With 60 minutes at 148°F, you probably didn't completely gelatinize the starch in the grain. Then when you mashed out at 170°F for 30 minutes, you finished gelatinizing the starch, and putting a lot of that recently gelatinized starch into suspension/solution. But, you also denatured the amylase enzymes during mash-out, so that none of the recently gelatinized starch could be hydrolyzed into sugar. Thus you were left with an unusually large amount of starch in suspension/solution in your wort.
The best way to insure that your mash is "finished" is to take SG readings every 10-15 minutes after the mash has been going for 45-60 minutes. When the SG stops rising between SG readings, your mash is done as far as gelatinization and solubilization of starch is concerned. It may not yet have reached maximum fermentability, depending on the concentration of amylase enzymes remaining towards the end of the mash.
A refractometer is extremely useful for taking quick SG readings during the mash. The mash should be stirred aggressively prior to taking each SG sample (to eliminate any stratification), and the samples drawn with an eyedropper or pipette to minimize evaporation of water from the hot wort. Evaporation from the sample will give you erroneously high SG readings, and with small samples, this can be a significant effect.
Brew on