by Dave Tucker February 18, 2010
A Little Urban Geology
Several readers of the website have asked about the large rock in the alley behind the Mount Baker Theater (MBT) in downtown Bellingham. Some have suggested that this is an erratic. The 12-foot-tall rock is the last surviving outcrop of Chuckanut Formation downtown, other than at Roeder Falls beside the main branch of the Post Office. Chuckanut underlies the downtown area, not too far below the streets and buildings. The MBT outcrop is an important geological landmark for Bellinghamsters, one that every local geologist should take a look at. It is sort of a natural heritage site, I guess. Please do not beat on it with your rock hammer- enjoy it as is. Visit the Chuckanut Formation page on this website to learn more about the rocks of this formation.
Why isn’t the rock an erratic? An erratic is a rock fragment (regardless of size, cobbles to huge boulders) that is ‘out of place’ relative to the local bedrock. You can visit some erratics on the website, too. The bedrock in the Bellingham area is Chuckanut Formation sandstone, mudstone, and shale. The local glacial erratics featured on this website are all ‘something else’- granitic or conglomerate, hauled by ice from elsewhere and deposited during the last great glaciation. The MBT rock is a bedrock outcrop, and hasn’t come from anywhere.
Getting there: The MBT rock is located behind the classic old theater (104 N. Commercial), in the alley between Commercial and Unity. The rock outcrop sticks out of the back of building built up against the rock outcrop.
This particular outcrop is of pebbly conglomerate and coarse sandstone. The outcrop is vaguely rounded, probably during one of the great glaciations that have buried the area repeatedly. The surface is weathered, and it is easy to brush away surface grains just with your fingers. At the extreme SE corner of the outcrop you can see some of the cross beds the ancient river deposited as it dropped its sediment load. The beds here dip (or slope downward) to the west. This means that the original beds have been tilted since they were deposited in a flood plain by rivers around 50 million years ago. Sometimes, sediment in rivers is NOT deposited in horizontal layers, in defiance of the ‘Law of Original Horizontality’. However, such initially dipping beds are typically very localized, and in reality they are usually curved, with rapid changes in their attitude, or dip. You can see that in some of the cross-beds in Silver Creek, or any number of places in the Chuckanut. The upper layer that makes up most of the MBT rock is a more massive sandstone layer, not as obviously bedded as is the slightly coarser pebble conglomerate at the SE corner.
At the northwest corner is an oblong concretion, or maybe its a fossilized log (see photo). All the little holes in this corner are something of a mystery to me, I haven’t seen this structure elsewhere; perhaps they are human-made. If you have insights, please let the rest of us know! Also at the NW corner are remains of long vertical drill holes used to blast apart this rock to make way for buildings. There was a lot more of this rock in the area that became downtown, but, alas, it was all removed in the name of progress. When the new sewers were installed along Cornwall in the summer of 2008, a lot of sandstone was excavated just below street level- the project took a lot longer than planned because of that. Shoulda asked a geologist, I guess…