In about 1/2% of these digs you find some layers are jumbled, but in the remaining 99% of these digs:
A, when present is always on top of B,
B, when present, is always on top of C
C, when present, is always on top of D
D, when present, is always on top of E
E, when present, is always on top of F
F, when present, is always on top of G
G, when present, is always on top of H
H, when present, is always on top of I
I, when present, is always on top of J
J, when present, is always on top of K
K, when present, is always on top of L
L, when present, is always on top of M
M, when present, is always on top of N
N, when present, is always on top of O
O, when present, is always on top of P
In none of your digs did you EVER find all 16 layers.
Making only the assumption that older layers are deeper, what is the oldest layer?