diff --git a/pcap/index.go b/pcap/index.go
index af4d381f13..cc5876f98a 100644
--- a/pcap/index.go
+++ b/pcap/index.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 	"io/ioutil"
+	"unsafe"
 
 	"github.com/brimsec/zq/pcap/pcapio"
 	"github.com/brimsec/zq/pkg/nano"
@@ -38,6 +39,15 @@ type Section struct {
 	Index  ranger.Envelope
 }
 
+const (
+	pointSize    = unsafe.Sizeof(ranger.Point{})
+	offsetMaxMem = 1024 * 1024 * 64 // 64MB
+)
+
+// offsetThresh is the max number of pcap offsets collected before offset array
+// in CreateIndex is condensed with ranger.NewEvelope.
+var offsetThresh = offsetMaxMem / int(pointSize)
+
 // CreateIndex creates an index for a pcap presented as an io.Reader.
 // The size parameter indicates how many bins the index should contain.
 func CreateIndex(r io.Reader, size int) (Index, error) {
@@ -79,6 +89,14 @@ func CreateIndex(r io.Reader, size int) (Index, error) {
 			}
 			y := uint64(ts)
 			offsets = append(offsets, ranger.Point{X: off, Y: y})
+			// In order to avoid running out of memory for large pcap sections,
+			// condense offsets with envelope once offsetThresh has been
+			// reached.
+			if len(offsets) > offsetThresh {
+				env := ranger.NewEnvelope(offsets, size)
+				section.Index = env.Merge(section.Index)
+				offsets = offsets[:0]
+			}
 
 		case pcapio.TypeSection:
 			// end previous section and start a new one
@@ -86,8 +104,11 @@ func CreateIndex(r io.Reader, size int) (Index, error) {
 				err := errors.New("missing section header")
 				return nil, pcapio.NewErrInvalidPcap(err)
 			}
-			if section != nil && offsets != nil {
-				section.Index = ranger.NewEnvelope(offsets, size)
+			if section != nil {
+				if offsets != nil {
+					env := ranger.NewEnvelope(offsets, size)
+					section.Index = env.Merge(section.Index)
+				}
 				index = append(index, *section)
 			}
 			slice := slicer.Slice{
@@ -97,12 +118,13 @@ func CreateIndex(r io.Reader, size int) (Index, error) {
 			section = &Section{
 				Blocks: []slicer.Slice{slice},
 			}
-			offsets = nil
+			offsets = offsets[:0]
 		}
 	}
 	// end last section
 	if section != nil && offsets != nil {
-		section.Index = ranger.NewEnvelope(offsets, size)
+		env := ranger.NewEnvelope(offsets, size)
+		section.Index = env.Merge(section.Index)
 		index = append(index, *section)
 	}
 	if len(index) == 0 {
diff --git a/pkg/ranger/envelope.go b/pkg/ranger/envelope.go
index c047249d3b..6e6db69c66 100644
--- a/pkg/ranger/envelope.go
+++ b/pkg/ranger/envelope.go
@@ -68,6 +68,20 @@ func rangeOf(offsets []Point) Range {
 	return r
 }
 
+func rangeOfBins(bins []Bin) Range {
+	r := Range{Y0: math.MaxUint64, Y1: 0}
+	for _, b := range bins {
+		y0, y1 := b.Range.Y0, b.Range.Y1
+		if r.Y0 > y0 {
+			r.Y0 = y0
+		}
+		if r.Y1 < y1 {
+			r.Y1 = y1
+		}
+	}
+	return r
+}
+
 // NewEnvelope creates a range envelope structure used by FindSmallestDomain.
 // The X field of the Points must be in non-decreasing order.
 func NewEnvelope(offsets []Point, nbin int) Envelope {
@@ -79,8 +93,8 @@ func NewEnvelope(offsets []Point, nbin int) Envelope {
 	nout := (n + stride - 1) / stride
 	bins := make([]Bin, nout)
 	for k := 0; k < nout; k++ {
-		bins[k].X = offsets[k*stride].X
 		m := k * stride
+		bins[k].X = offsets[m].X
 		end := m + stride
 		if end > len(offsets) {
 			end = len(offsets)
@@ -113,3 +127,70 @@ func (e Envelope) FindSmallestDomain(r Range) Domain {
 	}
 	return Domain{x0, x1}
 }
+
+type combiner struct {
+	envelopes []Envelope
+}
+
+func (c *combiner) next() (Bin, bool) {
+	idx := -1
+	for i := range c.envelopes {
+		if len(c.envelopes[i]) == 0 {
+			continue
+		}
+		if idx == -1 || c.envelopes[i][0].X < c.envelopes[idx][0].X {
+			idx = i
+		}
+	}
+	if idx == -1 {
+		return Bin{}, true
+	}
+	b := c.envelopes[idx][0]
+	c.envelopes[idx] = c.envelopes[idx][1:]
+	return b, false
+}
+
+func (c *combiner) nextN(n int) []Bin {
+	var bins []Bin
+	for i := 0; i < n; i++ {
+		b, done := c.next()
+		if done {
+			return bins
+		}
+		bins = append(bins, b)
+	}
+	return bins
+}
+
+func (c *combiner) size() (size int) {
+	for _, e := range c.envelopes {
+		size += len(e)
+	}
+	return
+}
+
+func (c *combiner) maxlen() (l int) {
+	for _, e := range c.envelopes {
+		if len(e) > l {
+			l = len(e)
+		}
+	}
+	return
+}
+
+// Merge squashes the two envelopes together returning a new Envelope the size
+// of the longest Envelope provided.
+func (e Envelope) Merge(u Envelope) Envelope {
+	c := &combiner{[]Envelope{e, u}}
+	n := c.size()
+	nbin := c.maxlen()
+	stride := strideSize(n, nbin)
+	nout := (n + stride - 1) / stride
+	out := make([]Bin, nout)
+	for k := 0; k < nout; k++ {
+		bins := c.nextN(stride)
+		out[k].X = bins[0].X
+		out[k].Range = rangeOfBins(bins)
+	}
+	return out
+}
diff --git a/pkg/ranger/ranger_test.go b/pkg/ranger/ranger_test.go
index b4615a5c19..2af90dcdc6 100644
--- a/pkg/ranger/ranger_test.go
+++ b/pkg/ranger/ranger_test.go
@@ -1,21 +1,34 @@
-package ranger_test
+package ranger
 
 import (
 	"math"
 	"testing"
 
