Part Three: Core Samples Help Explain Complex Geology
Urban geology also has its upsides. "At least some of the buildings contain coffee shops! A true advantage to working in the city," says Troost. In addition to caffe lattes, Troost and Booth also have access to data and technology that most field mappers can only dream about. To build modern structures, contractors have to drill hundreds of 1- to 3-inch-wide bore holes, between 30 and 290 feet deep on construction sites.
The drilling produces vertical shafts of rocka cross section of the geology that lies beneath. Geologists then analyze the data, looking at groundwater conditions, density of the deposits and where different layers occur. For example, the first round of borings for Sound Transit's light rail line found an ancient channel that ran east-west under the north end of Capitol Hill. Subsequent drilling found four parallel ancient channels that were naturally filled in by glacial and nonglacial deposits. The rising cost of the light rail tunnel under Capitol Hill and Portage Bay can be blamed, in part, on these deposits.
Usually this data just sits in a file drawer. "The people who obtain this information generally do not have the time or money to do a thorough and broad analysis. Kathy and Derek are able to go far beyond what most of us can do," says Bill Laprade of the geotechnical consulting firm Shannon & Wilson. "By integrating the disparate data and analyzing it across the city, they make it far more valuable to the general public."
These cores are helping Troost and Booth, in concert with other geologists, to clarify one of the most complicated and troublesome aspects of Seattle geology: which rocks make up the city's famed hills, an important concern because not all rocks are made the same. For instance, some act as a sieve, allowing water to flow through them, while other rocks block the flow of water, which can oversaturate the layer above and weaken it.
A quick synopsis of our recent geologic past explains the complexity of our hills. Around 15,000 years ago, a 3,000- to 4,000-foot-thick ice sheet flowed out of Canada, moved as far south as Olympia, stalled there for about 500-1,000 years, and then began to retreat, leaving behind three distinct rock layers. The oldest and lowermost unit is known as Lawton Clay, 80 feet or so of dark gray, relatively impermeable clay and silt. Sediment settling into a quiet lake, which formed far in advance of the ice sheet, produced this layer, which can be seen in the lower cliffs of Discovery Park.