I am currently working on a report for an archaeological survey that occurred in Madera County. During the course of the survey, several archaeological sites were found, as well as one location that is difficult.
Under the National Historic Preservation Act and the California Environmental Quality act, a number of criteria must be met in determining whether or not something is eligible for the National Register of Historic Places or the California Register of Historic Resources, and only if it is eligible will it be provided any legal protection*. Although not one of the formal four criteria, the age of a site does come into play. Remember that an archaeological site is just the past remains of human activity that is not solely a standing structure or object, regardless of age. The relevant regulations have placed the age of 50 years as the point in which we start seriously considering a site (some agencies use 45 years as there is often a several-year difference between the time the environmental studies get started and the time that work actually begins)**. So, as of the day that I write this, if a fallen building, a trash dump, or the remains of a camp site dates to 1961 or earlier, there is a (vanishingly small) chance that it might be eligible for the National Register. Of course, the older it gets, the more likely we are to consider it eligible, in large part because written records are less likely to provide information. So, a camp site from 1961 would have to be recorded, but a camp site from 1861 is much more likely to be subject to further study.
The difficulty is that so much of modern disposable consumer culture really got kicked up in the 1950s that it is often difficult to determine the age of sites that are post-WWII but still over 50 years old. Part of this is due to the narrow time frame (approximately 16 years), part of it is due to the fact that the labels on items tend to fade in the sun, often resulting in little more than blank bottles and cans (which, changes in manufacturing methods being what they are, we can often tell the age of by measuring them, but not always), and part of it is the fact that a sufficiently large number of people horde stuff in their garages, closets, etc, and then decide to dump it decades later, so a trash scatter or dump consisting primarily of materials from the 1940s might actually have been dumped some time in the 1970s, and you have to watch for clues that this might have been the case. Another part of the problem is that most trash deposits from the 1940s onward consist in large part of the remnants of consumer goods from brands that are still around today, leading the field worker to try to figure out whether a particular brand logo dates to 1946 or 1963 when trying to decide whether or not something needs to be recorded.
There are, of course, historic archaeologists who are experts in this. They are very good at the identification, and also can take basic information on brand distribution and show you fascinating things about how this reflects economic changes, the spread of different populations (sometimes the consumption of one brand of goods over another tells alot about who was doing the consuming), and the ways in which the creation of consumer goods in the modern sense led to an evolution of work and home life. If you have one of these people on hand, as I am sometimes lucky enough to, it can be a boon both to your efficiency (less time recording what you shouldn't, less time going back out to the field because you missed something) and your morale (when someone else is excited about a can scatter, it's harder to feel grumpy about having to record it).
But most of us in CRM are prehistoric archaeologists - we are into stone tools, bedrock milling features, and hunter-gatherer settlement patterns. We know enough about historic sites to be able to sufficiently record most pre-1940 sites that we encounter, and when we embark on a new project we do enough background research to identify unusual sites that we might otherwise miss. We also carry cheat sheets with corporate logos by year, maker's marks for items, and tables of can measurements so that we can judge how old things are. But we always find ourselves finding objects for which we don't have the knowledge or an information sheet ready at hand.
To get back to the case mentioned in the second sentence of this post, we recorded a site that appeared to be a dump of 1950s era household and industrial refuse. Once I returned to the office and began comparing photos and drawings from the field to information in our reference books and at some on-line resources that I use, it became clear that the materials were either from items that would have been in use any time between 1947 and 1976, or were produced in the 1970s but intentionally designed to look like they came from the 1940s and 1950s. A historics specialist wouldn't likely have made the error, but a crew of prehistoric archaeologists did.
It's one of the weird elements of CRM archaeology. Most of us attended undergraduate and graduate programs geared towards making us experts in a relatively narrow range of archaeological skills and knowledge, and then we end up in this profession where we have to work at broadening our knowledge to encompass a very wide range of site types. It also speaks to the value of double-checking your historic artifacts against your reference material before you go to the trouble of producing in-depth site records.
*I often hear people claim that the discovery of any archaeological site or historic structure will result in impacts to construction projects. This is not true. First off, the protections for archaeological sites are not particularly strong. In most cases you can, in fact, destroy them, you just may be required to pay to have them excavated first. Secondly, the only ones that even get this small measure of protection are those that are eligible for the relevant historic registers, and the vast majority do not meet eligibility criteria.
**This age criteria can be ignored if a site is considered to be sufficiently important to history, and this is clear before the 50 years is up. So, for example, the Cape Canaveral launch location, if evaluated before it was 50 years old (which it may have been, I haven't looked it up), was probably eligible for the register because of the huge, history-changing events that took place there.