-	"github.com/brimsec/zq/pkg/ranger"
 	"github.com/stretchr/testify/assert"
 )
 
-func find(pts []ranger.Point, nbin int, r ranger.Range) ranger.Domain {
-	e := ranger.NewEnvelope(pts, nbin)
+func find(pts []Point, nbin int, r Range) Domain {
+	e := NewEnvelope(pts, nbin)
 	return e.FindSmallestDomain(r)
 }
 
+// func TestEnvelope(t *testing.T) {
+// t.Parallel()
+// pts := []Point{
+// {1, 0x100},
+// {2, 0x120},
+// {3, 0x110},
+// {4, 0x130},
+// {5, 0x150},
+// {6, 0x150},
+// }
+// d := find(pts, 2, Range{0x100, 0x120})
+// assert.Exactly(t, Domain{1, 5}, d)
+// }
+
 func TestEnvelope(t *testing.T) {
 	t.Parallel()
-	pts := []ranger.Point{
+	pts := []Point{
 		{1, 0x100},
 		{2, 0x120},
 		{3, 0x110},
@@ -23,25 +36,46 @@ func TestEnvelope(t *testing.T) {
 		{5, 0x150},
 		{6, 0x150},
 	}
-	d := find(pts, 0, ranger.Range{0x151, 0x151})
-	assert.Exactly(t, ranger.Domain{}, d)
-	d = find(pts, 0, ranger.Range{0, 0x90})
-	assert.Exactly(t, ranger.Domain{}, d)
-	d = find(pts, 0, ranger.Range{0x90, 0x111})
-	assert.Exactly(t, ranger.Domain{1, 4}, d)
-	d = find(pts, 0, ranger.Range{0x115, 0x135})
-	assert.Exactly(t, ranger.Domain{2, 5}, d)
-	d = find(pts, 0, ranger.Range{0x150, 0x150})
-	assert.Exactly(t, ranger.Domain{5, math.MaxUint64}, d)
-	d = find(pts, 0, ranger.Range{0x151, 0x151})
-	assert.Exactly(t, ranger.Domain{}, d)
-	d = find(pts, 3, ranger.Range{0x100, 0x109})
-	assert.Exactly(t, ranger.Domain{1, 3}, d)
-	d = find(pts, 3, ranger.Range{0x100, 0x120})
-	assert.Exactly(t, ranger.Domain{1, 5}, d)
-	d = find(pts, 3, ranger.Range{0x100, 0x130})
-	assert.Exactly(t, ranger.Domain{1, 5}, d)
+	d := find(pts, 0, Range{0x151, 0x151})
+	assert.Exactly(t, Domain{}, d)
+	d = find(pts, 0, Range{0, 0x90})
+	assert.Exactly(t, Domain{}, d)
+	d = find(pts, 0, Range{0x90, 0x111})
+	assert.Exactly(t, Domain{1, 4}, d)
+	d = find(pts, 0, Range{0x115, 0x135})
+	assert.Exactly(t, Domain{2, 5}, d)
+	d = find(pts, 0, Range{0x150, 0x150})
+	assert.Exactly(t, Domain{5, math.MaxUint64}, d)
+	d = find(pts, 0, Range{0x151, 0x151})
+	assert.Exactly(t, Domain{}, d)
+	d = find(pts, 3, Range{0x100, 0x109})
+	assert.Exactly(t, Domain{1, 3}, d)
+	d = find(pts, 3, Range{0x100, 0x120})
+	assert.Exactly(t, Domain{1, 5}, d)
+	d = find(pts, 3, Range{0x100, 0x130})
+	assert.Exactly(t, Domain{1, 5}, d)
 	pts[5].Y = 0x149
-	d = find(pts, 3, ranger.Range{0x100, 0x149})
-	assert.Exactly(t, ranger.Domain{1, math.MaxUint64}, d)
+	d = find(pts, 3, Range{0x100, 0x149})
+	assert.Exactly(t, Domain{1, math.MaxUint64}, d)
+}
+
+func TestUnion(t *testing.T) {
+	env1 := Envelope{
+		{1, Range{0x100, 0x120}},
+		{3, Range{0x110, 0x130}},
+		{5, Range{0x150, 0x150}},
+	}
+	env2 := Envelope{
+		{7, Range{0x110, 0x160}},
+		{9, Range{0x170, 0x180}},
+		{11, Range{0x190, 0x190}},
+		{13, Range{0x180, 0x230}},
+	}
+	assert.Exactly(t, env1.Merge(env2), Envelope{
+		{1, Range{0x100, 0x130}},
+		{5, Range{0x110, 0x160}},
+		{9, Range{0x170, 0x190}},
+		{13, Range{0x180, 0x230}},
+	})
+	assert.Exactly(t, Envelope{}.Merge(env1), env1)
 }