A significant portions of the lowlands of northwest Washington are underlain by glacial marine drift. To get a sense of how glacial marine drift gets deposited, a little satellite imagery provides a good overview.
Plumes of silt and clay are poured into the marine waters from nearby melting glacial ice.
Another source of glacial marine sediment is from melting glacial ice floating on the sea surface. Ice bergs as well can provide sediment to the sea floor miles from the glacial front.
Southern coast of Chile
Plumes of silt and clay are poured into the marine waters from nearby melting glacial ice.
Northwest Greenland
Another source of glacial marine sediment is from melting glacial ice floating on the sea surface. Ice bergs as well can provide sediment to the sea floor miles from the glacial front.
In northwest Washington, the
glacial marine drift was deposited during the late stages of the last glacial
period. The mass of thick glacial ice that was on the order of 5,000 feet thick
in the area between 18,000 and 15,000 years ago had isostatically loaded the
land surface such that the current land surface was hundreds of feet lower than
the current elevation.
As the ice thinned the area became inundated with sea
water. Floating glacial ice on the water surface melted and dropped sediment
onto the sea floor. Sediment also was delivered by plumes of sediment rich
meltwater that entered into the shallow water from ice covered nearby upland
areas. Much of the glacial marine drift is predominantly clayey silt but also includes
drop stones and coarser sediment dropped out by the floating glacial ice.
Post glacial marine deposition, the land surface rapidly rebounded above sea level.
Post emergence, the upper glacial marine drift has undergone some compaction via wetting and drying, such that the upper layers of the glacial marine drift below the weathered top soil layer are very dense and compact.
Areas where the unit has remained saturated such in long term year round wetlands or at depth below the year round groundwater level have not undergone wetting and drying compaction and remain very soft.
The geotechnical properties of the marine drift differ from glacial till, and generally allow determination of the difference without the need for fossils. However, it is always fun to find fossil evidence to back up the interpretation.
What the lower Skagit valley may have looked like when inundated with sea water.
Ice would have been floating on the water and the ice margin on land would have been very nearby. Glacial marine drift covers areas from Marysville to the Canadian border and includes area in the San Juan Islands and near Port Townsend. The glacial marine drift is a predominant unit around Bellingham.
Post glacial marine deposition, the land surface rapidly rebounded above sea level.
DEM of Skagit Flats and Samish Flats with an upland area between
The upland area is where the Skagit Airport is located
This upland is underlain by clayey silt glacial marine drift
Post emergence, the upper glacial marine drift has undergone some compaction via wetting and drying, such that the upper layers of the glacial marine drift below the weathered top soil layer are very dense and compact.
Very hard glacial marine drift. Note the drop stone pebble.
Also note haw the compaction process has led fractures within the drift
Areas where the unit has remained saturated such in long term year round wetlands or at depth below the year round groundwater level have not undergone wetting and drying compaction and remain very soft.
Very soft glacial marine drift in sample tube from geoprobe boring
The geotechnical properties of the marine drift differ from glacial till, and generally allow determination of the difference without the need for fossils. However, it is always fun to find fossil evidence to back up the interpretation.
Worm tubes removed from soil boring in glacial marine drift
1 comment:
Thanks for this post, Dan. It gave me a better understanding of the geology where we live north of Thomas Creek on the upland between the Skagit and Samish Valleys.
We're at an elevation of about 300 feet. When we had our well drilled in 2004, their first try hit rock around 15 feet down or so and they ended up having to abandon a brand new, $10,000 bit. They moved the rig 10 or so feet away and then went down over 300 feet through solid, blue clay before hitting a good water-bearing gravel layer. (Incidentally, the well yields around 20 gpm, but the water is fairly alkaline and has high levels of hydrogen sulphide gas and "dissolved solids", along with somewhat high iron content and arsenic a little over the more recent EPA "safe" standard. We cook with and drink mostly water from a 4-stage under-sink filter system.)
Based on what you're saying here, the rock the drillers hit was probably a dropped boulder from a floating berg or ice margin, while the 300 feet of blue clay was glacial marine drift from when the surface was still depressed by the weight of the ice sheet. We have encountered a number of boulders on the property up to, say 5' by 3', and have used them for landscaping (with the help of an excavator for the bigger ones). The larger ones I'm guessing are mostly granite or granodiorite, presumably from somewhere up the Fraser Valley or the coast up towards Squamish. Interestingly, many of the smaller ones appear to me to be dunite, as they tend to be orangey-gold on the outside, weathered surfaces, but pale green on fresh, fractured surfaces. I assume these were carried down from the Sisters.
I understand that your DEM image showing what the area may have looked like with relation to sea level at some point during the ice sheet retreat is meant only to convey an idea of what it was like, not an exact picture. I see that where we live northeast of Butler Hill is not shown as being under sea level in your image and I'm curious as to why that might be. What difference from current sea level did you assume when you created the image? Would the fact that our location doesn't show as having been inundated be due to the sea levels having been higher at times than what you assumed for the image, different rates of isostatic rebound for different areas, or....?
Keep up the great writing! I enjoy the specific topics you cover, as well as their broad range.
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