ + HCO3– → CO32- + 2H+
So carbon dioxide dissolves in water which forms carbonic acid which further breaks down into carbonate ions (CO32- ). In conjunction with this reaction, photosynthesis from the plants in the lake removes non-dissolved C02 and adds oxygen which drives up the lake pH. The calcium to be added to the carbonate to make the marl is naturally present in the lake (as anyone who has well with hard water knows) and the marl precipitation occurs when the saturation point is reached. In technical terms, marl lakes are “characterized by alkaline water, high concentrations of divalent cations and low levels of dissolved organic compounds”. (Chemical pathway also taken from this link).
So how fast does this process occur?
One paper that I found for a lake in Northern Indiana measured the process at 1.5 to 4 inches of marl every 100 years. So not that fast. This was averaged over a period of a few thousand years so there is probably a lot of variation within that and many other factors at play. It cool to think though that this process is ongoing. As long as there is a good supply of alkaline groundwater, seasonally warm water and a steady supply of dissolved carbon dioxide, we will continue to see marl deposition in glacial lakes! (In addition to peat and other things which I will write about in another post).