I have 19914 lines (yes 19914 rows) of data that are separated into 3 columns

Column A: Known Depth

Column B: Known Age

Column C: Known Depth (but points fall in between those in column A)

Column D: Would like to know the ages for the depths in Column C using interpolation with columns A and B.

So, I have columns A and B, ages matched to depths. I have a third column with a depth, but no age. I would like to know the age of the depths in this column. The data is linear, but the linear part (slope) fluctuates throughout the data set. Excel freaks out and interpolates the data in both ascending and descending orders instead of just descending (ages go back in time as depth value gets higher).

I figured the easiest way to get the correct age would be to find the two points in column A where my depth in column C falls in between. Excel freaked out again because while the mid depths (column C) are rather evenly spread, the depths in column A are not.

So, is there anyone out there that can come up with a series of codes that will help, I would be forever grateful.