In 'Proust and the Squid', Maryanne Wolf talks about just that, how external reading and writing skills behave as a kind of storage area for brain contents. I can't remember the exact passage (I guess because I have it written down in a book at home) but she talks about how we don't write things down to remember them, but so that it's okay for us to forget them. She goes into an analysis of a few cultures and their strengths and weaknesses when it comes to writing, reading, and memory. Very related and a good read. It follows along a bit with Plato's Phaedrus, the story of Socrates' objection to the written word.
I think it's interesting the way what you have memorized, exactly, seems to change based on where you are or what you are doing. I'm sure most of us without eidetic memories have experienced the sudden loss of some memorized bit of information, only to remember it with ease a few hours later.
Memories of certain friends seem completely solid and close when you are around them but utter inaccessible otherwise, never entering into your day to day thought processes. I often wonder if this is even a brain-wide effect, with different tools in the toolbox other than just memory being triggered or non-triggered based on your environment. It would be strange if some environments caused tools in your brain to trigger that increased your skill at a task, tools that would go forgotten at another time. I think I ran into an example of that the other Friday. I got my arm stuck past the elbow in a narrow metal slat, reaching for something in a warehouse after hours. Legs off the ground, lacking leverage and totally unable to free myself, I struggled for a while then just sat around thinking, trying to figure out what to do. After half an hour, I realized I could spit on my arm to get it lubricated up and slip it out of the slat -- some gross struggling and a couple minutes later I was free, if really bruised up. I feel like I would have come across that solution faster if my worries weren't tending toward being stuck all weekend in the warehouse.
How many of the things you "know" do you have memorized?
Do you remember how to spell all of those words you let the spellcheck catch? Do you remember what fraction of a teaspoon of salt goes into that one recipe, or would you look at the list of ingredients to be sure? Do you remember what kinds of plastic they recycle in your neighborhood, or do you delegate that task to a list attached with a magnet to the fridge?
If I asked you what day of the month it is today, would you know, or would you look at your watch/computer clock/the posting date of this post?
Before I lost my Palm Pilot, I called it my "external brain". It didn't really fit the description; with no Internet access, it mostly held my contact list, class schedule, and grocery list. And a knockoff of Minesweeper. Still, in a real enough sense, it remembered things for me.The vast arena of knowledge at our fingertips in the era of constant computing has, ironically, brought it farther away. It seems nearer: after all, now, if you are curious about Zanzibar, Wikipedia is a few keystrokes away. Before the Internet, you'd probably have been looking at a trip to the library and a while wrestling with the card catalog; and that would be if you lived in an affluent, literate society. If you didn't, good luck knowing Zanzibar exists in the first place!
But if you were an illiterate random peasant farmer in some historical venue, and you needed to know the growing season of taro or barley or insert-your-favorite-staple-crop-here, Wikipedia would have been superfluous: you would already know it. It would be unlikely that you would find a song lyrics website of any use, because all of the songs you'd care about would be ones you really knew, in the sense of having heard them sung by real people who could clarify the words on request, as opposed to the "I think I heard half of this on the radio at the dentist's office last month" sense.
Everything you would need to know would be important enough to warrant - and keep - a spot in your memory.
So in a sense, propositional knowledge is being gradually supplanted by the procedural. You need only know how to find information, to be able to use it after a trivial delay. This requires some snippet of propositional data - to find a song lyric, you need a long enough string that you won't turn up 99% noise when you try to Google it! - but mostly, it's a skill, not a fact, that you need to act like you knew the fact.
It's not clear to me whether this means that we should be alarmed and seek to hone our factual memories... or whether we should devote our attention to honing our Google-fu, as our minds gradually become server-side operations.