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This comment only partially responds to Sam's, but after reading his thoughts, I became curious  about the process of memory "engraving" vs. memories that don't seem to really "make a mark."

I found myself led to a NY Times article entitled "Forgetting, With a Purpose" by Sindya N. Bhanoo (http://www.nytimes.com/2010/02/23/science/23obflies.html). The article explores how short-term memory might not just fade, but rather is actively erased in order to clear up space for the brain's storage system. Fruit flies, according to Bhanoo, exhibit behaviors that suggest that the "memory-eroding protein", Rac (which also is present in the human brain) allow for this constant emptying-the-trash of our minds. When Rac was obstructed for the fruit flies, the experiment showed that the flies tried to construct two different (and contrary memories) that layered upon one another and caused "chaos."

The protein, according to Bhanoo, acts such that "short-term memory is erased by the brain on purpose, so that new, more relevant memories can be recorded." I find this idea quite intriguing, that the brain takes an active role in determining the information that "matters" most. While this "relevant information" might just function in relevance to survival or health, the fact that the brain prioritizes and makes its own decisions about what is useful, useless, and worthy of its space seems to make our brains and what we consider to be ourselves peers.