I noted the makeup of the concrete pavement on I-5 while in slow moving traffic. A nice matrix supported conglomerate with rounded pebbles. I will say that my fellow passengers thought me a bit odd snapping a picture of the pavement, but pavement is part of the Washington Landscape and it is part of the landscape many of us interact with everyday. This particular patch of concrete landscape is 50 years old. Concrete is more expensive than asphalt and slower to lay down as pavement, but it lasts a much longer time.
Concrete requires mining. Two types of mines are required: lime for the cement and sand and gravel for the aggregate mixed into the cement. Washington State has extensive limestone deposits in northeast Washington. Small lime deposits in western Washington have been mostly if not completely mined out. The town of Concrete built up to process limestone from nearby mines but that industry is no longer active as the mines were exhausted. In Whatcom County extensive limestone mining took place in the early 1900s north of Kendal and Maple Falls. The extent of the ruins and the number of people that once lived in the area for the mining is surprising. Limestone mining took place on the west side of San Juan Island. One of the mines on San Juan Island is now a state park, Lime Kiln Point State Park. Much of the lime for cement in Washington is now imported via rail and ship with some coming into Puget Sound via barge from Texada Island between mainland British Columbia and Vancouver Island.
Sand and gravel for concrete is certainly more common, but quality considerations make some sand and gravel deposits significantly more valuable than others. For concrete, roundedness is a good thing for most concrete as it helps for mixing. The hardness and strength of the pebbles is important as well. Soft rock pebbles would ware down too fast. And certain rock types do not react well with cement and thus can not be used. Too much andesite is generally not good.
Besides quality the value of deposit is based on accessibility and proximity to markets. Being next to good roads and near cities is a big bonus. Water access is useful as well. Trucking gravel is expensive but barging is cheaper. This issues has caused significant debate for the sources of gravel for expanding the Seattle-Tacoma airport with mining of Murray Island being one of the proposed sources. Hence the other major factors are environmental impacts and ease of mining and processing.
Eric Cheney my UW economic geology professor noted that though aggregate mining may not appeal as much as gold, but a good aggregate deposit can be very valuable not only to the deposit owner but to the engineers that utilize the product. As it turns out in my career I have spent a lot more time evaluating aggregate and quarry rock than other economic deposits. I sometimes describe my job as getting paid to give people bad news, but when it comes to aggregate valuation, I have had the pleasure of delivering good news on numerous occasions. A high quality deposit in the right location and with the right market conditions can be very valuable.